Study of Z Boson Production in PbPb Collisions at p ffiffiffiffiffiffiffiffiffi s
NN¼ 2:76 TeV
S. Chatrchyan et al.*
(CMS Collaboration)
(Received 1 March 2011; published 24 May 2011)
A search for Z bosons in the þdecay channel has been performed in PbPb collisions at ffiffiffiffiffiffiffiffi sNN
p ¼
2:76 TeV with the CMS detector at the LHC, in a 7:2 b1 data sample. The number of opposite-sign muon pairs observed in the60–120 GeV=c2invariant mass range is 39, corresponding to a yield per unit of rapidity (y) and per minimum bias event of ½33:8 5:5ðstatÞ 4:4ðsystÞ 108, in the jyj < 2:0 range. Rapidity, transverse momentum, and centrality dependencies are also measured. The results agree with next-to-leading order QCD calculations, scaled by the number of incoherent nucleon-nucleon collisions.
DOI:10.1103/PhysRevLett.106.212301 PACS numbers: 25.75.Cj, 12.38.Bx, 14.70.Hp
The hot and dense matter produced in heavy-ion colli- sions, often referred to as the quark-gluon plasma (QGP), can be studied in various ways. One approach is to compare measurements made in heavy-ion (AA) collisions to those in proton-proton (pp) and proton- (or deuteron-)nucleus collisions. Another way is to compare in the same AA sample the yields of particles that are modified by the QGP to those of unmodified reference particles. At the Relativistic Heavy Ion Collider (RHIC), direct photons play the reference role [1], although their measurement is complicated by copious background from 0 and other decays, and by the existence of a parton fragmentation component which is potentially modified by the medium [2]. At the Large Hadron Collider (LHC) energies, a new and cleaner reference becomes available: the Z boson, decaying into leptons [3,4].
Electroweak boson production is an important bench- mark process at hadron colliders. At 7 TeV center-of-mass energy, measurements in pp collisions at the LHC [5,6] are well described by calculations based on higher-order per- turbative quantum chromodynamics (pQCD), using recent parton distribution functions (PDFs). In AA collisions, Z boson production can be affected by various initial-state effects, though predictions indicate that these contributions are rather small [3,7–10]. First, the mix of protons and neutrons in AA collisions (the so-called isospin effect) is estimated to modify the Z yield by less than 3% compared to pp collisions [9]. Second, energy loss and multiple scattering of the initial partons can also alter the Z produc- tion, by about 3% [10]. The PDFs however are modified in nuclei and a depletion (shadowing) is expected for Z bosons at the LHC, modifying their yield by as much as
20% [9]. Precise measurements of Z production in heavy- ion collisions can therefore help to constrain nuclear PDFs.
Once produced, Z bosons decay within the medium, with a lifetime of 0:1 fm=c. Their leptonic decays are of particular interest since leptons lose negligible energy in the produced medium regardless of its nature (partonic or hadronic) and properties [4]. Dileptons from Z bosons can thus serve as a reference to the processes expected to be heavily modified in the QGP, such as quarkonia produc- tion, or the production of an opposite-side jet in Z þ jet processes [3,11]. The Z bosons are therefore ideally suited to serve as a standard candle of the initial state in PbPb collisions at the LHC energies.
During the first PbPb LHC run at the end of 2010, at a center-of-mass energy per nucleon pair of ffiffiffiffiffiffiffiffi
sNN
p ¼
2:76 TeV, Z bosons were observed by the Compact Muon Solenoid (CMS) experiment. The measurement reported in this Letter is performed with a 55 106 minimum bias (MB) event sample, corresponding to an integrated lumi- nosity of7:2 b1.
A detailed description of the CMS detector can be found in [12]. Its central feature is a superconducting solenoid of 6 m internal diameter, providing a magnetic field of 3.8 T.
Within the field volume are the silicon pixel and strip tracker, the crystal electromagnetic calorimeter, and the brass or scintillator hadron calorimeter. Muons are mea- sured in gas-ionization detectors embedded in the steel return yoke. In addition, CMS has extensive forward calo- rimetry, in particular, two steel or quartz-fiber Cˇ erenkov, hadron forward (HF) calorimeters, which cover the pseu- dorapidity range2:9 < jj < 5:2.
In this analysis, Z bosons are measured through their dimuon decays. The silicon pixel and strip tracker mea- sures charged particle trajectories in the rangejj < 2:5. It consists of 66 M pixel and 10 M strip detector channels. It provides a distance-to-vertex resolution of15 m in the transverse plane. Muons are detected in the jj < 2:4 range, with detection planes based on three technologies:
drift tubes, cathode strip chambers, and resistive plate chambers. A matching of the muons to the tracks measured
*Full author list given at the end of the article.
Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distri- bution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
in the silicon tracker results in a pTresolution between 1%
and 2%, for pT values up to100 GeV=c.
The centrality of AA collisions, i.e., the geometrical overlap of the incoming nuclei, is related to the energy released in the collisions. In CMS, centrality is defined as percentiles of the distribution of the energy deposited in the HFs [13,14]. The centrality classes used in this analysis are 30%–100%, 10%–30%, and 0%–10% (most central), or- dered from the lowest to the highest HF energy deposit.
Events are preselected if they contain a reconstructed primary vertex made of at least two tracks, and an offline coincidence of both of the HFs with a total deposited energy of at least 9 GeV. These criteria reduce contribu- tions from single-beam interactions with the environment (e.g., beam-gas and beam halo collisions with the beam pipe), ultraperipheral electromagnetic collisions, and cosmic-ray muons. The acceptance of this selection is (97 3)% of the hadronic inelastic cross section [13].
