Search for heavy stable charged particles in the CMS experiment using the RPC detectors

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(1)Search for Heavy Stable Charged Particles in the CMS Experiment using the RPC Detectors. Camilo Andrés Carrillo Montoya PhD Thesis in Physics. Universidad de los Andes. Advisor: Juan Carlos Sanabria PhD Universidad de los Andes. Co-Advisor: Marcello Maggi PhD INFN Sezione di Bari. Geneva, November 10th 2010..

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(3) iii. Agradecimientos En los túneles del ISR en donde las RPCs que serı́an instaladas en el CMS eran ensambladas, conocı́ a mis primeros amigos y colegas: Archana Sharma, Andrey Marinov, Serguei Akiamenko, Ian Crotty, Walter Vandoninck, Ban Yong, Sohaib Alam, Supriya Muraleedharanv y Juan Arduino. Con ellos compartı́ la primera experiencia de ensamblar un detector. A pesar que el tiempo que estuve trabajando en la construcción de las RPCs fue relativamente corto, el hecho de conocer el detector de primera mano representó una gran ventaja a la hora de estudiarlo con el software. Quiero agradecer las enseñanzas y lecciones sobre el mundo del hardware que aprendı́ a este grupo de gente.. A mediados de 2006 mi primer encuentro con el software de CMS, CMSSW, fue de la mano de Ilaria Segoni, quiero agradecer su paciencia y diversas explicaciones sobre este complejo pero completo software, también agradezco a Ricardo Bellan, Nicola Amapane, Tim Cox, Gregory Rakness y Zoltan Gecse por sus múltiples respuestas y ayudas en el mismo tema.. A mis amigos y colegas de las RPCs, Salvatore Tupputi, Raffaello Trentadue, Giuseppe Rosselli, Giovanni Polese, Anna Cimmino, Claudio Viviani, Michele De Gruttola, Annapaola de Cosa y Orso Iorio; quiero agradecer no solo el haber compartido conmigo la complicidad del estudiante y este largo periodo de aprendizaje sino las amenas clases de italiano. En el grupo de RPCs también están Anton Dimitrov, Filip Thyssen y Piet Verwilligen a los que agradezco haber compartido todas estas experiencias conmigo.. A mis guı́as en el grupo de las RPCs; Davide Piccolo, Gabriella Pugliese y Anna Colaleo; quiero agradecer su constante apoyo e interes por mi trabajo en la puesta en servicio de las RPCs. En especial quiero agradecer a Giusseppe Iaselli por siempre creer en mi trabajo. Al grupo CMS INFN de Nápoles guiado por Luca Lista y.

(4) iv Pierluigi Paolucci quiero agradecer su paciencia e invaluable apoyo en los últimos meses de trabajo para culminar mi doctorado.. A Marcello Maggi le quiero agradecer en varios aspectos. La idea de la extapolación de segmentos de las DTs y CSCs a las RPCs es originalmente suya, este simple pero poderozo algoritmo fue la herramienta para todo mi trabajo de servicio con las RPCs. Trabajando con él he aprendido a no subestimar cualquier resultado experimental, esto es el núcleo de un análisis exitoso. De él también aprendı́ que mantener presente la conexión entre el detector y la fı́sica que hacemos es la primera pista para la solución de cualquier problema. Su guı́a en el CERN siempre fue sin presiones, dando plena libertad a la imaginación y a la creatividad, esto fue la clave para el exitoso paso por el periodo de puesta en servicio de las RPCs y el análisis de HSCPs.. A Giacommo Bruno, Andrea Rizzi, Jie Chen, Loic Quentermont, Seth Cooper, Malgorzata Kazana y todo el grupo de HSCPs quiero agradecer su ayuda especialmente en la producción y validación de simulaciones Monte Carlo al igual que su ayuda en el camino al entendimiento de los diferentes modelos teóricos que predicen HSCPs.. A los profesores Carlos Ávila y Bernardo Gómez quiero agradecerles la confianza que depositaron en mi al designarme como uno de los representates del grupo ante el CERN y su apoyo constante durante el desarrollo de mi doctorado. Al profesor Juan Carlos Sanabria quiero agradecerle su invaluable ayuda en la escritura y corrección del documento de tesis y sus innumerables consejos para la ciencia y para la vida. A los profesores del departamnteo de fı́sica de la universidad de los Andes quiero agradecer las lecciones de fı́sica impartidas durante los primeros meses de doctorado y al personal administrativo y en espeial a Julieta Rodrı́guez agradezco su pronta ayuda en todos los procesos administrativos y académicos que se presentaron a lo largo de mi doctorado.. A John Ellis le quiero agradecer haber sugerido este tema de tesis, pues se ajustó.

(5) v perfectamente a los conocimientos que habı́a adquirido previamente con mi trabajo en las RPCs.. Agradezco a Verónica Riquer, Luciano Maiani y a todo el proyecto HELEN, el apoyo económico durante los primeros años de mi programa de doctorado, el cual hizo posible mi llegada al CERN. A la Universidad de los Andes, la facultad de ciencias, el departamento de fı́sica y a COLCIENCIAS agradezco también el apoyo económico durante el desarrollo de este proyecto de investigación. A la “colaboración CMS” en especial a Kirsti Aspola y Dawn Hudson agradezco su colaboración con todas las operaciones administrativas y burocráticas durante el desarrollo de mi trabajo de investigación en el CERN.. A mis amigos y colegas colombianos que están o que estuvieron en el CERN: Alberto Ocampo, Andres Osorio, Diego Reyes, Ariel Gómez, Luis Lopera, Marcelo Baquero, Sebastian Bonilla, Juan Sebastian Rodrı́guez, Andrés Flores, Guillermo Zamudio, Carlos Medina, Heather Brown, Carlos Sandoval, Carlos Bula y Leandro Franco agradezco su cálida compañı́a durante todo este tiempo.. A mis amigos de Carpe-Diem y del club de ping-pong, les agradezco haber llenado de amistad mis tiempos libres. A los Carriage y a los Piccino les agradezco haber sido mi familia en Suiza.. Finalmente agradezco a todos mis primos, tios y amigos por su afecto siempre latente sin importar la distancia, a mis Padres por su constante e infinito amor y a Isabel y a Germán les agradezco haber llegado hasta acá para ser mi familia y mis mejores amigos en estas tierras lejanas..

(6) vi .. “Nunca releo mis libros, porque me da miedo” Gabriel Garcı́a Márquez.

(7) 1 Abstract In this work the results of a search for heavy stable charged particles in protonproton collisions at a center-of-mass energy of 7 TeV is presented. The search was performed with the CMS detector at the CERN LHC accelerator using the data from the 2010 run. The RPC detectors of the Muon System of CMS were used to reconstruct tracks consistent with massive and slow-moving charged particles, using what is know as the time-of-flight technique. These particles are predicted by several theoretical scenarios beyond the Standard Model that include long-lived massive charged particles as part of their spectrum, for instance different flavors of supersymmetry, universal extra-dimension models etc. These charged particles can be hadrons that have long-lived gluinos or stops in their partonic content, known as R-Hadrons. The integrated luminosity of the LHC during the 2010 run and the predicted cross sections for this kind of states indicate that only R-Hadrons and staus could be observed. The R-Hadrons can be the result of long-lived stops predicted by the Minimal Supersymmetric Standard Model or long-lived gluinos predicted by split SUSY. The staus can be the result of the Minimal Supersymmetric Standard Model with minimal gauge mediated symmetry breaking. Since these particles are massive and have electric charge, they behave as slow muons, being very penetrating and leaving traces in the detectors, therefore they can be registered by the Muon System of CMS. The CMS RPCs record timing information for these tracks allowing for the estimation of the speed β. The curvature of the tracks in the CMS magnetic field is used for the determination of the momentum p of the particle. Given β and p, the mass of the state can be determined. States with mass of the order of hundreds of GeV represent the discovery of physics beyond the Standard Model. The first run of the LHC finished on October 28 2010, the LHC has delivered a total integrated luminosity of 46.36 pb−1 , of those CMS has recorded 42.51 pb−1 , Using the certified data (30.68 pb−1 by November 7) and the RPC time-of-flight technique, limits on the mass for HSCPs predicted by three theoretical models were set with 95% of confidence level (split-SUSY gluino: 299.5 GeV/c2 , MSSM s-top: 167.5 GeV/c2 and mGMSB SUSY stau:107.5 GeV/c2 ). The complete set of results and a detailed description of the analysis are presented in this document..

