One of the first measurements will be made at the LHC by ATLAS on properties of inelastic collisions, analysing the central charged particle density and transverse momentum distributions. Current predictions of these distributions by models have large uncertainties in the LHC energy range. We describe the ATLAS minimum bias triggers, designed to select all kinds of inelastic interactions, and the performance of the track reconstruction software which was adpapted to soft particle track reconstruction. The precision with which the minimum bias distributions can be measured with early data is presented and the uncertainties on the inelastic distributions due to trigger bias is discussed

Kwee, R

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ATL-PHYS-PROC-2010-00827January2010CPC(HEP & NP), 2009, 33(X): 1—4 Chinese Physics C Vol. 33, No. X, Xxx, 2009Prospectives of the Hadron Program in ATLASR. Kwee1,2) on behalf of the ATLAS Collaboration1 (CERN, 1211 Geneve 23, Switzerland)2 (Humboldt University of Berlin: Physics Department, Experimental Particle Physics, 12489 Berlin, Germany)Abstract One of the first measurements that will be made at the LHC by ATLAS deals with the propertiesof inelastic collisions, namely the central charged particle density and transverse momentum distributions.Current predictions of these distributions have large uncertainties in the LHC energy range. We describe theATLAS minimum bias triggers, designed to select all kind of inelastic interactions, and the performance ofthe track reconstruction software which was adapted to soft particle track reconstruction. The precision withwhich the minimum bias distributions can be measured with early data is presented and the uncertainties onthe inelastic distributions due to trigger bias is discussed.Key words ATLAS, LHC, Minimum Bias, Minimum Bias Trigger, Underlying Event, early measurementsPACS 13.85.Hd Inelastic scattering: many-particle final states1 IntroductionATLAS is one of the four major experiments atthe Large Hadron Collider (LHC). The machine op-eration has successfully re-started in November 2009,providing proton-proton collisions to the four experi-ments. A wide range of physics analyses is envisagedto exploit the physics potential when the acceleratoris operated at its nominal parameters of energy√s =14 TeV and high luminosities of L = 1034 cm−2s−1.Searches for Higgs bosons of masses up to the elec-troweak energy scale of 1 TeV, possible deviationsfrom the Standard Model and also unknown signa-tures are the objectives of the ATLAS physics pro-gram. Their studies and expectations can be foundin ref.[1]. However, ATLAS has the potential alreadyat initial lower energies of√s = 7 and 10 TeV andluminosities of L = 1031 cm−2s−1, to improve theknowledge of the inelastic scattering processes and ofthe multi-parton interactions by performing measure-ments of interactions with a small momentum trans-fer.Minimum Bias (MB) and Underlying Event (UE)analyses are the main hadron studies of soft interac-tions with early data. In both cases the motivationis that soft interaction processes are yet not well un-derstood. It is therefore important to quantify theproperties of the soft processes at all available centerof mass energies. The best currently available de-scriptions come from phenomenological models wheremultiple parton interaction is one of the key ingredi-ents to simulate charged particle multiplicities. Withdata, we will be able to improve the descriptions,which are used in Monte-Carlo event generators likePythia[6] or Phojet[7]. The ultimate goal is to de-scribe soft interactions in the fundamental languageof QCD. Results of the analyses of soft processeswill have a crucial impact on all analysis of high-pTprocesses, where e.g. the determination of the jetenergy and missing transverse energy scale is needed.Also lepton isolation or the direct Higgs-search in thevector-boson-fusion channel require the understand-ing of low-pT QCD analysis beforehand.This paper focuses on early measurements of theproperties of inelastic interactions. In particular, thestudies of the kinematic spectra of charged particledistributions dNch/dpT and dNch/dη are presented.2 Minimum Bias AnalysisEvents associated with MB physics comprise con-tributions from inelastic interactions. They canbe furthermore classified as events with diffractiveand non-diffractive dissociation of the nucleon. Thediffractive dissociation, short diffractives, can be sub-divided into single and double diffractives (sd, dd).These events are characterized by the presence of theReceived December 20091)E-mail: rkwee@mail.cern.chc©2009Chinese Physical Society and the Institute of High Energy Physics of the ChineseAcademy of Sciences and the Instituteof Modern Physics of the Chinese Academy of Sciences and IOP Publishing LtdNo. X R. Kwee for the ATLAS Collaboration: Prospectives of the Hadron Program in ATLAS 2so called rapidity gaps which are regions, where finalstate particles are exponentially suppressed. In con-trast, in a non-diffractive (nd) process a color fieldover the whole range in rapidity is formed allowingfor