The events are also selected by the two-level trigger of CMS. At the first hardware level, two muon candidates in the muon detectors are required. At the software-based higher level, two reconstructed tracks in the muon detec- tors are required, each with a pT of at least3 GeV=c. In order to study the dimuon trigger efficiency, events are also collected with a single-muon trigger, requiring pT >
20 GeV=c. For Z bosons, the trigger efficiency is estimated to be’ 94%.
Muon offline reconstruction is seeded with’ 99% effi- ciency by tracks in the muon detectors, called stand-alone muons. These tracks are then matched to tracks recon- structed in the silicon tracker by means of an algorithm optimized for the heavy-ion environment [14,15]. For a muon from Z decays the tracking efficiency is ’ 85%, less than in the pp case, as the track reconstruction requires more pixel hits to lower the number of combinations, due to the high multiplicity. Global fits of the muon and tracker tracks, called global muons, are used to obtain the results presented in this Letter.
Background muons from cosmic rays and heavy-quark semileptonic decays are rejected by requiring a transverse (longitudinal) impact parameter of less than 0.3 (1.5) mm from the measured vertex. Loose criteria applied on the reconstructed muons result in the dimuon mass spectrum shown in Fig.1. No muon isolation criteria are applied, as they are expected to have reduced efficiency in the high particle density of the PbPb environment. The fraction of Z decays removed by the applied selection criteria is esti- mated to be’ 2:6%. A conservative upper limit of 4% for the residual background is estimated by extrapolations of various shapes from the low mass region, and no correction is applied. Thirty-nine Z candidates are observed in the mass interval 60–120 GeV=c2. Their distribution is con- sistent with the one from pp data at 7 TeV [6], scaled down to 39 counts and limited to the60–120 GeV=c2mass range as displayed by the histogram in Fig.1.
Muon trigger, reconstruction, and selection efficiencies, as well as acceptance, are estimated using thePYTHIA6.424 simulation [16] with CTEQ6L PDFs [17] and fullGEANT4
[18] detector simulation. To take into account the effect of the higher PbPb underlying-event activity, simulated Z decays are embedded in measured PbPb events at the level of detector hits and with generated vertices matched to the measured ones. These events were processed through the trigger emulation and event reconstruction chain. Track characteristics, such as the number of hits and the 2 of the track fit, have similar distributions in data and simula- tion. The detector acceptance , defined as the fraction of Z bosons produced at rapidity jyj < 2:0 that decay into muons withjj < 2:4 and pT> 10 GeV=c, is estimated to be 78%. Within this acceptance, the overall trigger, recon- struction, and identification efficiency " averages to 67%, and varies by less than 10% as a function of centrality.
The individual components of this efficiency are also estimated with a data-driven technique, called tag-and- probe, similar to the one used for the corresponding pp measurement [6]. It consists in counting the Z candidates with and without applying the probed selection on one of the muons: (1) the stand-alone muon reconstruction effi- ciency is probed with tracker tracks; (2) the silicon tracker reconstruction efficiency is probed with stand-alone muons; (3) the trigger efficiency is probed by testing the trigger response to global muons from a sample triggered by a single-muon requirement. The last is also checked with high-quality reconstructed muons from MB events. In all cases, these data-driven efficiencies agree with those derived from simulation within the statistical uncertainties.
The total systematic uncertainty on the Z yield is esti- mated to be 13% by summing in quadrature the following contributions. The largest one is associated with the tracking efficiency and taken as the 9.8% precision of the above-mentioned data-driven efficiency determination.
Similarly, the uncertainty associated with the dimuon
2) Dimuon mass (GeV/c
30 40 50 60 70 80 90 100 110 120
)2Events/(2 GeV/c
0 5 10 15
Opposite-sign Same-sign CMS pp 7 TeV 2.9 pb-1
= 2.76 TeV sNN
CMS PbPb b-1 µ Ldt = 7.2
∫
< 2.4 ηµ > 10 GeV/c, µ
pT
2) Dimuon mass (GeV/c
30 40 50 60 70 80 90 100 110 120
)2Events/(2 GeV/c
0 5 10 15
FIG. 1 (color online). Dimuon invariant mass spectra. Full squares are opposite-sign dimuons, while the empty circle shows a unique like-sign dimuon candidate. The histogram shows the corresponding distribution measured in pp collisions at 7 TeV within60–120 GeV=c2, scaled to the 39 PbPb candidates.
trigger is 4.5%. The 4% maximum contribution from un- subtracted background is taken as a systematic uncertainty.
The uncertainty associated with the muon-pair selection is considered to be equal to the 2.6% loss of events. The MB trigger efficiency is known at the 3% level. The uncertainty coming from the acceptance correction is estimated to be less than 3%, by varying the underlying generated kinematics (y, pT) beyond reasonable modifications.
Other systematic uncertainties are estimated to sum to less than 1.5%.
The yield of Z ! þdecays per MB event is defined as dN=dyðjyj < 2:0Þ ¼ NZ=ð"NMByÞ, where NZ¼ 39 is the number of dimuons counted in the mass window of 60–120 GeV=c2, NMB¼ 55 106is the number of corre- sponding MB events, corrected for trigger efficiency, and
" are the acceptance and overall efficiency, and y ¼ 4:0 is the rapidity bin width. We find dN=dyðjyj < 2:0Þ ¼ ð33:8 5:5 4:4Þ 108, where the first uncertainty is statistical and the second systematic. The analysis de- scribed above is repeated after subdividing the data into three bins for each of the following variables: event cen- trality and Z boson y and pT. The total systematic uncer- tainty does not vary significantly with these variables and is considered to be constant and dominantly uncorrelated.