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(9) Contents 1 Heavy Stable Charged Particles. 13. 1.1. HSCPs in the Minimal Supersymmetric Standard Model . . . . . . . 18. 1.2. HSCPs in Gauge Mediated Symmetry Breaking . . . . . . . . . . . . 21. 1.3. HSCPs in Split-SUSY Models . . . . . . . . . . . . . . . . . . . . . . 24. 1.4. Previous Searches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27. 2 The CMS Detector. 31. 2.1. The Large Hadron Collider (LHC) . . . . . . . . . . . . . . . . . . . . 31. 2.2. The Compact Muon Solenoid Detector . . . . . . . . . . . . . . . . . 34 2.2.1. Solenoid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35. 2.2.2. Silicon Tracker . . . . . . . . . . . . . . . . . . . . . . . . . . 38. 2.2.3. Electromagnetic Calorimeter [1] . . . . . . . . . . . . . . . . . 41. 2.2.4. Hadron Calorimeter . . . . . . . . . . . . . . . . . . . . . . . . 44. 2.2.5. CMS Muon Detectors . . . . . . . . . . . . . . . . . . . . . . . 48. 2.2.6. Drift tube system . . . . . . . . . . . . . . . . . . . . . . . . . 50. 2.2.7. Cathode strip chambers . . . . . . . . . . . . . . . . . . . . . 52. 2.2.8. Resistive Plate Chambers . . . . . . . . . . . . . . . . . . . . 56. 2.2.9. CMS Trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . 60. 2.2.10 L1 Trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 2.2.11 The Event Filter . . . . . . . . . . . . . . . . . . . . . . . . . 65 3 Search for HSCPs 3.1. 69. Signals of HSCPs in CMS . . . . . . . . . . . . . . . . . . . . . . . . 69 3.1.1. dE/dx . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70. 3.1.2. Time of flight technique for RPCs . . . . . . . . . . . . . . . . 72 3.

(10) CONTENTS. 4 3.1.3. Measurement of velocity . . . . . . . . . . . . . . . . . . . . . 75. 3.1.4. Measurement of momentum . . . . . . . . . . . . . . . . . . . 79. 3.1.5. Measurement of HSCP mass . . . . . . . . . . . . . . . . . . . 81. 3.1.6. Monte Carlo simulation studies . . . . . . . . . . . . . . . . . 84. 3.2. Triggering of HSCPs . . . . . . . . . . . . . . . . . . . . . . . . . . . 85. 3.3. Offline Muon Reconstruction . . . . . . . . . . . . . . . . . . . . . . . 96. 3.4. Reconstruction of HSCP Tracks . . . . . . . . . . . . . . . . . . . . . 97. 3.5. 3.4.1. HSCP identification for different models as a function of β . . 98. 3.4.2. HSCP identification for different models as a function of η . . 106. 3.4.3. HSCP identification for different models as a function of φ . . 111. 3.4.4. HSCP identification for different models as a function of p . . 116. Summary on HSCP efficiency reconstruction . . . . . . . . . . . . . . 121. 4 CMS RPC System Performance. 123. 4.1. DT and CSC Segment Extrapolation . . . . . . . . . . . . . . . . . . 123. 4.2. RPC-detector Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . 125. 4.3. 4.4. 4.2.1. RPC detector efficiency definition . . . . . . . . . . . . . . . . 126. 4.2.2. Efficiency studies with Monte Carlo . . . . . . . . . . . . . . . 127. 4.2.3. Efficiency studies with cosmic ray data . . . . . . . . . . . . . 129. 4.2.4. Efficiency studies with collision data . . . . . . . . . . . . . . 135. 4.2.5. High-Level-Trigger for commissioning . . . . . . . . . . . . . . 136. 4.2.6. HV Scan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139. Software Geometry Surveys . . . . . . . . . . . . . . . . . . . . . . . 140 4.3.1. Strip lengths. 4.3.2. Chamber alignment . . . . . . . . . . . . . . . . . . . . . . . . 141. 4.3.3. FEBs connectivity test . . . . . . . . . . . . . . . . . . . . . . 142. Data quality monitoring . . . . . . . . . . . . . . . . . . . . . . . . . 142 4.4.1. 4.5. 4.6. . . . . . . . . . . . . . . . . . . . . . . . . . . . 140. Offline chamber-detection efficiency . . . . . . . . . . . . . . . 143. RPC local reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . 144 4.5.1. RPC Cluster studies . . . . . . . . . . . . . . . . . . . . . . . 145. 4.5.2. Masked-strips reclusterization . . . . . . . . . . . . . . . . . . 147. RPC Performance and HSCP searches . . . . . . . . . . . . . . . . . 148.

(11) CONTENTS. 5. 5 Data Analysis 5.1. Collected Luminosity . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 5.1.1. 5.2. 153. Quality bit distribution for different runs . . . . . . . . . . . . 156. Datasets for HSCPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 5.2.1. Mu Primary Data Set . . . . . . . . . . . . . . . . . . . . . . 157. 5.3. Expected events from different models . . . . . . . . . . . . . . . . . 160. 5.4. Event Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162. 5.5. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169. 6 Conclusions. 173. A CMS conventions. 177. A.1 The CMS coordinate system . . . . . . . . . . . . . . . . . . . . . . . 177 A.2 CMS units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 B HLT-paths present in CMSSW 35X. 179. C Publications by the author during his PhD work. 183. D Presentations. 189. E Developed Software. 191. F Certified lumi-sections.. 193. G CMS Achievement Award. 195.

(12) 6. CONTENTS.

(13) Introduction The Standard Model of Particles have represented during the last 30 years our best description of the laws of physics at the most fundamental level. This model has been put to the test by several experiments during the last 3 decades. Its predictions are in excellent agreement with the data, specially in the electroweak sector. The accuracy of the results obtained by LEP during the 90’s (see Figs. 1 and 2) attest the success of this model [2].. Figure 1:. Agreement between SM predictions and different experiments. (LEP,SLC,KEKB).. In spite of its phenomenological predicting power, there are several theoretical aspects that lead to believe that this may not represent the ultimate theory of particles and their interactions. Among the main problems of the model, one 7.

(14) 8. CONTENTS. Figure 2: Recent results from the LEP Electroweak Working Group. LEP-I+SLD lept measurements of sin2 θef f and Γll and the SM prediction. The point with the arrow. labelled ∆α shows the prediction if among the electroweak radiative corrections only the photon vacuum polarisation is included. The associated arrow shows variation of this prediction if α(m2Z ) is changed by one standard deviation. This variation gives an additional uncertainty to the SM prediction shown in the figure. Taken from [2].

(15) CONTENTS. 9. could mention the high number of free parameters; the absence of an explanation for the dark matter problem and the difficulties when extending the theory to a higher energy scale, due mostly to the appearance of quadratic divergences in the normalization of scalar fields (Higgs). In recent times one of the emphasis has been the search for evidences of physics beyond the standard model, for instance: different versions of the supersymmetry hypothesis and of models with extra-dimensions. Some of these theoretical scenarios predict results that can be tested experimentally in the TeV energy scale, which is the energy scale covered by the LHC. The main scientific goals of this accelerator and its associated experiments is to search for the Higgs Boson and any physics beyond the standard model. The energy of the proton-proton collisions produced at the LHC accelerator will explore a kinematical region where evidences of supersymmetry and even extra-dimensions should appear. The CMS and ATLAS detectors have been optimized in their design for the search and discovery of this new physics. In march of 2010 the LHC (Large Hadron Collider) started its first run for physics producing p-p collisions with the center of mass energy of 7 TeV, an energy never reached before in previous particles accelerators. At this energy evidences of production of supersymmetric matter are expected and with lesser probability of extra-dimension. Some of the most interesting candidates, from the experimental point of view are heavy stable (or quasi-stable) charged particles (HSCPs) since they would leave a direct signal in the detectors due to their electric charge and possible long life-time. Their mass could be in the range of hundreds of GeV, causing they to move with a speed significantly lower than the speed of light when produced with momentum of the order of 1 TeV/c. These particles are predicted by parameterizations of the minimal supersymmetric standard model (MSSM) in which the next to the lightest supersymmetric particle has a reduce phase space to decay into, giving it a long life time. One of the most probable scenarios is the production of long-lived stops that will have enough time to form hadronic bound states known as R-Hadrons. Another possibility within the framework of supersymmetry comes from split-SUSY in which the gluino may be long-lived, again having enough time to form R-Hadrons. These R-Hadrons are massive, electrically charged and long-lived as well, therefore.