mostly soft particle production. In former MBanalyses e.g. made by the CDF collaboration[5] MBevents were only associated to non-single diffractiveevents, excluding double-diffractive events. The ex-act definition of MB events is given by the triggersystem. In ATLAS, the MB trigger systems are sen-sitive to all kinds of diffractive events, in additionto the non-diffractive processes. To compare alsoto previous results, ATLAS will measure also thenon-single-diffractive distributions. These studies areshown here.2.1 Monte Carlo PredictionsTwo different Monte Carlo generators were usedto compare predictions of cross-sections and kine-matic spectra of charged particles, Pythia 6.420 andPhojet 1.12. They have a different way of modellingthe soft QCD processes: while Pythia uses an ap-proach combining Regge theory and an eikonalizedQCD model, the dual parton model is implementedin Phojet. The cross-sections for√s = 10 and 14TeV are shown in Tab. 1 for the single inelastic pro-cesses. One can clearly see significant differences,which reach about 50% for (dd) events.A similar difference is also visible in the kinematicspectra of charged stable particles (shown here for√s= 14 TeV only). The distribution for pseudorapid-ity∗ η is shown in Fig. 1, for the transverse momentumpT distribution in Fig. 2. While Phojet seems to gen-erate less charged particles per η- and pT -unit in thend-events, the different modelling of the diffractiveprocesses seems to be reflected especially in the shapeof the pT -distribution, showing a much larger tail tohigher pT values in Phojet generated events. Also aclear difference is visible for the diffractive events.Table 1. Cross-section predictions for√s= 10 TeV(14 TeV).σ/mb Pythia 6.420 Phojet 1.12σtotal 96(101) 113(119)σinel 75(79) 81(84)σnd 52(55) 65(68)σdd 9.8(10.3) 4.0(4.1)σsd 14.0(14.3) 10.9(11.0)Fig. 1. Pseudorapidity distribution of charged particles fromPythia 6.420 and Phojet 1.12 shown for different inelasticprocesses, see ref. [1].Fig. 2. Transverse momentum distribution of charged par-ticles from Pythia 6.420 and Phojet 1.12 shown for differentinelastic processes, see ref. [1].2.2 Analysis ProcedureATLAS will perform these measurements rightfrom the beginning of data-taking, making use of min-imum bias triggers. With increasing luminosities, azero bias trigger will replace them. Several factorshave to be considered in order to correct for inefficien-cies caused by the trigger and track reconstruction.The systematic uncertainties need to be estimated.The procedure had been studied with√s= 14 TeVMC data and is outlined in the following sections.Full details are in ref. [1].2.3 Minimum Bias TriggerATLAS possesses two main independent mini-mum bias trigger systems, both optimized to effi-∗η=− ln(tanθ/2), where θ is the polar angleNo. X R. Kwee for the ATLAS Collaboration: Prospectives of the Hadron Program in ATLAS 3ciently suppress empty bunch-crossing events, as theyform the main background source at low luminosi-ties, while triggering efficiently on inelastic interac-tions. One of such systems consists of Minimum BiasTrigger Scintillators (MBTS) that is made up of twodetector parts with 16 scintillator modules, belongingto the first trigger level (L1) trigger†. They are situ-ated at about 3.5 m away from the interaction point(IP) on each side of the ATLAS detector and coveran η range of 2.1 < |η|< 3.8.An alternative approach is realised by the Inner De-tector (ID) Minimum Bias Trigger, covering the IDrange of |η| < 2.5, which is the region where trackbased measurements are performed. This trigger isseeded by a random trigger at L1 to minimize initalbias in the event selection. The actual event selec-tion takes place at the high-level trigger (HLT), wherein two trigger levels signals of the pixel and semi-conductor tracker (SCT) detectors[3] are required. Atthe second trigger level (L2) silicon hits above elec-tronic noise are required, while at the third triggerlevel, the Event Filter (EF), a track requirement isimposed, such that tracks close to the nominal IPhave to be found.The trigger efficiency, here defined as the ratio ofevents with and without trigger requirement, is shownas a function of the trigger threshold: for MBTS theL1 requirement to have a multiplicity of 2 hits in theMBTS counters in Fig. 3 and for the ID MinimumBias trigger passing the L2 silicon hit requirement inFig. 4.MBTS Lvl 1 Threshold (mV)   0 50 100 150 200 250 300 350 400Trigger Efficiency   00.20.40.60.81emptysdiffddiffndiffbeamgasATLASFig. 3. MBTS trigger efficiency with a 2-hit multiplicity re-quirement for signal (inelastic processes) and background(empty events, beamgas events). Already at a very lowthreshold the empty events are completely suppressed.Number of Tracks Cut   0 1 2 3 4 5 6 7 8 9 10Trigger Efficiency   00.20.40.60.81emptysdiffddiffndiffbeamgasATLASFig. 