In the absence of in-medium modifications, the yield of perturbative processes such as the Z boson production is supposed to scale with the number of incoherent nucleon- nucleon binary collisions [19]. In order to compare the PbPb measured yields to available pp cross-section calcu- lations, a scaling factor TAB is necessary. This nuclear overlap function is equal to the number of elementary nucleon-nucleon binary collisions divided by the elemen- tary NN cross section, and can be interpreted as the NN equivalent integrated luminosity per AA collision, at a given centrality. In units ofmb1, the average TABamounts to1:45 0:18, 11:6 0:7, and 23:2 1:0, for the central- ity ranges 30%–100%, 10%–30%, and 0%–10%, respec- tively, and5:66 0:35 for MB events. These numbers are computed with a Glauber model calculation [19], using the same parameters as in [13]. The quoted uncertainties are derived by varying within uncertainties the Glauber pa- rameters and the MB trigger and selection efficiency.
The full circles in Fig.2(a)show the centrality depen- dence of the Z yield divided by TAB, while the open square is for MB events. The variable used on the abscissa is the average number of participating nucleons Npart corre- sponding to the selected centrality intervals, computed in the same Glauber model. No centrality dependence of the binary-scaled Z yields is observed in data. A similar result was recently published by the ATLAS collaboration [20].
The normalized yields ðdN=dyÞ=TAB are compared to various calculations: (1) using the nucleon CT10 and modified nuclear EPS09 PDFs [9,21], (2) using MSTW08 PDFs [22] and modeling incoming-parton en- ergy loss [11], and (3) provided by the POWHEG [23]
Npart
0 50 100 150 200 250 300 350 400 (pb) ABdN/dy (|y|<2.0) / T
0 10 20 30 40 50 60 70 80 90 100
POWHEG + PYTHIA 6.4 Paukkunen et al., CT10+isospin Paukkunen et al., idem+EPS09 Neufeld et al., MSTW+isospin Neufeld et al., idem+eloss
a) CMS PbPb 7.2 µb-1 at sNN = 2.76 TeV [30-100]%
[10-30]%
[0-10]%
[0-100]%
Rapidity
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
dN/dy
0 10 20 30 40 50 60
CMS
POWHEG + PYTHIA 6.4 Paukkunen et al., CT10+isospin Paukkunen et al., idem+EPS09 Neufeld et al., MSTW+isospin Neufeld et al., idem+eloss
b) CMS PbPb 7.2 µb-1 at sNN = 2.76 TeV x10-8
Transverse momentum (GeV/c)
0 5 10 15 20 25 30 35
-1 (GeV/c) TN/dydp2 d
0 0.5 1 1.5
2 2.5 3
CMS, |y|<2.0 POWHEG + PYTHIA 6.4
c) CMS PbPb 7.2 µb-1 at sNN = 2.76 TeV x10-8
FIG. 2 (color online). The yields of Z ! per event:
(a) dN=dy divided by the expected nuclear overlap function TAB and as a function of event centrality parametrized as the number of participating nucleons Npart, (b) dN=dy versus the Z boson y, (c) d2N=dydpT versus the Z boson pT. Data points are located horizontally at average values measured within a given bin. Vertical lines (bands) correspond to statistical (systematic) uncertainties. Theoretical predictions are computed within the same bins as the data, and are described in the text.
generator interfaced with thePYTHIA parton-shower gen- erator and using CTEQ6.6 PDFs [17]. Only a marginal centrality dependence is predicted: the inhomogeneous (i.e., depending on the radial position in nuclei) shadowing is predicted to have negligible impact [7] and the energy- loss prediction drops by 3% from peripheral to central collisions [11].
Figures 2(b) and 2(c) show the differential yields, dN=dy and d2N=dydpT, as a function of the Z boson y and pT. They are compared to the same theoretical calcu- lations as used for the centrality distribution (when avail- able) multiplied by the minimum bias TAB value. In all bins, no significant deviations from binary-collision scal- ing are observed.
Nuclear modification factors, RAA¼ dN=ðTABdppÞ, are computed from the AA measured yields dN, the nuclear overlap function TAB, and the pp ! Z cross sections dpp given by thePOWHEGcalculation (solid lines on Fig.2, e.g., dpp=dy ¼ 59:6 pb in jyj < 2:0). The RAAsystematic un- certainty includes TABuncertainties, but no uncertainty is assigned to the theoretical pp cross section. All RAAvalues are found compatible with unity. They are reported in Table I, together with the number of observed Z bosons and their yield per event.
In conclusion, the Z boson yield in PbPb collisions at ffiffiffiffiffiffiffiffi
sNN
p ¼ 2:76 TeV has been measured inclusively and as a function of rapidity, transverse momentum, and centrality.
Within uncertainties, no modification is observed with respect to theoretical next-to-leading order perturbative quantum chromodynamics proton-proton cross sections scaled by the number of elementary nucleon-nucleon collisions. This measurement confirms the validity of the Glauber scaling for perturbative cross sections in nucleus-nucleus collisions at the LHC and establishes the
feasibility of carrying out detailed Z physics studies in heavy-ion collisions with the CMS detector. With upcom- ing PbPb collisions at higher luminosity, the Z boson promises to be a powerful reference tool for final-state heavy-ion related signatures as well as providing a means to study the modifications of the parton distribution functions.