(16) CONTENTS. 10. they would behave as HSCPs. Another possibility is the production of staus in models with Gauge Meditated Symmetry Breaking (GMSB), in all these cases, the production cross section is large enough to be probed during the first run of the LHC. In the case of the minimal extra-dimensions model a long lived kktau is expected however with cross section too small to be tested at this stage. The HSCPs, regardless of their nature, would behave as “heavy muons” because of their mass and electric charge. They would travel at a speed significantly slower than light, depositing a higher amount of energy as they go through matter, allowing for the use of the time-of-flight (ToF) and the dE/dx experimental techniques in their search. The CMS experiment is conducting several searches for HSCPs, for example using the dE/dx technique with the silicon tracker and the electromagnetic calorimeter, or the ToF technique with the DTs and RPCs. Another possibility is the measurement by the calorimeters of the late decay products of R-Hadrons stopped in the material of the detector (once the LHC beams are off). In this work, the search for HSCPs using the Time of Flight technique and the RPC detectors is presented. The ToF technique is based on measuring the time of flight of a particle through a known distance to extract β and to get information of the momentum p by studding the curvature of the trajectory of the particle in a magnetic field. With β and p the mass of the state can be determined. The mass extraction is only possible for particles with β less than one, which can be de case of HSCPs. The CMS experiment was not designed to measure the time of flight of any particle since in most cases they move at the speed of light. The only global timing information is related with the crossing of bunches of protons through the interaction point every 25 ns (at full luminosity). Every time information is measure in units of 25 ns. In the case of slow moving particles like HSCPs moving through the outer layers of the muon system their time delay can be higher than 25 ns. This delay information can be used to implement a time of flight technique, since the RPC are trigger detector with high time resolution ≈ 1 ns, they are suitable for this purpose, so that even though was not conceived for this kind of analysis it can be done with enough resolution to make discoveries. In summary in this document a search for stop R-Hadrons, gluino R-Hadrons and staus using the time of flight technique with RPCs is reported. In chapter 1.

(17) CONTENTS. 11. the different models that predict HSCPs are discussed, including tables and plots of masses and cross sections. Making an emphasis on the LHC potential for their discovery. Summary of the result of previous searches at LEP and Tevatron are also presented. In Chapter 2 a general description of the CMS and all its subtedetector is included. The emphasis in this chapter is the RPCs as well as the timing and trigger information that is needed to implement the time of flight technique. In chapter 3 a very detailed description of the Time of Flight method developed for this search is discussed. In chapter 4 a thorough description of the CMS RPC system and its performance is presented. All this information is crucial in understanding and determining trigger efficiencies, reconstruction efficiencies and geometrical acceptances related with the method and with the search. In chapter 5 the data samples and the results of the data analysis are presented. Finally the conclusions of this research are discussed in chapter 6..

(18) 12. CONTENTS.

(19) Chapter 1 Heavy Stable Charged Particles The range of energies that are available at the LHC open the possibility for production of new and exotic forms of matter [3]. Many theoretical models predict the existence of new particles whose signature could be searched by detectors like CMS[4]. Some of the most interesting candidates, from the experimental point of view, are heavy stable (or quasi-stable) charged particles (HSCPs) since they would leave a direct signal in the detectors due to their electric charge and possible long life-time. Their mass could be in the range of hundreds of GeV, therefore they will move with a speed significantly lower than the speed of light when produced with momenta less than a TeV/c. From the mere experimental point of view they should behave as “slow muons”, and could be identified by the muon detectors. The HSCPs are predicted by several models like R-parity conserving supersymmetry, split supersymmetry, universal extra dimension models with Kaluza-Klein parity (KK-Parity), F-theory, etc [5]. Since these hypothetical HSCPs can be observed directly, their search is model-independent.. Several theoretical models inspired in the idea of supersymmetry (SUSY) accommodate the possibility of HSCPs for some parameterizations, that is the case of R-Parity-conserving models like the minimal standard supersymmetric model (MSSM). The R-Parity is a multiplicative quantum number that results in a value of +1 for particles of the Standard Model (SM), and −1 for their SUSY partners [6, 7]. The conservation of the R-Parity implies that the SUSY partners should be 13.

(20) CHAPTER 1. HEAVY STABLE CHARGED PARTICLES. 14. produced in pairs in p−p collisions, the decay of each of them should involve another SUSY state, and as a consequence of all this, the lightest supersymmetric particle (LSP) should be stable, since it does not have a lighter SUSY particle to decay into. This LSP could have electric charge, however, astrophysical and cosmological arguments point to neutral massive and weakly-interacting stable particles (WIMPs), that would become candidates for dark matter [6]. In this case, if the next particle in mass to the LSP (NLSP) has a long life time (quasi-stable) and electric charge, it becomes the “HSCP” to be looked for.. In order to be consistent with the phenomenology of the SM, SUSY has to be a broken symmetry, because for instance no bosons of the same mass of the SM fermions have ever been observed, therefore there should be a mass gap between SUSY partners, and this gap can be produced by breaking SUSY. There are different mechanisms through which this SUSY breaking can be achieved in such a way that the theory preserves all the good field-theory features regarding removal of pathological higher order divergences etc. The minimum SUSY model that includes the SM has more than 100 free parameters. The ignorance about the value of all these parameters allows for a large number of possibilities, and among those, several that predict long-lived charged NLSPs. For example, the τ̃ would be an HSCP in the MSSM when the lightest neutralino, χo1 , is the LSP; or in gravity-mediated-SUSYbreaking models where the LSP is the gravitino. For some other parameterizations of the MSSM the lightest chargino χ+ 1 would be a HSCP.. Some SUSY models could have an s-quark like the t̃ or the b̃ as quasi-stable NLSPs. Even the g̃ could be long-lived, like for example in the case of split SUSY [8] [9]. In these scenarios since q̃ and g̃ interact strongly, they will be subject to the confining strong forces that form hadrons. They will go through a hadronization stage during which they will end up being part of hadrons generically known as R-Hadrons [10]. These could be R-Baryons, R-Mesons, or R-Glueballs (see Table 1.1)..

(21) 15 Particle. gluino (g̃). stop (t̃). R-Baryons. g̃qqq. R-AntiBarions. g̃ q̄ q̄ q̄. t̃qq ˜t̄q̄ q̄. R-Mesons. g̃q q̄. t̃q̄. R-Glue Balls. g̃g. R-Anti Mesons. -. ˜t̄q. Table 1.1: Clasification of R-Hadrons according to their partonic content.. As it can be seen from the table 1.1, some of the R-hadrons are charged and some are neutral. The main experimental feature that distinguishes R-Hadrons from other HSCPs is the possibility of electric charge flipping, since due to hadronic interactions with surrounding matter, R-hadrons crossing the detector could change charge by “replacing” a light quarks bounded to the heavy parton by a different quark from the medium [4]. A schematic view of this process is show in Fig. 1.1.. Figure 1.1: Track of an R-Hadron that flips charge while crossing the CMS detector. (Taken from [11]). Other candidates to HSCPs are KK-leptons. These massive charged and longlived particles are predicted in models with extra dimensions like the Universal Extra Dimensions Model (UED) [12]. In these models the SM fields can get excited and propagate in compacted extra dimensions as KK states. The lightest of these KK excitations is expected to be stable and neutral, becoming a candidate for dark.

(22) CHAPTER 1. HEAVY STABLE CHARGED PARTICLES. 16. matter. In this kind of theories there is also the possibility that KK modes of light SM particles would have a long life time, for instance the KK-τ could behave as HSCP, and would be detected by the muon system of the CMS as a slow moving particle. In proton proton collisions there are several possibilities of production channels for HSCPs. In Fig. 1.2 it is presented one example for every type of HSCP.. Figure 1.2: Feynman diagrams of production of HSCPs in all the presented models (stop, gluino and stau), taken from [3].. As it was mentioned earlier, regardless of the nature of the HSCPs, they will leave a direct signal in the tracker, calorimeters and muon systems of CMS. Their nature as massive and charged particles will allow for the use of techniques like Time of Flight (ToF) and Energy Loss (dE/dX) to identify them [13], making the search model-independent and only driven by the mass of the state.. In Table 1.2 a summary of the most plausible models is presented, together with the predicted HSCP, possible values for its mass and the corresponding production cross section at 7 TeV. As it can be seen from the table, the largest cross sections are for light gluinos (in split SUSY models) and s-tops (in MSSM parameterizations). In both scenarios the observed state would be an R-Hadron. The order of magnitude of the cross sections in this cases are such that there is a significant probability of observing them during the first run of the LHC. In the case of mGMSB models with a light τ̃ as HSCP, the value of the cross section indicates that, even though small, there is some probability of observing a few of them during the first run. The case of the τKK is different since the cross section is too small and there is no hope to detect any signal at this stage. In view of these numbers, the cross section production limits that have been performed in this work only involved R-Hadrons.

(23) 17 resulting from gluinos or s-tops, as well τ̃ , all of them predicted in different flavors of SUSY. In the next section will be discussed some details of these theoretical scenarios.. Theoretical Model. mGMSB. mUED. HSCP. Mass (GeV/c2 ). Expected Cross Section (pb) √ at s = 7 T eV. τ̃1. 100. 1.34 × 100. τ̃1. 126. 3.35 × 10−1. τ̃1. 156. 9.85 × 10−2. τ̃1. 200. 2.26 × 10−2. τ̃1. 247. 7.75 × 10−3. τ̃1. 308. 2.14 × 10−3. τ̃k. 300. 5.70 × 10−3. g̃. 200. 3.27 × 10+2. g̃. 300. 2.77 × 10+1. g̃. 600. 1.71 × 10−1. g̃. 900. 3.94 × 10−3. g̃. 1200. 1.69 × 10−4. g̃. 1500. 1.11 × 10−5. t̃1. 130. 6.55 × 10+1. t̃1. 200. 6.83 × 10+0. t̃1. 300. 6.48 × 10−1. t̃1. 500. 2.29 × 10−2. t̃1. 800. 5.42 × 10−4. Split SUSY. MSSM. Table 1.2: Models considered in this work in the search for HSCP and its cross section computed by Pythia 6 wrapped in CMSSW..