4. ID Minimum Bias Trigger efficiency as a function ofa track multiplicity requirement.2.4 Event and Track SelectionFor the analysis, the events were selected by theL1 MBTS trigger with the 2-hit multiplicity require-ment. Additionally, oﬄine selection criteria wereapplied, such that only events containing a recon-structed primary vertex and tracks with a pT ≥ 150MeV satisfying quality criteria were considered in theanalysis.2.5 CorrectionsThe selection criteria imply that corrections haveto be taken into account. The complexity arises as de-pendencies exist amongst various requirements. Forthe measurement of dNch/dpT and dNch/dη, the cor-rections account for trigger bias in the event selectionand track and vertex reconstruction efficiencies. Atevent-level the corrections for trigger bias were calcu-lated by relating the number of events of a particularinelastic processed to the number of events selectedby the trigger. For this study, the bias correction ismodel-dependent, since it takes into account the rel-ative cross-sections of the different inelastic physicsprocesses. The vertex correction accounts for ineffi-ciency of the vertex reconstruction, to not reconstructprimary vertex, even though charged particles weregenerated in the measured region.A track-to-particle correction at track-level con-siders the track reconstruction efficiency, when gen-erated charged primary particles were not recon-structed or fake and secondary tracks contribute tothe primary tracks. The tracking efficiency is de-fined as the number of reconstructed primary tracksmatched to the generated track particle over the num-ber of generated charged primary particles, and isshown as function of pT in Fig. 5, where the defaultreconstruction cut has been lowered to pT = 100 MeV.†For details of the ATLAS trigger system, see [2].No. X R. Kwee for the ATLAS Collaboration: Prospectives of the Hadron Program in ATLAS 4   (GeV)Tp0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2Efficiency   00.20.40.60.81ATLASFig. 5. Tracking efficiency as a function of pT in |η|< 2.5The result for the track-to-particle corrections isa function of η, pT and the number of charged parti-cles Nch, here shown projected to the η-axis in Fig. 6.One can see, that the corrections are smaller in thecentral part of |η|< 1.Fig. 6. Track-to-particle corrections as a function of η.Systematic uncertainties were evaluated by chang-ing parameters in the particle generation and recon-struction and re-calculating the corrections. It wasfound that the major source of systematic uncertain-ties comes from the diffractive cross-section ∼4%.The results of the studies confirm that this procedureis able to reproduce the dNch/dpT , dNch/dη distribu-tions which agree well with the Pythia MC predicteddistributions.3 Underlying EventThe activity of the UE manifests itself in addi-tional production of mostly soft particles: similar toMB physics, the study of the UE also deals with mul-tiple parton interactions and soft hadronic particleproduction. Accordnig to the common definition, theUE comprises all particles accompanying the hardscattering component of the collision. However, ex-perimentally this separation is not possible. There-fore, topological regions are defined which are sensi-tive to the UE activity. Based on the direction of theleading jet or track pT , the plane orthogonal to thebeam direction is divided into 3 regions: the forward,away and transverse region. The transverse area issensitive to the UE activity and typical observablesin that region are e.g. the charged particle density,pT , mean pT and scalar sum of pT of tracks and jets.As example, the mean multiplicity of charged parti-cles in the transverse region is shown in Fig. 7, whereit is clearly visible that also for the UE activity thepreditions are highly uncertain [8].Fig. 7. MC predictions of UE activity for LHC energies. Dif-ferent Pythia tunes and both MC generators show distinctactivity levels in the mean multiplicity of charged particles.3.1 Analysis GoalsMeasuring the UE activity will help to distinguishdifferent physics models. An inital beam energy of√s = 7 TeV and an integrated luminosity of 10 pb−1are already useful for such studies. Nevertheless thesystematic uncertainties need to be understood first.4 ConclusionThe hadron program of ATLAS starts with thevery first measurements of the properties of the in-elastic interactions, which will help understandingsoft QCD models. Additional hadron analyses aredescribed in ref.[4].References1 The ATLAS Collaboration, Expected Performance of the ATLAS Exper-iment, Detector, Trigger and Physics, CERN-OPEN-2008-020, Dec. 20082 Technical Design Report, ATLAS High-Level Trigger Data Aquisition andControls, CERN/LHCC/2003-22, ATLAS TDR 016, 30 June 20033 Inner Detector Technical Design Report, ATLAS TDR 5,CERN/LHCC/97-17, ISBN 92-9083-103-0, 30. April 19974 K. Hara for the ATLAS Collaboration, ATLAS commissioning and earlyphysics of resonance and jet production, this conference.5 The CDF Collaboration, Measurement of Particle Production and In-clusive differential cross-sections in pp¯ Collisions at√s =1.96 TeV,arXiv:0904.1098 [hep-ex]6 PYTHIA 6.400 manual, JHEP 05 (2006) 0267 R. Engel, PHOJET manual (ver. 1.05c, June 1996)8 A. Moraes, C. Buttar, D. Clements, Measuring the underlying event atATLAS, CERN-ATL-PHYS-PUB-2005-015

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