We thank Bryon Neufeld, Hannu Paukkunen, Carlos Salgado, Ivan Vitev, and Ramona Vogt for fruitful theoreti- cal inputs on the nuclear effects involved in Z production.
We wish to congratulate our colleagues in the CERN accelerator departments for the excellent performance of the LHC machine in 2010. We thank the technical and administrative staff at CERN and other CMS institutes, and acknowledge support from FMSR (Austria); FNRS and FWO (Belgium); CNPq, CAPES, FAPERJ, and FAPESP (Brazil); MES (Bulgaria); CERN; CAS, MoST, and NSFC (China); COLCIENCIAS (Colombia); MSES (Croatia); RPF (Cyprus); Academy of Sciences and NICPB (Estonia); Academy of Finland, ME, and HIP (Finland);
CEA and CNRS/IN2P3 (France); BMBF, DFG, and HGF (Germany); GSRT (Greece); OTKA and NKTH (Hungary);
DAE and DST (India); IPM (Iran); SFI (Ireland); INFN (Italy); NRF and WCU (Korea); LAS (Lithuania);
CINVESTAV, CONACYT, SEP, and UASLP-FAI (Mexico); PAEC (Pakistan); SCSR (Poland); FCT (Portugal); JINR (Armenia, Belarus, Georgia, Ukraine, Uzbekistan); MST and MAE (Russia); MSTD (Serbia);
MICINN and CPAN (Spain); Swiss Funding Agencies (Switzerland); NSC (Taipei); TUBITAK and TAEK (Turkey); STFC (United Kingdom); DOE and NSF (USA).
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TABLE I. For eachjyj, pT, and centrality interval, number of Z bosons NZ, associated yield per event dN=dy, and nuclear modification factor RAA derived by using aPOWHEG pp refer- ence. The quantity d2N=dydpTis given in units ofðGeV=cÞ1. The first uncertainty is statistical and the second systematic.
jyj NZ dN=dy (108) RAA
[0, 2.0] 39 33:8 5:5 4:4 1:00 0:16 0:14 [0, 0.5] 13 38:1 10:7 5:0 1:03 0:29 0:15 [0.5, 1.0] 12 35:6 10:4 4:6 0:98 0:29 0:14 [1.0, 2.0] 14 30:0 8:1 3:9 0:97 0:26 0:14 pTðGeV=cÞ NZ d2N=dydpT (108) RAA
[0, 6] 11 1:65 0:50 0:22 0:84 0:26 0:12 [6, 12] 15 2:05 0:54 0:27 1:32 0:34 0:19 [12, 36] 12 0:44 0:13 0:06 1:06 0:31 0:15
Centrality NZ dN=dy (108) RAA
[30, 100]% 7 7:9 3:0 1:0 0:92 0:35 0:16 [10, 30]% 14 59:5 16:0 7:7 0:86 0:23 0:12 [0, 10]% 18 165 40 22 1:20 0:29 0:16
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S. Chatrchyan,1V. Khachatryan,1A. M. Sirunyan,1A. Tumasyan,1W. Adam,2T. Bergauer,2M. Dragicevic,2J. Ero¨,2 C. Fabjan,2M. Friedl,2R. Fru¨hwirth,2V. M. Ghete,2J. Hammer,2,bS. Ha¨nsel,2C. Hartl,2M. Hoch,2N. Ho¨rmann,2
J. Hrubec,2M. Jeitler,2G. Kasieczka,2W. Kiesenhofer,2M. Krammer,2D. Liko,2I. Mikulec,2M. Pernicka,2 H. Rohringer,2R. Scho¨fbeck,2J. Strauss,2F. Teischinger,2P. Wagner,2W. Waltenberger,2G. Walzel,2E. Widl,2 C.-E. Wulz,2V. Mossolov,3N. Shumeiko,3J. Suarez Gonzalez,3L. Benucci,4E. A. De Wolf,4X. Janssen,4T. Maes,4
L. Mucibello,4S. Ochesanu,4B. Roland,4R. Rougny,4M. Selvaggi,4H. Van Haevermaet,4P. Van Mechelen,4 N. Van Remortel,4F. Blekman,5S. Blyweert,5J. D’Hondt,5O. Devroede,5R. Gonzalez Suarez,5A. Kalogeropoulos,5 J. Maes,5M. Maes,5W. Van Doninck,5P. Van Mulders,5G. P. Van Onsem,5I. Villella,5O. Charaf,6B. Clerbaux,6 G. De Lentdecker,6V. Dero,6A. P. R. Gay,6G. H. Hammad,6T. Hreus,6P. E. Marage,6L. Thomas,6C. Vander Velde,6
P. Vanlaer,6J. Wickens,6V. Adler,7S. Costantini,7M. Grunewald,7B. Klein,7A. Marinov,7J. Mccartin,7 D. Ryckbosch,7F. Thyssen,7M. Tytgat,7L. Vanelderen,7P. Verwilligen,7S. Walsh,7N. Zaganidis,7S. Basegmez,8 G. Bruno,8J. Caudron,8L. Ceard,8E. Cortina Gil,8C. Delaere,8D. Favart,8A. Giammanco,8G. Gre´goire,8J. Hollar,8