(24) CHAPTER 1. HEAVY STABLE CHARGED PARTICLES. 18. 1.1. HSCPs in the Minimal Supersymmetric Standard Model. The supersymmetry represents an extension of the fundamental Poincare space-time symmetry that promotes fermionic and bosonic fields into superfields. Each superfield includes an equal number of bosonic and fermionic degrees of freedom, all of them with the same mass. Filed theories can be constructed for these superfields, solving theoretical problems like for example the so-called hierarchy problem associated with the ratio MZ2 /MP2 lanck , the removal of quadratic divergences in the renormalization of scalar field masses, etc. The absence of partner particles with the same mass but with spin that differs by 1/2 to the SM ones, suggests that the supersymmetry must be broken at the weak scale, generating mass gaps between super-partners. This mass gaps should be of the order of ∼ 1 TeV to preserve the good features of the theory. Arguments coming from cosmology and from grand unification also point to this order of magnitude for the mass gaps. The breaking of SUSY allows for the inclusion of the SM fields within this framework playing the role of the light super-partners. In the most general versions of SUSY models, that include the SM, lepton and barion numbers are violated, resulting on proton decay. To implement the barion (B) and lepton number (L) conservation at the SUSY level, the conservation of the quantum number R = (−1)3B+L+2S is postulated (R-Parity conservation). Given the multiplicative nature of R and since the SM particles have R = +1 and the SUSY partners have R = −1 the lightest supersymmetric particle (LSP) should be stable. If the R-Parity is violated, as happens in some models, the LSP can be unstable but could be long-lived.. At this point there are no phenomenological, or even theoretical arguments that could provide solid information about the needed SUSY-breaking mechanism. The most economical extension of the SM into a SUSY theory, the so-called Minimal Supersymmetric Standard Model (MSSM), considers SUSY-breaking’s that preserve the gauge invariance of the theory, as well as the Poincare invariance. This kind of mechanisms is referred to as soft SUSY-breaking. The large number of free parameters in the MSSM can be reduced drastically by resorting to a grand unification of.

(25) 1.1. HSCPS IN THE MINIMAL SUPERSYMMETRIC STANDARD MODEL 19 interactions at an energy scale of ∼ 1016 GeV (GUT scale).. The freedom in the choice of input parameters in the MSSM allows for several possible HSCPs. In models where χ̃01 is the LSP and the candidate for dark matter, a HSCP results from an NLSP that is very close in mass to the LSP, so that it acquires a long life-time due to the reduced phase space for its decay. One of the possible scenarios involves the t̃ as the NLSP, that is light enough so that the channel t̃ → bχ̃+ 1 is kinematically suppressed, resulting in a long-lived stop. As mentioned above, there are several possibilities for HSCPs within the framework of the MSSM, but as shown in Table 1.2, the one with large enough production cross section is precisely the t̃, therefore, as far as this work is concerned, in the case of the MSSM the R-Hadrons associated with stops were searched for in the data. In Fig. 1.3 the predicted cross sections for different masses on this model are shown. Figure 1.3: Cross Section for different masses of stops t̃1 at 7 TeV, using pythia and CMSSW358patch2. From the experimental point of view, the most important kinematical value is the speed, β, of the particle. For this model the β distribution for different masses is shown in figure Fig. 1.4. The β of the resulting R-hadron will be very similar to the.

(26) CHAPTER 1. HEAVY STABLE CHARGED PARTICLES. 20. one of the t̃, due to the very high mass of this parton that will therefore carry most of the momentum of the state. In Fig. 1.5 the momentum distribution for different masses is shown and in Fig. 1.6 is shown the eta distribution for the t̃.. Figure 1.4: β distribution for different masses of stops t̃1. Figure 1.5: Momentum distribution for different masses of stops t̃1 Note that the majority of the particles will be produced for low η values Fig. 1.6, where the muon system in CMS has a better resolution, compared with the one in the endcaps..

(27) 1.2. HSCPS IN GAUGE MEDIATED SYMMETRY BREAKING. 21. Figure 1.6: Eta distribution for different masses of stops t̃1. 1.2. HSCPs in Gauge Mediated Symmetry Breaking. When the supersymmetric transformation is gauged (promoted to a local symmetry), a new gauge superfield ought to be included. This superfield is constituted by a spin 2 boson and a spin 3/2 fermion, the graviton and the gravitino. This local version of SUSY is called Supergravity (SUGRA). Within SUGRA there are also several options to implement the SUSY-breaking, for instance the Gauge-MediatedSymmetry-Breaking mechanism (GMSB). In this scenario the SUSY-breaking is mediated by chiral SU (5) multiplets added to the theory as coupling between the MSSM fields via SM-gauge-interactions, and a hidden sector that is the source of the breaking. As a result of all this, the gravitino becomes very light and constitutes the LSP of the theory. The NLSP decays into the gravitino in a process mediated by gravity and since the gravitational coupling is very weak, this state acquires long life-time and becomes an HSCP. In the minimum version of this kind of models, the so-called mGMSB, a light τ̃ becomes the HSCP of the theory. Table 1.2 shows how, even though the production cross section is one to two orders of magnitude smaller than that for R-Hadrons (stops and gluinos), it is still within reach of the first run of the LHC. There is an experimental advantage for the search of this kind of lepton-like-HSCPs due to the fact that the efficiency for the.

(28) CHAPTER 1. HEAVY STABLE CHARGED PARTICLES. 22. presented technique is much higher due to the fact that since they behaves more like muons, there is no hadronization stage and no charge flipping. Considering this, a search for mGMSB τ̃ was performed as part of this work. In Fig. 1.7 the cross section for different masses on this model is presented. Two benchmark points selected of this model are on the SPS line 7 [14] and the benchmark parameter values corresponding to these two points are:. τ̃ (156) : N = 3, Λ = 50000 GeV, M = 100000 GeV, tan β = 10, sign(µ) = 1,. cg rav = 10000 τ̃ (247) : N = 3, Λ = 80000 GeV, M = 160000 GeV, tan β = 10, sign(µ) = 1,. cg rav = 10000. As it can be seen the corresponding τ̃ masses are 156 and 247 GeV/c2 , and the squark/gluino masses are of order 1.0 TeV/c2 and 1.6 TeV/c2 respectively.. Figure 1.7: Cross Section for different masses of stau τ̃1 at 7 TeV, using pythia and CMSSW358patch2. The speed distribution for different masses on this model is shown in Fig. 1.8. Momentum p and η distribution for τ̃ in this model is shown in Figs. 1.9 and 1.10..

(29) 1.2. HSCPS IN GAUGE MEDIATED SYMMETRY BREAKING. Figure 1.8: β distribution for different masses of stau τ̃1. Figure 1.9: P distribution for different masses of stau τ̃1. 23.

(30) CHAPTER 1. HEAVY STABLE CHARGED PARTICLES. 24. Figure 1.10: Eta distribution for different masses of stau τ̃1. 1.3. HSCPs in Split-SUSY Models. Another SUSY-breaking scenario that is relevant in the early searches for HSCPs is split-SUSY. In this case the breaking mechanism splits the s-particle spectrum into scalars with masses around the GUT scale and fermions with masses around the electroweak scale. The only light scalar of the theory would be a Higgs. As a result, the s-particles with masses in the TeV scale would be gauginos and higgsinos, and among them, the gluinos. Since the gluinos decay into s-quarks, that have very large masses, they become very long-lived. These quasi-stable gluinos will interact strongly with quarks and gluons forming R-Hadrons that would behave as HSCP. The production cross sections of gluino R-Hadrons are again quite large, and very much within reach of the LHC during its first run, as presented in Table 1.2. In summary, in this work the search for HSCPs in the form of stop R-Hadrons (from MSSM), gluino R-Hadrons (from split-SUSY) and staus (from mGMSBSUSY), was performed. The prediced cross section for all the models is presented in Fig. 1.15 for a wide range of masses. In the next chapter detailed Monte Carlo studies of production and detection of these states by the CMS detector and in particular by the muon RPCs, using the ToF technique, will be presented..

(31) 1.3. HSCPS IN SPLIT-SUSY MODELS. Figure 1.11: β distribution for different masses of gluino g̃. Figure 1.12: P distribution for different masses of gluino g̃. 25.

(32) 26. CHAPTER 1. HEAVY STABLE CHARGED PARTICLES. Figure 1.13: Eta distribution for different masses of gluino g̃. Figure 1.14: Cross Section for different masses of gluino g̃.

