INGENIERÍA DEL PROYECTO.

experiments are located. Further details about the LHC, a comprehensive description of the machine can be found in Ref. [59].

3.1. The LHC experiments

In total there are seven experiments attached to the LHC, each of the four collision points accommodates one of the four big experiments ATLAS, CMS, ALICE, and LHCb.

ATLAS [60] and CMS [61], A Toroidal LHC ApparatuS and the Compact Muon Solenoid, are both multi-purpose detectors. Their main physics goals are to perform direct searches of new particles, such as candidates for dark matter or supersymmetry, and to look for the missing piece of the Standard Model, the Higgs boson. Both experiments successfully discovered the Higgs boson in July 2012 [4,5].

The ALICE detector [62], A Large Ion Colliding Experiment, is specialised for heavy-ion collisions. The physics program fo- cusses on the physics of strong interactions and the properties of a new phase of matter, the quark-gluon-plasma (QGP). This phase emerges at extremely high densities and temperatures, which can be achieved in high energetic collisions of heavy ions.

The Large Hadron Collider beauty experiment, LHCb [63], is a specialised experiment dedicated to study CP violation and rare decays in the system of b-hadrons. The detector has proven to be suitable to successfully extent the physics program to a variety of other fields up to also covering proton-ion physics, as it will be presented in the course of this thesis.

There are three additional smaller experiments attached to the LHC. The TOTEM [64] experiment is installed next to the CMS detector. The main goal is to precisely determine the size of pro- tons and to perform cross-section and luminosity measurements in pp collisions. LHCf [65] is a very small detector located at the interaction point of ATLAS. It is designed to detect particles that are produced in the extreme forward region of a collision. The results of this experiment can be used to calibrate and simulate cosmic-ray detectors. MoEDAL [66] is the most recent LHC experiment, it shares the cavern of the LHCb experiment. The physics goal is to search for magnetic monopoles.

3.2. A multi-purpose collider

Figure 3.2.:Picture of the LHC with its superconducting magnets placed in the tunnel of the

former LEP collider. Figure taken from Ref. [58]

3.2. A multi-purpose collider

For the LHC, three different operation modes are distinguished, which allow colliding either protons, lead-ions, or a mixed setup of protons and lead-ions. The three machine setups are briefly discussed below:

Proton-proton mode The design target of the LHC is to collide two proton beams at

a maximum centre-of-mass energy of √s = 14 TeV. The corresponding energy of 7 TeV

per beam is defined by the peak dipole field of the superconducting magnets which keep the protons on a circular orbit along the collider. A picture of the LHC and its magnets is depicted in Fig. 3.2.

At nominal running conditions, bunches are equally distributed along the LHC, corresponding to a bunch spacing of 25 ns. This results in a maximum frequency of proton-proton collisions of 40 MHz. Due to restrictions of the injection procedure and reserved empty bunches used to operate the beam, 2808 bunches can be populated

with protons (proton bunches). The nominal peak luminosity of L = 1034_cm−2_s−1 _is

obtained by filling 1.1 × 1011 _{protons in each of proton bunches.}

Due to an incident during the commissioning of the LHC in 2008, where a large number of the superconducting magnets had been damaged, the LHC started to provide first pp collisions only in 2009 and at a lower centre-of-mass energy of 900 GeV. In the years 2010 to 2012 the energy had been increased up to 7 and 8 TeV, leading to a maximum

luminosity of 0.76 × 1034_cm−2_s−1 _{at a total number of 1374 proton bunches per beam}

and a 50 ns bunch spacing. In addition, short periods of pp collisions at√s = 1.36 TeV

and 2.76 TeV have been provided.

After the current ongoing upgrade of the LHC magnets the energy will be further increase towards the nominal values of 13 or 14 TeV.