V. Lemaitre,8J. Liao,8O. Militaru,8S. Ovyn,8D. Pagano,8A. Pin,8K. Piotrzkowski,8N. Schul,8N. Beliy,9 T. Caebergs,9E. Daubie,9G. A. Alves,10D. De Jesus Damiao,10M. E. Pol,10M. H. G. Souza,10W. Carvalho,11
E. M. Da Costa,11C. De Oliveira Martins,11S. Fonseca De Souza,11L. Mundim,11H. Nogima,11V. Oguri,11 W. L. Prado Da Silva,11A. Santoro,11S. M. Silva Do Amaral,11A. Sznajder,11F. Torres Da Silva De Araujo,11 F. A. Dias,12T. R. Fernandez Perez Tomei,12E. M. Gregores,12,cC. Lagana,12F. Marinho,12P. G. Mercadante,12,c
S. F. Novaes,12Sandra S. Padula,12N. Darmenov,13,bL. Dimitrov,13V. Genchev,13,bP. Iaydjiev,13,bS. Piperov,13 M. Rodozov,13S. Stoykova,13G. Sultanov,13V. Tcholakov,13R. Trayanov,13I. Vankov,13M. Dyulendarova,14 R. Hadjiiska,14V. Kozhuharov,14L. Litov,14E. Marinova,14M. Mateev,14B. Pavlov,14P. Petkov,14J. G. Bian,15
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S. Choudhury,29M. Dejardin,29D. Denegri,29B. Fabbro,29J. L. Faure,29F. Ferri,29S. Ganjour,29F. X. Gentit,29 A. Givernaud,29P. Gras,29G. Hamel de Monchenault,29P. Jarry,29E. Locci,29J. Malcles,29M. Marionneau,29 L. Millischer,29J. Rander,29A. Rosowsky,29I. Shreyber,29M. Titov,29P. Verrecchia,29S. Baffioni,30F. Beaudette,30
L. Benhabib,30L. Bianchini,30M. Bluj,30,gC. Broutin,30P. Busson,30C. Charlot,30T. Dahms,30L. Dobrzynski,30 S. Elgammal,30R. Granier de Cassagnac,30M. Haguenauer,30P. Mine´,30C. Mironov,30C. Ochando,30P. Paganini,30
D. Sabes,30R. Salerno,30Y. Sirois,30C. Thiebaux,30B. Wyslouch,30,hA. Zabi,30J.-L. Agram,31,iJ. Andrea,31 D. Bloch,31D. Bodin,31J.-M. Brom,31M. Cardaci,31E. C. Chabert,31C. Collard,31E. Conte,31,iF. Drouhin,31,i C. Ferro,31J.-C. Fontaine,31,iD. Gele´,31U. Goerlach,31S. Greder,31P. Juillot,31M. Karim,31,iA.-C. Le Bihan,31 Y. Mikami,31P. Van Hove,31F. Fassi,32D. Mercier,32C. Baty,33S. Beauceron,33N. Beaupere,33M. Bedjidian,33 O. Bondu,33G. Boudoul,33D. Boumediene,33H. Brun,33N. Chanon,33R. Chierici,33D. Contardo,33P. Depasse,33
H. El Mamouni,33A. Falkiewicz,33J. Fay,33S. Gascon,33B. Ille,33T. Kurca,33T. Le Grand,33M. Lethuillier,33 L. Mirabito,33S. Perries,33V. Sordini,33S. Tosi,33Y. Tschudi,33P. Verdier,33V. Roinishvili,34D. Lomidze,35 G. Anagnostou,36M. Edelhoff,36L. Feld,36N. Heracleous,36O. Hindrichs,36R. Jussen,36K. Klein,36J. Merz,36
N. Mohr,36A. Ostapchuk,36A. Perieanu,36F. Raupach,36J. Sammet,36S. Schael,36D. Sprenger,36H. Weber,36
M. Weber,36B. Wittmer,36M. Ata,37W. Bender,37M. Erdmann,37J. Frangenheim,37T. Hebbeker,37A. Hinzmann,37 K. Hoepfner,37C. Hof,37T. Klimkovich,37D. Klingebiel,37P. Kreuzer,37D. Lanske,37,aC. Magass,37G. Masetti,37
M. Merschmeyer,37A. Meyer,37P. Papacz,37H. Pieta,37H. Reithler,37S. A. Schmitz,37L. Sonnenschein,37 J. Steggemann,37D. Teyssier,37M. Tonutti,37M. Bontenackels,38M. Davids,38M. Duda,38G. Flu¨gge,38 H. Geenen,38M. Giffels,38W. Haj Ahmad,38D. Heydhausen,38T. Kress,38Y. Kuessel,38A. Linn,38A. Nowack,38 L. Perchalla,38O. Pooth,38J. Rennefeld,38P. Sauerland,38A. Stahl,38M. Thomas,38D. Tornier,38M. H. Zoeller,38 M. Aldaya Martin,39W. Behrenhoff,39U. Behrens,39M. Bergholz,39,jK. Borras,39A. Cakir,39A. Campbell,39 E. Castro,39D. Dammann,39G. Eckerlin,39D. Eckstein,39A. Flossdorf,39G. Flucke,39A. Geiser,39J. Hauk,39 H. Jung,39M. Kasemann,39I. Katkov,39P. Katsas,39C. Kleinwort,39H. Kluge,39A. Knutsson,39M. Kra¨mer,39