(33) 1.4. PREVIOUS SEARCHES. 27. Figure 1.15: Cross Section for different HSCP models as a function of mass.. 1.4. Previous Searches. For the last two decades searches for HSCPs have been made in different accelerators, the most relevant ones are D0 [15] and CDF [16] [17] at Tevatron (FNAL) and L3 [18], Delphi [19], OPAL [20] and ALEPH [21] at LEP (CERN). In the Tevatron detectors, one of the most active areas is the searches for t̃. The task force dedicated to this analysis is extending new exclusion limits for the mass of this particle in recent years [22]. One of the searches was performed by CDF using the timing capabilities of its outer tracker. The selection of events was done requiring muon triggers for muons with pT > 20 GeV/c coming from the primary vertex. The cosmic background was rejected by avoiding events with second tracks consistent with the first one. For every candidate its velocity was measured from its path length and the time of arrival to the time-of-flight detector (TOF), that is a part of the tracking system. The muon velocity is β = 1 while massive particles would have β < 1. The momentum and velocity were used to calculate the candidate mass. Only one candidate was found with m > 100 GeV /c2 , consistent with the predicted background. CDF preliminary limits on the stable massive stop production cross section are shown in Fig. 1.16. From the calculated next to leading order cross section, a limit on the stop mass of 240 GeV /c2 was set by CDF [22] in 2008. However the most recent publication.

(34) CHAPTER 1. HEAVY STABLE CHARGED PARTICLES. 28. Figure 1.16: Observed limit on stop production cross section as a function of mass. Figure taken from [22] . by the same experiment goes beyond this limit [17], using the same time-of-flight technique the new stablished limit is at 249 GeV /c2 at 95% confidence level see Fig.1.17 The most recent public analysis published by the D0 regarding GMSB models (τ̃ ) do not extend the limits imposed by the LEP experiments as shown in Fig. 1.18. This search was done selecting events with two muons. For each muon the average speed was calculated using a ToF technique with the three layers of muon detector of D0. The sensitivity in the low mass region was limited by the time resolution and for high masses it was restricted by the trigger that only accepts muons within its timing window [22]. For all the masses that were considered the expected background agrees with the observed data. The combined results from all LEP experiments (LEP II run) for the search of τ̃ [23] assumes that the particles are stable or sufficiently long lived to cross the detectors without decay, the results presented as limiting case within the GMSB (for s-muons and staus), and it is expected masses > 99.5 GeV /c2 . This limit has a 95% confidence level. Finally searches for gluino-like R-Hadrons have not been performed up to now.

(35) 1.4. PREVIOUS SEARCHES. 29. Figure 1.17: The observed 95% C.L. limits on the cross-section for production of a stable top-squark pair (points), compared to the theoretical cross-section (curve). The band represents theoretical and parton distribution function uncertainties. The intersection of the band with the limit curve yields a lower mass limit for a stable stop squark of 249 GeV /c2 , taken from [17].. Figure 1.18: Limits on pair production of GMSB staus, by LEP overlapped with recent study from Tevatron. Fig taken from [22].

(36) 30. CHAPTER 1. HEAVY STABLE CHARGED PARTICLES. [24] nevertheless the signal would be similar to the one predicted for stops (RHadrons). Doing a simple scaling it is possible to estimate that Tevatron could have done an exlusion of the order of ≈ 350 GeV /c2 for the models predicting gluino.. In conclusions no “HSCPs” have been observed neither at LEP or at the TEVATRON, and the exclusion limits on the masses are: 249 GeV /c2 for stops, and 99.5 GeV /c2 for τ̃ at 95% of confidence level..

(37) Chapter 2 The CMS Detector The search for HSCPs reported in this document was done with the CMS detector. In this chapter the detector and its components will be described in some detail and the emphasis will be made in the muon system and its response to HSCPs.. 2.1. The Large Hadron Collider (LHC). The CMS detector is one of the 4 experiments at the Large Hadron Collider (LHC) which is the biggest and most powerful particle accelerator built up to date, the main scientific goals of the LHC and its detectors are: The search for the Higgs particle(s). The search for physics beyond the standard model, in particular, the search. for Supersymmetry (SUSY). The study of the physics of the Quark-Gluon Plasma (QGP). The improve of our understanding of the standard model.. The LHC is a double ring proton and heavy-ions accelerator. Currently it collides beams of protons at a center-of-mass energy of 7 TeV, and it is expected to increase that energy up to 14 TeV by 2012. In each of the rings bunches of protons will travel in opposite directions crossing in four points along the circumference that correspond to the interaction points of the CMS, ATLAS, LHCb and ALICE detectors. As the bunches cross each other proton-proton collisions will take place. 31.

(38) CHAPTER 2. THE CMS DETECTOR. 32. The LHC is projected to reach a peak luminosity of 1034 cm−2 sec−1 at 14 TeV, operating with a bunch-crossing rate of 40M Hz corresponding to Bunch crossings every 25 ns. At this peak luminosity an average of 20 interactions per crossing are expected each of them with a multiplicity of 100 charged particles, resulting on an average of 2000 detected particles per bunch crossing. During 2010 the LHC has been commissioned and the instantaneous luminosity has increased from 8.9 ×1026 cm−2 s−1 in the first day of run (March 30t h 2010) to 1.01 ×1032 cm−2 s−1 by the end of the year. At full luminosity each proton-beam is distributed in ≈ 2800 bunches of 1011 protons and they are separated by 7 m (≈25ns×c). During the first year of commissioning the number of bunches has been gradually increased from one bunch per beam the first days of collisions up to 368 × 368. In table 2.1 the most relevant parameters for the LHC during 2010 are compared with the values at full luminosity.. Run 2010. Full luminosity. Beam Energy at collision. 7 TeV. 14TeV. Beam Energy at injection. 0.45 TeV. 0.45 TeV. Max Luminosity. 1.01 ×1032 cm−2 s−1. 1034 cm−2 s−1. Protons per Bunch. ≈ 1011. 10 × 1011. Number of bunches per beam. 368 (max). 2800. Operating temperature. 1.9 K. 1.9 K. Table 2.1: Some relevant LHC parameters. The start of the LHC physics runs was on March 30th 2010. Beams of 3.5 TeV protons collided for the first time on that date setting a new record for the highest energy proton proton collisions. Since then, LHC has been colliding bunches at √ s = 7 TeV, and the integrated luminosity delivered by the LHC up to now is ≈ 46 pb−1 . The parameters of the LHC during the period of data taking have changed, specially the bunches configuration, in table 2.2 it is presented the evolution of the most important parameters during 2010..

(39) 2.1. THE LARGE HADRON COLLIDER (LHC). fill. day. month. #. 33. stable. energy. protons. bunches. bunches. β∗. beam(h). (TeV). per bunch. per beam. colliding. (m). 1005. 30. 3. 3.12. 3.50. 1e+10. 2. 1. 11.00. 1013. 31. 3. 8.03. 3.50. 1e+10. 2. 1. 11.00. 1019. 3. 4. 3.00. 3.50. 1e+10. 2. 1. 11.00. 1022. 4. 4. 20.05. 3.50. 1.1e+10. 2. 1. 11.00. 1023. 6. 4. 12.25. 3.50. 1.1e+10. 2. 1. 11.00. 1026. 7. 4. 2.40. 3.50. 1.1e+10. 2. 1. 11.00. 1059. 26. 4. 4.97. 3.50. 1.1e+10. 2. 1. 2.00. 1068. 2. 5. 7.18. 0.45. 9e+10. 2. 1. 11.00. 1069. 3. 5. 7.25. 0.45. 9e+10. 2. 1. 11.00. 1089. 8. 5. 20.37. 3.50. 2e+10. 2. 1. 2.00. 1128. 27. 5. 0.93. 0.45. 9e+10. 7. 4. 11.00. 1134. 5. 6. 3.77. 3.50. 2e+10. 13. 8. 2.00. 1179. 25. 6. 2.37. 3.50. 8e+10. 3. 2. 3.50. 1186. 30. 6. 2.35. 3.50. 1e+11. 3. 2. 3.50. 1188. 1. 7. 7.85. 3.50. 8e+10. 3. 2. 3.50. 1192. 2. 7. 0.57. 3.50. 9e+10. 7. 4. 3.50. 1284. 14. 8. 3.48. 3.50. 1e+11. 25. 16. 3.50. 1285. 15. 8. 12.38. 3.50. 1e+11. 25. 16. 3.50. 1375. 28. 9. 9.00. 3.50. 1e+11. 104. 93. 3.50. 1381. 30. 9. 3.05. 3.50. 1e+11. 152. 140. 3.50. 1386. 1. 10. 2.90. 3.50. 8e+10. 19. 6. 3.50. 1400. 8. 10. 6.57. 3.50. 1e+11. 248. 233. 3.50. 1408. 11. 10. 9.95. 3.50. 1e+11. 248. 233. 3.50. 1424. 16. 10. 0.88. 3.50. 1e+11. 312. 295. 3.50. 1430. 18. 10. 0.63. 3.50. 1e+11. 312. 295. 3.50. 1443. 26. 10. 2.23. 3.50. 1e+11. 368. 348. 3.50. 1453. 29. 10. 6.33. 3.50. 1e+11. 368. 348. 3.50. Table 2.2: Main LHC parameters during the 2010 Run, for some fills..