Heavy-ion mode In addition to protons, the design of the machine also allows to

accelerate and collide heavy ions. For this purpose, fully stripped lead ions (208_Pb82+₎

are used. In nominal conditions, the 82 protons per nucleus can be accelerated to 7 TeV corresponding to an energy of 2.76 TeV per nucleon. The nucleon-nucleon centre-of-mass

3. The Large Hadron Collider at CERN

energy of two colliding lead-lead (PbPb) beams then amounts to √sN N = 5.52 TeV. The

target peak luminosity in PbPb runs is 1.0 × 1027_cm−2_s−1_{, obtained with 592 bunches}

filled with 7 × 107 _{lead ions.}

Besides the pp running periods, also two dedicated heavy-ion runs took place in 2010 and 2011. Due to the mentioned problems with the superconducting magnets also the energy during the heavy-ion runs was decreased to half of the design value leading to

centre-of-mass energy of √sN N = 2.76 TeV in the nucleon-nucleon system.

Proton-ion mode A third operation mode, which was not foreseen in the original

design of the LHC, involves an asymmetric beam configuration of colliding protons and lead ions. The main motivation for adding this type of collision is to provide benchmark measurements for PbPb collisions. The energy of both beams in this configuration is different. While the proton beam had an energy of 4 TeV, the lead beam is accelerated to an energy of 1.58 TeV/nucleon, accounting for the ratio of protons to nucleons

(Z/A = 82/208) within the ion. The nucleon-nucleon centre-of-mass energy √sN N of a

proton (p) in one beam and a nucleon (N ) in the other beam can be calculated in the lab system as follows:

sN N = (Ep+ EN)2− (pz,p− pz,N)2,with EN= (Z/A)Ep

= E_p2· (1 + Z/A) + p2z,p· (1 − Z/A) , pz,N≈ pz,p≈ Ep

√

sN N = 2EppZ/A (3.1)

The resulting energy for proton-ion beams amounts to √sN N = 5.02 TeV.

Since the momenta of the nucleons in the ion- and proton-beam are different, the centre-of-mass system (cms) of the collision is boosted. It follows, that the measured rapidity of particles in the detector’s rest frame is shifted by a factor ∆y with respect to the rapidity in the cms. This boost acts along the direction of the proton and amount to

∆y = 1/2 ln A/Z _{≈ +0.465.} (3.2)

The LHCb detector, designed to originally collect proton-proton data only, also participated in the proton-lead program of the LHC. The detector design and its key features are presented in the next chapter.

CHAPTER

4

The LHCb experiment at the LHC

LHCb is the dedicated experiment for heavy flavour physics at the LHC. Its design is driven by the properties of heavy quark production and their decay characteristics. The geometry of the LHCb detector is different compared to most of other particle detectors.

As discussed previously (c.f. Chap. 2.2.2), the production of boosted b¯b pairs at the LHC predominantly takes place in the forward and backward direction. Therefore, the LHCb detector is a single-arm magnetic dipole spectrometer in the forward region, the layout is given in Fig. 4.1. The LHCb coordinate system is defined as a right-handed

250mrad 100mrad M1 M3 M2 M4 M5 RICH2 HCAL ECAL SPD/PS Magnet T1T2 T3 z 5m y 5m − 5m 10m 15m 20m TT Vertex Locator RICH1

Р и с.1.1 Д етек тор LHCb состоит из вершинного детек тора (Vert ex Locat or - V ELO), дипольного магнита, трек овой системы (T T , T 1, T 2, T 3), аэрогелевого и газового детек торов черенк овск ого излучения (RICH 1 и 2), сцинтилляционных счетчик ов(SPD), предливневого детек тора (PS), элек тромагнитного к алориметра (ECA L), адронного к алориметра (HCA L) и пяти мюонных к амер (M 1-M 5) 0 1 2 3 1 2 3 θb [rad] θ_b [rad_] Р и с.1.2 К орреляция полярных углов b- и ¯b-адронов, рапределение моделировано с помощью PY T HIA .

1.3 М аг нит

Магнит позволяет получить большой интеграл поля 4Тм на относительно

небольшой длине. Поле направлено вертикально и достигает в максимуме

Figure 4.1.:Schematic overview of the LHCb detector in the y-z plane. The detector’s subsys-

tems are described in the text. Figure is taken from Ref. [63].