D. Kru¨cker,39E. Kuznetsova,39W. Lange,39W. Lohmann,39,jR. Mankel,39M. Marienfeld,39 I.-A. Melzer-Pellmann,39A. B. Meyer,39J. Mnich,39A. Mussgiller,39J. Olzem,39D. Pitzl,39A. Raspereza,39
A. Raval,39M. Rosin,39R. Schmidt,39,jT. Schoerner-Sadenius,39N. Sen,39A. Spiridonov,39M. Stein,39 J. Tomaszewska,39R. Walsh,39C. Wissing,39C. Autermann,40S. Bobrovskyi,40J. Draeger,40H. Enderle,40 U. Gebbert,40K. Kaschube,40G. Kaussen,40J. Lange,40B. Mura,40S. Naumann-Emme,40F. Nowak,40N. Pietsch,40 C. Sander,40H. Schettler,40P. Schleper,40M. Schro¨der,40T. Schum,40J. Schwandt,40H. Stadie,40G. Steinbru¨ck,40
J. Thomsen,40C. Barth,41J. Bauer,41V. Buege,41T. Chwalek,41W. De Boer,41A. Dierlamm,41G. Dirkes,41 M. Feindt,41J. Gruschke,41C. Hackstein,41F. Hartmann,41S. M. Heindl,41M. Heinrich,41H. Held,41 K. H. Hoffmann,41S. Honc,41T. Kuhr,41D. Martschei,41S. Mueller,41Th. Mu¨ller,41M. Niegel,41O. Oberst,41 A. Oehler,41J. Ott,41T. Peiffer,41D. Piparo,41G. Quast,41K. Rabbertz,41F. Ratnikov,41N. Ratnikova,41M. Renz,41
C. Saout,41A. Scheurer,41P. Schieferdecker,41F.-P. Schilling,41M. Schmanau,41G. Schott,41H. J. Simonis,41 F. M. Stober,41D. Troendle,41J. Wagner-Kuhr,41T. Weiler,41M. Zeise,41V. Zhukov,41,kE. B. Ziebarth,41 G. Daskalakis,42T. Geralis,42K. Karafasoulis,42S. Kesisoglou,42A. Kyriakis,42D. Loukas,42I. Manolakos,42 A. Markou,42C. Markou,42C. Mavrommatis,42E. Ntomari,42E. Petrakou,42L. Gouskos,43T. J. Mertzimekis,43
A. Panagiotou,43I. Evangelou,44C. Foudas,44P. Kokkas,44N. Manthos,44I. Papadopoulos,44V. Patras,44 F. A. Triantis,44A. Aranyi,45G. Bencze,45L. Boldizsar,45C. Hajdu,45,bP. Hidas,45D. Horvath,45,lA. Kapusi,45 K. Krajczar,45,mF. Sikler,45G. I. Veres,45,mG. Vesztergombi,45,mN. Beni,46J. Molnar,46J. Palinkas,46Z. Szillasi,46
V. Veszpremi,46P. Raics,47Z. L. Trocsanyi,47B. Ujvari,47S. Bansal,48S. B. Beri,48V. Bhatnagar,48N. Dhingra,48 R. Gupta,48M. Jindal,48M. Kaur,48J. M. Kohli,48M. Z. Mehta,48N. Nishu,48L. K. Saini,48A. Sharma,48 A. P. Singh,48J. B. Singh,48S. P. Singh,48S. Ahuja,49S. Bhattacharya,49B. C. Choudhary,49P. Gupta,49S. Jain,49
S. Jain,49A. Kumar,49K. Ranjan,49R. K. Shivpuri,49R. K. Choudhury,50D. Dutta,50S. Kailas,50V. Kumar,50 A. K. Mohanty,50,bL. M. Pant,50P. Shukla,50T. Aziz,51M. Guchait,51,nA. Gurtu,51M. Maity,51,oD. Majumder,51
G. Majumder,51K. Mazumdar,51G. B. Mohanty,51A. Saha,51K. Sudhakar,51N. Wickramage,51S. Banerjee,52 S. Dugad,52N. K. Mondal,52H. Arfaei,53H. Bakhshiansohi,53S. M. Etesami,53A. Fahim,53M. Hashemi,53
A. Jafari,53M. Khakzad,53A. Mohammadi,53M. Mohammadi Najafabadi,53S. Paktinat Mehdiabadi,53 B. Safarzadeh,53M. Zeinali,53M. Abbrescia,54a,54bL. Barbone,54a,54bC. Calabria,54a,54bA. Colaleo,54a D. Creanza,54a,54cN. De Filippis,54a,54cM. De Palma,54a,54bA. Dimitrov,54aL. Fiore,54aG. Iaselli,54a,54c L. Lusito,54a,54b,bG. Maggi,54a,54cM. Maggi,54aN. Manna,54a,54bB. Marangelli,54a,54bS. My,54a,54cS. Nuzzo,54a,54b
N. Pacifico,54a,54bG. A. Pierro,54aA. Pompili,54a,54bG. Pugliese,54a,54cF. Romano,54a,54cG. Roselli,54a,54b G. Selvaggi,54a,54bL. Silvestris,54aR. Trentadue,54aS. Tupputi,54a,54bG. Zito,54aG. Abbiendi,55aA. C. Benvenuti,55a