(40) CHAPTER 2. THE CMS DETECTOR. 34. The total integrated luminosity during the 2010 Run in P5 (where the CMS experiment is located) and the integrated luminosity per day as a function of time are presented in Figs. 2.1 and 2.2.. Figure 2.1: Integrated luminossity as a function of time in CMS.. 2.2. The Compact Muon Solenoid Detector. The CMS detector is located in one of the four interaction points of the LHC, called “Point 5”, (near the French village of Cessy on the north side of the LHC ring). It is a general propose detector, conceived for the search and discovery of new physics. The CMS detector has a cylindrical shape with a length of 21.6 m and a diameter of 14.6 m. It is located in a cavern 100 m underground and consist of 4 sub-detector systems: a Central Tracker, an electromagnetic calorimeter (ECAL), a hadron calorimeter (HCAL), and a Muon System. The tracker and the calorimeters are inside a solenoidal magnet of 13 m long and 6 m in diameter, that provides magnetic field up to 3.8 T. The total mass of the whole detector is ≈ 12.5 kt..

(41) 2.2. THE COMPACT MUON SOLENOID DETECTOR. 35. Figure 2.2: Integrated luminossity per Day as a function of time in CMS. The CMS main detection features are [25]: Very good muon resolution over a large range of momenta and angles, with unambiguous charge identification for muons with p < 1 TeV. Very good charged-particle momentum resolution in the inner tracker, with high trigger efficiency and high τ and b-jets tagging efficiencies. Very good electromagnetic energy resolution for di-photons and di-electrons (1% at 100 GeV). Resulting on excellent π 0 rejection and efficient photon and lepton isolation. Good missing-transverse-energy and dijet-mass resolution. The overall layout of CMS is shown in figure 2.3. Due to the cylindrical geometry of CMS its design and construction is divided in to a Barrel and two endcaps as can be seen in the figure 2.3.. 2.2.1. Solenoid. The momentum of charged particles as muons, electrons or HSCPs is measured using the bending of their trajectory in the magnetic field produced by a super-conducting.

(42) CHAPTER 2. THE CMS DETECTOR. 36. Figure 2.3: Layout of the CMS Detector [1]. solenoid, the biggest one ever constructed. The solenoid is 13 m long and 6 m in diameter with niobium-titanium coils operated at a temperature of 3 K producing a 3.8 T magnetic field in the inner part. The field coming out of the solenoid is collected by a surrounding iron yoke that compact the field lines resulting on a field strength of about 2 T [26] in the outer part of the solenoid where the Muon Chambers are embedded inside the yoke. With this design the field produced by the solenoid is used to bend the muons in the outer region so that no extra-magnets are required. At 3.8 T the current is 19500 A, there are dump circuits to safely dissipate the 2.6 GJ of energy able to store. In table 2.3 the main parameters of the magnet are shown and a sketch of the magnet is show in Fig. 2.4. The Tracker, the ECAL and the HCAL are located inside the solenoid where the magnetic field is 3.8 T and goes in a direction parallel to the beam line. In the outer region where the muon detectors are the magnetic field is 2 T and goes in the opposite direction. This change in intensity and specially in direction of the magnetic field causes a double bend of the muon tracks that is characteristic of the CMS detector..

(43) 2.2. THE COMPACT MUON SOLENOID DETECTOR. 37. Table 2.3: Main parameters of the CMS magnet.[25]. General parameters Magnetic length. 12.5 m. Cold bore diameter. 6.3 m. Central magnetic induction. 3.8 T. Total Ampere-turns. 41.7 MA-turns. Nominal current. 19.14 kA. Inductance. 14.2 H. Stored energy. 2.6 GJ Cold mass. Radial thickness of cold mass. 312 mm. Radiation thickness of cold mass. 3.9 X0. Weight of cold mass. 220 t. Maximum induction on conductor. 4.6 T. Temperature margin with respect to operating temperature. 1.8 K. Stored energy/unit cold mass. 11.6 kJ/kg Iron yoke. Outer diameter of the iron flats. 14 m. Length of Barrel. 13 m. Mass of iron in Barrel. 6000 t. Mass of iron in each endcap. 2000 t. Total mass of iron in return yoke. 10 000 t.

(44) CHAPTER 2. THE CMS DETECTOR. 38. Figure 2.4: General artistic view of the CMS solenoid showing the 5 modules composing the cold mass inside the cryostat, with details of the supporting system (vertical, radial and longitudinal tie rods). [1]. 2.2.2. Silicon Tracker. In the inner-most part of the CMS detector, just outside the beam pipe and sourronding the interaction point a high resolution tracker made out of silicon strips and pixels is located. As explained before, at full luminosity, ≈ 20 p-p collisions are expected every 25 ns, generating ≈ 1000 charged particles that will produce signals in the tracker. This requires a very fast response to correctly reconstruct tracks and give them the correct bunch-crossing assignment (timing information). The Silicon Tracker is finely segmented in silicon sensors (strips and pixels) this allows to track charged particles emerging from the LHC collisions and to identify secondary vertices. The overall dimensions of the Silicon Tracker are 5.8 m length and 2.5 m in diameter. As mentioned above the magnetic field in this region is 3.8 T. The expected life of the Silicon Tracker is about 10 years or to be able to record with good performance ≈ 500 fb−1 from the LHC. A sketch of the CMS Silicon Tracker is shown in figure 2.5. A sketch of the Silicon Tracker is shown in Fig. 2.5, surrounding the interaction.

(45) 2.2. THE COMPACT MUON SOLENOID DETECTOR. 39. Figure 2.5: Schematic cross section through the CMS tracker. Each line represents a detector module. Double lines indicate back-to-back modules which deliver stereo hits.. point there are three cylindrical layer of silicon pixels and two disks in each Endcap region, this subset of silicon detectors is called the “pixel detector”. It covers an area of 1 m2 with 66 millions channels and provides 3 high precision points for each charged particle trajectory. Outside the pixel detector the sensors used are silicon strips and they conform what is called “the silicon strip tracker” composed by 3 different subsystems: The Tracker Inner Barrel and Disks (TIB/TID): It is an array of silicon. strip detectors that extends in radius up to 55 cm is composed of 4 Barrel layers, supplemented by 3 disks at each end. It delivers up to 4 measurements on a trajectory using 320 µm thick silicon micro-strip sensors with their strips parallel to the beam axis in the Barrel and radially oriented on the disks. Tracker Outer Barrel (TOB): Surrounds the TIB and TID system and. consists of 6 Barrel layers of 500 µm strip sensors with strip pitch of 183 µm on the first 4 layers and 122 µm on layers 5 and 6. It provides another 6 measurements with single point resolution of 53 µm and 35 µm, respectively. Tracker Endcaps (TEC+/TEC-): Each TEC is composed of 9 disks with. 7 rings of silicon micro-strip detectors, with radial strips that have an average pitch that goes from 97 µm up to 184 µm. Thus, they provide up to 9 measurements per trajectory (TEC+ and TEC- indicates the location along the z.

(46) CHAPTER 2. THE CMS DETECTOR. 40. global axis, a more detailed description of the conventions in the CMS global coordinate system is presented in Appendix A).. Figure 2.6: Number of hits expected in the Silicon Tracker as a function of pseudorapidity η. Filled circles show the total number (back-to-back modules count as one) while open squares show the number of stereo layers.[1]. The number of expected hits in the tracker system for a charged particle as a function of the pseudo-rapidity is shown in figure Fig. 2.6, and the material budget in units of radiation length X0 as a function of the pseudo-rapidity η for different sub-detectors is shown in figure 2.7.. Figure 2.7: Material budget in units of radiation length as a function of pseudorapidity η for the different sub-detectors. [1] Search for HSCPs using the Tracker information is also performed by CMS. A detailed description of this technique are presented in chapter 3 subsection 3.1.1..

(47) 2.2. THE COMPACT MUON SOLENOID DETECTOR. 2.2.3. 41. Electromagnetic Calorimeter [1]. The electromagnetic calorimeter of CMS is a homogeneous calorimeter made of 61 200 lead tungsten (PbWO4 ) crystals in the central Barrel part, closed by 7 324 crystals in each of the two Endcaps. Additional detectors placed in front of the two Endcaps called preshower detectors are used to increase the resolution in the forward region. Avalanche photo-diodes (APDs) are used as photo-detectors in the Barrel and vacuum photo-triodes (VPTs) in the Endcaps. The high PbWO4 density crystals allows the design of a fast calorimeter, with fine granularity and radiation resistant, all important characteristics in the LHC environment.. Figure 2.8: PbWO4 crystals with photo-detectors attached. Left panel: A Barrel crystal with the upper face depolished and the APD capsule. In the insert, a capsule with the two APDs. Right panel: An Endcap crystal and VPT.. ECAL Barrel (EB) The Barrel part of the ECAL (EB) covers the pseudo-rapidity range |η| < 1.479 with a granularity of 360 sections in φ and (2×85) sections in η, resulting in a total of 61 200 crystal. They are mounted in a quasi-projective geometry to avoid cracks aligned with particle trajectories, so that their axes make a small angle, 3 degree, with respect to the vector from the nominal direction interaction vertex, in both the φ and η projections. The crystal cross-section corresponds to approximately 0.0174 × 0.0174 in η-φ or 22×22 mm2 at the front face, and 26×26 mm2 at the rear face. The crystal length is 230 mm corresponding to 25.8 X0 . The Barrel crystal volume is 8.14 m3 and the weight is 67.4 t..