D. Bonacorsi,55aS. Braibant-Giacomelli,55a,55bL. Brigliadori,55aP. Capiluppi,55a,55bA. Castro,55a,55b F. R. Cavallo,55aM. Cuffiani,55a,55bG. M. Dallavalle,55aF. Fabbri,55aA. Fanfani,55a,55bD. Fasanella,55a P. Giacomelli,55aM. Giunta,55aS. Marcellini,55aM. Meneghelli,55a,55bA. Montanari,55aF. L. Navarria,55a,55b
F. Odorici,55aA. Perrotta,55aF. Primavera,55aA. M. Rossi,55a,55bT. Rovelli,55a,55bG. Siroli,55a,55b R. Travaglini,55a,55bS. Albergo,56a,56bG. Cappello,56a,56bM. Chiorboli,56a,56b,bS. Costa,56a,56bA. Tricomi,56a,56b C. Tuve,56aG. Barbagli,57aV. Ciulli,57a,57bC. Civinini,57aR. D’Alessandro,57a,57bE. Focardi,57a,57bS. Frosali,57a,57b
E. Gallo,57aS. Gonzi,57a,57bP. Lenzi,57a,57bM. Meschini,57aS. Paoletti,57aG. Sguazzoni,57aA. Tropiano,57a,b L. Benussi,58S. Bianco,58S. Colafranceschi,58,pF. Fabbri,58D. Piccolo,58P. Fabbricatore,59R. Musenich,59 A. Benaglia,60a,60bF. De Guio,60a,60b,bL. Di Matteo,60a,60bA. Ghezzi,60a,60b,bM. Malberti,60a,60bS. Malvezzi,60a
A. Martelli,60a,60bA. Massironi,60a,60bD. Menasce,60aL. Moroni,60aM. Paganoni,60a,60bD. Pedrini,60a S. Ragazzi,60a,60bN. Redaelli,60aS. Sala,60aT. Tabarelli de Fatis,60a,60bV. Tancini,60a,60bS. Buontempo,61a
C. A. Carrillo Montoya,61aN. Cavallo,61a,qA. Cimmino,61a,61bA. De Cosa,61a,61bM. De Gruttola,61a,61b
F. Fabozzi,61a,qA. O. M. Iorio,61aL. Lista,61aM. Merola,61a,61bP. Noli,61a,61bP. Paolucci,61aP. Azzi,62a N. Bacchetta,62aP. Bellan,62a,62bD. Bisello,62a,62bA. Branca,62aR. Carlin,62a,62bP. Checchia,62aM. De Mattia,62a,62b
T. Dorigo,62aU. Dosselli,62aF. Fanzago,62aF. Gasparini,62a,62bU. Gasparini,62a,62bS. Lacaprara,62a,r I. Lazzizzera,62a,62cM. Margoni,62a,62bM. Mazzucato,62aA. T. Meneguzzo,62a,62bM. Nespolo,62aL. Perrozzi,62a,b
N. Pozzobon,62a,62bP. Ronchese,62a,62bF. Simonetto,62a,62bE. Torassa,62aM. Tosi,62a,62bS. Vanini,62a,62b P. Zotto,62a,62bG. Zumerle,62a,62bP. Baesso,63a,63bU. Berzano,63aS. P. Ratti,63a,63bC. Riccardi,63a,63bP. Torre,63a,63b
P. Vitulo,63a,63bC. Viviani,63a,63bM. Biasini,64a,64bG. M. Bilei,64aB. Caponeri,64a,64bL. Fano`,64a,64b P. Lariccia,64a,64bA. Lucaroni,64a,64b,bG. Mantovani,64a,64bM. Menichelli,64aA. Nappi,64a,64bA. Santocchia,64a,64b
S. Taroni,64a,64bM. Valdata,64a,64bR. Volpe,64a,64b,bP. Azzurri,65a,65cG. Bagliesi,65aJ. Bernardini,65a,65b T. Boccali,65a,bG. Broccolo,65a,65cR. Castaldi,65aR. T. D’Agnolo,65a,65cR. Dell’Orso,65aF. Fiori,65a,65bL. Foa`,65a,65c
A. Giassi,65aA. Kraan,65aF. Ligabue,65a,65cT. Lomtadze,65aL. Martini,65a,sA. Messineo,65a,65bF. Palla,65a F. Palmonari,65aG. Segneri,65aA. T. Serban,65aP. Spagnolo,65aR. Tenchini,65aG. Tonelli,65a,65b,bA. Venturi,65a,b P. G. Verdini,65aL. Barone,66a,66bF. Cavallari,66aD. Del Re,66a,66bE. Di Marco,66a,66bM. Diemoz,66aD. Franci,66a,66b
M. Grassi,66aE. Longo,66a,66bS. Nourbakhsh,66aG. Organtini,66a,66bA. Palma,66a,66bF. Pandolfi,66a,66b,b R. Paramatti,66aS. Rahatlou,66a,66bN. Amapane,67a,67bR. Arcidiacono,67a,67cS. Argiro,67a,67bM. Arneodo,67a,67c C. Biino,67aC. Botta,67a,67b,bN. Cartiglia,67aR. Castello,67a,67bM. Costa,67a,67bN. Demaria,67aA. Graziano,67a,67b,b C. Mariotti,67aM. Marone,67a,67bS. Maselli,67aE. Migliore,67a,67bG. Mila,67a,67bV. Monaco,67a,67bM. Musich,67a,67b M. M. Obertino,67a,67cN. Pastrone,67aM. Pelliccioni,67a,67b,bA. Romero,67a,67bM. Ruspa,67a,67cR. Sacchi,67a,67b
V. Sola,67a,67bA. Solano,67a,67bA. Staiano,67aD. Trocino,67a,67bA. Vilela Pereira,67a,67b,bS. Belforte,68a F. Cossutti,68aG. Della Ricca,68a,68bB. Gobbo,68aD. Montanino,68a,68bA. Penzo,68aS. G. Heo,69S. K. Nam,69
S. Chang,70J. Chung,70D. H. Kim,70G. N. Kim,70J. E. Kim,70D. J. Kong,70H. Park,70S. R. Ro,70D. Son,70 D. C. Son,70Zero Kim,71J. Y. Kim,71S. Song,71S. Choi,72B. Hong,72M. S. Jeong,72M. Jo,72H. Kim,72J. H. Kim,72
T. J. Kim,72K. S. Lee,72D. H. Moon,72S. K. Park,72H. B. Rhee,72E. Seo,72S. Shin,72K. S. Sim,72M. Choi,73 S. Kang,73H. Kim,73C. Park,73I. C. Park,73S. Park,73G. Ryu,73Y. Choi,74Y. K. Choi,74J. Goh,74M. S. Kim,74 J. Lee,74S. Lee,74H. Seo,74I. Yu,74M. J. Bilinskas,75I. Grigelionis,75M. Janulis,75D. Martisiute,75P. Petrov,75 T. Sabonis,75H. Castilla-Valdez,76E. De La Cruz-Burelo,76R. Lopez-Fernandez,76A. Sa´nchez-Herna´ndez,76
L. M. Villasenor-Cendejas,76S. Carrillo Moreno,77F. Vazquez Valencia,77H. A. Salazar Ibarguen,78 E. Casimiro Linares,79A. Morelos Pineda,79M. A. Reyes-Santos,79D. Krofcheck,80P. H. Butler,81R. Doesburg,81 H. Silverwood,81M. Ahmad,82I. Ahmed,82M. I. Asghar,82H. R. Hoorani,82W. A. Khan,82T. Khurshid,82S. Qazi,82
M. Cwiok,83W. Dominik,83K. Doroba,83A. Kalinowski,83M. Konecki,83J. Krolikowski,83T. Frueboes,84 R. Gokieli,84M. Go´rski,84M. Kazana,84K. Nawrocki,84K. Romanowska-Rybinska,84M. Szleper,84G. Wrochna,84
P. Zalewski,84N. Almeida,85P. Bargassa,85A. David,85P. Faccioli,85P. G. Ferreira Parracho,85M. Gallinaro,85 P. Musella,85A. Nayak,85J. Seixas,85J. Varela,85S. Afanasiev,86I. Belotelov,86P. Bunin,86I. Golutvin,86 A. Kamenev,86V. Karjavin,86G. Kozlov,86A. Lanev,86P. Moisenz,86V. Palichik,86V. Perelygin,86S. Shmatov,86
V. Smirnov,86A. Volodko,86A. Zarubin,86V. Golovtsov,87Y. Ivanov,87V. Kim,87P. Levchenko,87V. Murzin,87 V. Oreshkin,87I. Smirnov,87V. Sulimov,87L. Uvarov,87S. Vavilov,87A. Vorobyev,87A. Vorobyev,87Yu. Andreev,88
A. Dermenev,88S. Gninenko,88N. Golubev,88M. Kirsanov,88N. Krasnikov,88V. Matveev,88A. Pashenkov,88 A. Toropin,88S. Troitsky,88V. Epshteyn,89V. Gavrilov,89V. Kaftanov,89,aM. Kossov,89,bA. Krokhotin,89 N. Lychkovskaya,89V. Popov,89G. Safronov,89S. Semenov,89V. Stolin,89E. Vlasov,89A. Zhokin,89E. Boos,90
A. Demiyanov,90A. Ershov,90A. Gribushin,90O. Kodolova,90I. Lokhtin,90S. Obraztsov,90S. Petrushanko,90 L. Sarycheva,90V. Savrin,90A. Snigirev,90I. Vardanyan,90V. Andreev,91M. Azarkin,91I. Dremin,91 M. Kirakosyan,91A. Leonidov,91S. V. Rusakov,91A. Vinogradov,91I. Azhgirey,92S. Bitioukov,92V. Grishin,92,b
V. Kachanov,92D. Konstantinov,92A. Korablev,92V. Krychkine,92V. Petrov,92R. Ryutin,92S. Slabospitsky,92 A. Sobol,92L. Tourtchanovitch,92S. Troshin,92N. Tyurin,92A. Uzunian,92A. Volkov,92P. Adzic,93,tM. Djordjevic,93
D. Krpic,93,tJ. Milosevic,93M. Aguilar-Benitez,94J. Alcaraz Maestre,94P. Arce,94C. Battilana,94E. Calvo,94 M. Cepeda,94M. Cerrada,94N. Colino,94B. De La Cruz,94A. Delgado Peris,94C. Diez Pardos,94
D. Domı´nguez Va´zquez,94C. Fernandez Bedoya,94J. P. Ferna´ndez Ramos,94A. Ferrando,94J. Flix,94M. C. Fouz,94 P. Garcia-Abia,94O. Gonzalez Lopez,94S. Goy Lopez,94J. M. Hernandez,94M. I. Josa,94G. Merino,94 J. Puerta Pelayo,94I. Redondo,94L. Romero,94J. Santaolalla,94C. Willmott,94C. Albajar,95G. Codispoti,95
J. F. de Troco´niz,95J. Cuevas,96J. Fernandez Menendez,96S. Folgueras,96I. Gonzalez Caballero,96 L. Lloret Iglesias,96J. M. Vizan Garcia,96J. A. Brochero Cifuentes,97I. J. Cabrillo,97A. Calderon,97