(48) CHAPTER 2. THE CMS DETECTOR. 42. The centers of the front faces of the crystals are at a distance of 1.29 m from the beam line. The crystal to crystal distance is 0.35 mm inside a sub-module, and 0.5 mm between sub-modules. The sub-modules are assembled into modules of different types, according to the position in η, each containing 400 or 500 crystals. Four modules, separated by aluminum conical webs 4-mm thick, are assembled in a super-module, which contains 1700 crystals. ECAL Endcap (EE) The Endcaps (EE) cover the rapidity range 1.479 < |η| < 3.0. The longitudinal distance between the interaction point and the Endcap envelope is 315.4 cm, with the magnetic field on (when the magnetic field is turned on the endcaps shift toward the interaction point by 1.6 cm). The Endcaps consists of identically shaped crystals grouped in mechanical units of 5×5 crystals (super-crystals, or SCs). Each Endcap is divided into two halves, each of them holds 3 662 crystals. These are contained in 138 standard SCs and 18 special partial super-crystals on the inner and outer circumference. The crystals and SCs are arranged in a rectangular x-y grid, with the crystals pointing at a focus 1 300 mm beyond the interaction point, giving offpointing angles ranging from 2 to 8 degrees. The crystals have a rear face cross section 30×30 mm2 , a front face cross section 28.62×28.62 mm2 and a length of 220 mm (24.7 X0 ). The Endcaps crystal volume is 2.90 m3 and the weight is 24 t. The layout of the calorimeter is shown in Fig. 2.9. Figure 2.10 shows the Barrel already mounted inside the hadron calorimeter that will be described in the next section. The purpose of the Preshower sub-detector shown in Fig. 2.9 is to identify neutral pions decaying into two photons in the end-caps (1.653 < |η| < 2.6). The preshower also provides information about the identification of electrons against minimum ionizing particles, and improves the resolution in position of electrons and photons. The Preshower has two layers: lead radiators initiate electromagnetic showers from incoming photons/electrons while silicon strip sensors placed after each radiator measure the deposited energy and the transverse shower profiles. The Preshower is 20 cm thick and the material of Preshower traversed at η = 1.653 before reaching the first sensor plane is 2 X0 , followed by a further 1 X0 before reaching the second.

(49) 2.2. THE COMPACT MUON SOLENOID DETECTOR. 43. Figure 2.9: Layout of the CMS electromagnetic calorimeter showing the arrangement of crystal modules, super-modules and Endcaps, with the preshower in front. [1]. Figure 2.10: The ECAL Barrel positioned inside the hadron calorimeter..

(50) CHAPTER 2. THE CMS DETECTOR. 44. plane. Thus about 95% of single incident photons start showering before the second sensor plane. The orientation of the strips in the two planes is orthogonal. The lead planes are arranged in two halves, one on each side of the beam pipe, with the same orientation as the crystals in the ECAL.. 2.2.4. Hadron Calorimeter. Figure 2.11: Longitudinal view of the CMS detector showing the locations of the hadron calorimeter Barrel (HB), Endcap (HE), outer (HO) and forward (HF) calorimeters.. The Hadron Calorimeter (HCAL) was designed to study hadron jets and particles resulting in missing transverse energy over a wide range of energies [27]. A sketch of CMS showing the different components of the Hadron Calorimeter is presented in Fig. 2.11. The hadron calorimeter Barrel and Endcaps sit behind the tracker and the electromagnetic calorimeter as seen from the interaction point. The Barrel is radially restricted between the outer extent of the electromagnetic calorimeter (r = 1.77 m) and the inner extent of the magnet coil (r = 2.95 m). This constrains the total amount of material which can be put to absorb the hadronic shower. Therefore, an outer hadron calorimeter is placed outside the solenoid complementing the Barrel one. Beyond |η| = 3, the forward hadron calorimeters placed at 11.2 m from the.

(51) 2.2. THE COMPACT MUON SOLENOID DETECTOR. 45. interaction point extend the pseudo-rapidity coverage down to |η| = 5.2. HCAL Barrel (HB) The Barrel region of the HCAL is composed by wedges made of flat brass absorber plates aligned parallel to the beam axis. Each wedge is segmented into four azimuthal angle (φ) sectors. The plates are bolted together in a staggered geometry resulting in a configuration that contains no projective dead material for the full radial extent of a wedge. The innermost and outermost plates are made of stainless steel for structural strength. The absorber consists of a 40-mm-thick front steel plate, followed by eight 50.5mm-thick brass plates, six 56.5-mm-thick brass plates, and a 75-mm-thick steel back plate. The total absorber thickness at 90◦ is 5.82. The active medium uses the well known tile and wavelength shifting fiber concept to bring out the light. The CMS hadron calorimeter consists of about 70 000 tiles. The tiles of a given φ layer are grouped into a single mechanical scintillator tray unit. Each HB wedge has four φ divisions (φ-index = 1–4). The η towers 1–14 have a single longitudinal read-out. This tower segmentation is presented in Fig. 2.12.. HCAL Endcap (HE) The Endcaps of the Hadron Calorimeter (HE) cover the pseudo-rapidity range between 1.3 and 3, a region containing about 34% of the particles produced in the final state of a collision. Since the calorimeter is inserted into the ends of a 3.8-T solenoidal magnet, the absorber must be made from a non-magnetic material. The Endcaps are attached to the muon Endcap yoke as shown in Fig. 2.13. The 10 t electromagnetic calorimeter (EE) with a 2 t preshower detector (ES) is attached at the front face of HE.. HCAL Outer calorimeter (HO) The combined stopping power of EB plus HB does not provide sufficient containment for hadron showers. To ensure adequate sampling depth for |η| < 1.3, the hadron calorimeter is extended outside the solenoid called the HO or outer calorimeter. The.

(52) 46. CHAPTER 2. THE CMS DETECTOR. Figure 2.12: The HCAL tower segmentation in the r, z plane for 1/4 of the HB, HO, and HE detectors. The shading represents the optical grouping of scintillator layers into different longitudinal readouts.. Figure 2.13: Hadron Endcap (HE) calorimeter mounted on the Endcap iron yoke..

(53) 2.2. THE COMPACT MUON SOLENOID DETECTOR. 47. HO utilizes the solenoid coil as an additional absorber equal to (1.4/sinθ) Xo and is used to identify late starting showers and to measure the shower energy deposited after the HB. Outside the vacuum tank of the solenoid, the magnetic field is returned through an iron yoke designed in the form of five rings. The HO is placed as the first sensitive layer in each of these five rings. The rings are identified by the numbers −2, −1, 0, +1,+2. The numbering increases with z and the nominal central z positions of the five rings are respectively −5.342 m, −2.686 m, 0, +2.686 m and +5.342 m. At η = 0, HB has the minimal absorber depth, therefore in order to compensate, the central ring (ring 0) has two layers on either side of a 19.5 cm thick piece of iron “the tail catcher iron” at radial distances of 3.82 m and 4.07 m, respectively. All other rings have a single HO layer at a radial distance of 4.07 m. The total depth of the calorimeter system is thus extended to a minimum of 11.8 λI except at the Barrel-Endcap boundary region. The HO is constrained by the geometry of the muon system. Fig. 2.14 shows the position of the HO layers in the rings of the muon stations in the overall CMS setup. The segmentation of these detectors closely follows that of the Barrel muon system. Each ring has 12 identical φ-sectors.. Figure 2.14: Longitudinal and transverse views of the CMS detector showing the position of the HO layers. [1] The HCAL plays an important roll when triggering events due to missing energy, this kind of signature characterizes models predicting R-Hadrons. More details about the Trigger and R-Hadron will be presented in section 3.2..

(54) CHAPTER 2. THE CMS DETECTOR. 48. 2.2.5. CMS Muon Detectors. The detection of muons is of central importance to CMS. The muon system has three functions: triggering, muon identification and momentum measurement. Good muon momentum resolution and trigger capability are enabled by the high-field solenoidal magnet and its flux-return yoke. The yoke also serves as a hadron absorber cleaning up the muon signal. The CMS muon system was designed to have the capability of reconstructing the momentum and charge of muons over the the entire kinematic range of the LHC. CMS uses three types of gaseous particle detectors for muon identification: Drift Tubes (DTs), Cathode Strip Chambers (CSCs) and Resistive Plate Chambers (RPCs) see Fig. 2.15 [28].. Figure 2.15: Muon System layout, showing the three subsystem of the muon spectrometer RPCs, DTs and CSCs. In the Barrel region where the neutron-induced background is small, the muon rate is low and the 2 T magnetic field is contained in the steel yoke, drift chambers with standard rectangular drift cells (“drift tubes”) are used. These DT chambers cover the pseudo-rapidity region |η| < 1.2 and are organized into 4 stations inserted among the layers of the yoke plates. Each of the first three stations contains 8 chambers, in 2 groups of 4, which measure the muon coordinate in the r-φ bending plane, and 4 chambers which provide a measurement in the z direction (along the beam line see Appendix A). The fourth station does not contain the z-measuring.

(55) 2.2. THE COMPACT MUON SOLENOID DETECTOR. 49. planes. The two sets of four chambers in each station are separated as much as possible to achieve the best angular resolution. The drift cells of each chamber are offset by a half-cell with respect to their neighbor to eliminate detection dead spots. This arrangement provides a space resolution of about 200 µm. The number of chambers in each station and their orientation were chosen to provide good efficiency for linking together muon hits from different stations into a single muon track and for rejecting background hits. For the forward regions of the muon system cathode strip chambers (CSC) are used instead of DT. With their fast response time, fine segmentation, and radiation resistance, the CSCs are more suitable for the very high multiplicity environment of the forward regions. This detector identify muons between 0.9 < |η| < 2.4. There are 4 stations of CSCs in each Endcap, with chambers positioned perpendicular to the beam line. The cathode strips of each chamber run radially outward and provide a precision measurement in the r-φ bending plane. The anode wires run approximately perpendicular to the strips and are also read out in order to provide measurements of η as well as the bunch-crossing time of a muon. Each CSC has 6 detection layer providing robust pattern recognition for rejection of non-muon backgrounds and efficient matching of hits to those in other stations and with the CMS inner tracker. Taking into account all its sub-detectors the muon system covers the full pseudorapidity |η| < 2.4 with no acceptance gaps. Offline reconstruction efficiency of simulated single-muon samples (shown in Fig. 2.16) is typically between 95% to 99% except in the regions |η| = 0.25, 0.8 and 1.2 due to the mechanical structure of CMS (joints between wheels and disks). An important characteristic of the DT and CSC subsystems is that they can trigger on the pT of muons with good efficiency and high background rejection, independent of the rest of the CMS detector. The Level-1 trigger pT resolution is about 15% in the Barrel and 25% in the Endcap [1]. In the next section the CMS trigger system will be discused in detail. A total of six layers of RPCs are embedded in the Barrel muon system, two in each of the first two stations, and one in each of the last two stations. The redundancy in the first 2 stations allows the trigger algorithm to work even for low-.

(56) CHAPTER 2. THE CMS DETECTOR. 50. Figure 2.16: Muon reconstruction efficiency as a function of pseudo-rapidity for selected values of pT . Left panel: stand-alone reconstruction (using only hits from the muon system with a vertex constraint). Right panel: global reconstruction (using hits from both the muon system and the tracker).. pT tracks that may stop before reaching the outer 2 stations. In the Endcap region, there is a plane of RPCs in each of the first 3 stations in order for the trigger to use the coincidences between stations to reduce background, to improve the time resolution for bunch-crossing identification, and to achieve a good pT resolution. A complete description of the RPC system that is the main sub-detector to be used in the HSCP analysis presented in this document will be done in the section 2.2.8. 2.2.6. Drift tube system. Drift Chambers can be used as tracking detectors in despite its long dead time, since the multiplicity of particles outside the calorimeters drops significantly. In this region the magnetic field is lower and most of the magnetic field lines go through the yoke allowing for the safe operation of this kind of detectors that rely on detailed knowledge of the electric field. The drift tube barrel tracking system consists of 4 stations forming concentric cylinders around the beam line: the 3 inner cylinders have 60 drift chambers each and the outer cylinder has 70. There are about 172 000 sensitive wires. The wire.

(57) 2.2. THE COMPACT MUON SOLENOID DETECTOR. 51. length, is about 2.4 m and is constrained by the segmentation of the iron Barrel yoke. The transverse dimension of the drift cell, i.e. the maximum drift path, was chosen to be 21 mm (corresponding to a drift time of 380 ns in a gas mixture of 85% Ar + 15% CO2 ). This value is small enough to produce a negligible occupancy and to avoid the need for multi-hit electronics, yet the cell is large enough to limit the number of active channels to an affordable value.. Figure 2.17: Layout of the CMS Barrel muon DT chambers in one of the 5 wheels. The chambers in each wheel are identical with the exception of wheels -1 and +1 where the presence of cryogenic chimneys for the magnet shortens the chambers in 2 sectors, the same happens for the RPC system. Note that in sectors 4 (top) and 10 (bottom) the MB4 chambers (outermost layer) are cut in half to simplify the mechanical assembly.. In each of the 12 sectors of the yoke there are 4 muon chambers per wheel,.

(58) CHAPTER 2. THE CMS DETECTOR. 52. labeled MB1, MB2, MB3, and MB4 (Fig. 2.17). A drift-tube (DT) chamber is made of 3 (or 2) super-layers (SL, see Fig. 2.18), each made of 4 layers of rectangular drift cells staggered by half a cell. The SL is the smallest independent unit of the design.. Figure 2.18: Left: Schematic view of a DT chamber. Right: Section of a drift tube cell showing drift lines and isochrones (equal drift time). The voltages applied are +3600 V for wires, +1800 V for electrode strips, and −1200 V for cathode strips.. The wires in the 2 outer SLs are parallel to the beam line and provide a track measurement in the magnetic bending plane (r-φ). In the inner SL, the wires are orthogonal to the beam line and measure the z position. The third SL that measures z is not present in the fourth station, which therefore measures only the φ coordinate. A muon coming from the interaction point first encounters a φ-measuring SL, passes through the honeycomb plate, then crosses the z-measuring SL and the second φmeasuring SL, resulting in two measurements in r − φ and one measurement in z per station. The good space resolution of the chambers and a precise alignment allows the track segments to be reconstructed with a resolution better than 250 µm. This resolution is needed for the estimation of high momentum muons and HSCPs. As well as the determination of the RPC detection efficiency using the segment extrapolation technique explained in chapter 4.. 2.2.7. Cathode strip chambers. The Endcap Muon system consists of 468 cathode strip chambers (CSC) arranged in groups as follows: 72 ME1/1, 72 ME1/2, 72 ME1/3, 36 ME2/1, 72 ME2/2,.

(59) 2.2. THE COMPACT MUON SOLENOID DETECTOR. 53. 36 ME3/1, 72 ME3/2, and 36 ME4/1 (Figs. 2.19 and 2.20). The chambers are trapezoidal and cover either 10◦ or 20◦ in φ; all chambers, except for the ME1/3 ring, overlap and provide contiguous φ-coverage. A muon in the pseudo-rapidity range 1.2 < |η| < 2.4 crosses 3 or 4 CSCs. In the region where the Endcap and Barrel overlap, corresponding to 0.9 < |η| < 1.2, muons are detected by both the Barrel DT and Endcap CSCs. Muons with |η| < 1.6 are also detected by RPCs.. Figure 2.19: Quarter-view of the CMS detector. Cathode strip chambers of the Endcap Muon system are highlighted. CSCs are composed by 6 anode wire planes interleaved among 7 cathode panels (Fig. 2.22). Wires run azimuthally and define a track’s radial coordinate. Strips are on cathode panels and run lengthwise at a constant φ width. The muon coordinate along the wires (φ) is obtained by interpolating charges induced on strips (Fig. 2.21). The largest chambers, ME2/2 and ME3/2, are about 3.4 × 1.5 m2 in size. The overall area covered by the sensitive planes of all chambers is about 5000 m2 , the gas volume is of the order of 50 m3 , and the number of wires is about 2 million. There are about 9000 high-voltage channels in the system, about 220 000 cathode strip read-out channels, and about 180 000 anode wire read-out channels. The space resolution achieved with the CSC system is of the same order of magnitude of the DTs ≈ 200 µm. As the DTs, the CSCs system is a tracking detectors, they both contribute to.

(60) 54. CHAPTER 2. THE CMS DETECTOR. Figure 2.20: The ME2 station of CSCs. The outer ring consists of 36 ME2/2 chambers, each spanning 10◦ in φ, and the inner ring of eighteen 20◦ ME2/1 chambers. The chambers overlap to provide contiguous coverage in φ.. Figure 2.21: CSC Scheme, charge induced on different strips..

(61) 2.2. THE COMPACT MUON SOLENOID DETECTOR. 55. Figure 2.22: Trapezoidal form in CSC the reconstruction of muons and play an important roll in the estimation of their momentum..