Soft-QCD measurements in proton-proton collisions at √s =

900 GeV and √s = 7TeV recorded with the ATLAS detector at the LHC using a single-arm minimum bias trigger are presented. The charged particle multiplicity distribution, its dependence on transverse momentum and pseudorapidity and its correlation with the average transverse momentum will be shown. Moreover, the measurement of the underlying event properties using charged particles is discussed.

Finally the measurement of the angular correlations between charged particles is presented. All the measurements are compared with Monte Carlo predictions

Di Nardo, R.

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DOI 10.1393/ncc/i2011-11009-6Colloquia: LC10IL NUOVO CIMENTO Vol. 34 C, N. 5 Suppl. 1 Settembre-Ottobre 2011Minimum bias and soft QCD ATLAS resultsR. Di NardoUniversita` di Roma “Tor Vergata” and INFNVia della Ricerca Scientifica 1, 00133 Rome, Italy(ricevuto il 20 Luglio 2011; pubblicato online il 19 Ottobre 2011)Summary. — Soft-QCD measurements in proton-proton collisions at√s =900GeV and√s = 7TeV recorded with the ATLAS detector at the LHC usinga single-arm minimum bias trigger are presented. The charged particle multiplicitydistribution, its dependence on transverse momentum and pseudorapidity and itscorrelation with the average transverse momentum will be shown. Moreover, themeasurement of the underlying event properties using charged particles is discussed.Finally the measurement of the angular correlations between charged particles ispresented. All the measurements are compared with Monte Carlo predictions.PACS 12.38.Lg – Other nonperturbative calculations.PACS 12.38.Qk – Experimental tests.PACS 13.85.Hd – Inelastic scattering: many-particle final states.PACS 14.40.Df – Strange mesons (|S| > 0, C = B = 0).1. – IntroductionThe understanding at LHC energies of low-pT processes in proton-proton collisionsis fundamental in order to characterize the underlying event that is an important back-ground for the high-pT collisions. These soft processes are described by phenomenolog-ical models that have to be tuned to the data since perturbative QCD can correctlydescribe only the hard scattering between partons. Due to the high LHC luminosities,pileup events are unavoidable and they are composed essentially of soft particle pro-duction [1, 2] that overlay the interesting hard-scattering events: these can introduceexperimental biases in isolation criteria, jet trigger and jet energy scale. It is also impor-tant to describe properly the underlying event (UE) that consists of everything exceptthe hard-scattering process in a hadron-hadron collision (the contribution from initialand final state radiation, beam-beam remnants, multi-parton interactions). The chargedparticle distributions, UE [3,4] and angular correlation [5] measurements performed withthe ATLAS experiment [6] at√s = 900GeV and√s = 7TeV in proton-proton collisionsare presented.c© Societa` Italiana di Fisica 12 R. DI NARDO [MeV]-π+πM400 420 440 460 480 500 520 540 560 580 600Entries / 1 MeV=7 TeV)sMinimum Bias Stream, Data 2010 (ATLAS Preliminary20406080100310×Entries / 1 MeVDatadouble Gauss + poly fitPythia MC09 signalPythia MC09 background [MeV]-πpM1110 1120 1130 1140 1150Entries / 1 MeV5000100001500020000250003000035000Entries / 1 MeV=7 TeV)sMinimum Bias Stream, Data 2010 (ATLAS PreliminaryDatadouble Gauss + poly fitPythia MC09 signalPythia MC09 backgroundFig. 1. – K0s (left) and Λ (right) reconstructed mass spectra in proton-proton collision at√s =7TeV in the barrel region of the ATLAS Inner Detector (|η| < 1.2 for both tracks) comparedwith Monte Carlo simulation.2. – The ATLAS experimentThe ATLAS experiment is one of two general purpose detectors at the Large HadronCollider. The relevant ATLAS detector components to these measurements include theInner Detector consisting of three inner subdetectors: the pixel and microstrip subde-tectors, which use silicon technology, are complemented with the transition radiationtracker. These detectors allow to reconstruct the tracks of charged particles within|η| < 2.5. The reconstruction of known particle decays has been used as an importanttool to understand the performance of the ATLAS Inner Detector, the track and vertexreconstruction and particle identification capabilities. Figure 1 shows the mass spec-tra for K0s (left) and Λ (right) reconstructed in the barrel region of the ATLAS InnerDetector (|η| < 1.2 for both tracks) in proton-proton collision at √s = 7TeV. Theseare compared with Monte Carlo simulation in which signal and background componentsare separately normalized to data. The agreement demonstrates accuracy of the trackmomentum scale and excellent modeling of the Inner Detectors 2T solenoid magneticfield. The Minimum Bias Trigger Scintillator (MBTS [7]) disks are used to trigger in thepresence of charged particles in the region 2.09 < |η| < 3.84 and are located on both sidesof the detector. The events used in the analyses presented in this paper were triggeredrequiring the presence of activity on either of the two opposite sides of the MBTS disks.3. – Charged particle multiplicitiesThe charged particle multiplicity distributions [1] measured by the ATLAS experimentare1NevdNevdnch,1NevdNchdη,1Nev12πpTd2Nchdη dpT, 〈pT 〉 vs. nch,(1)where nch represent the number of charged particles for a given event, Nev is the numberof events with a minimum number of charged particles within the kinematic range se-lected, Nch is the total number of charged particles in the sample and 〈pT 〉 is the averagetransverse momentum for events with a specific value of nch. Various phase-spaces havebeen considered for these measurements:MINIMUM BIAS AND SOFT QCD ATLAS RESULTS 3η / dchN d⋅ evN1/12345678Data 2010PYTHIA ATLAS AMBT1PYTHIA ATLAS MC09PYTHIA DWPYTHIA 8PHOJET | < 2.5η > 100 MeV, | Tp 2, ≥ chn = 7 TeVsATLAS  η / dchN d⋅ evN1/η-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5Ratio0.811.2 Data UncertaintiesMC / DataRatio ]-2 [ GeVTpdη/dchN2) dTpπ 1/(2evN1/-1110-1010-910-810-710-610-510-410-310-210-110110210Data 2010PYTHIA ATLAS AMBT1PYTHIA ATLAS MC09PYTHIA DWPYTHIA 8PHOJET | < 2.5η > 100 MeV, | Tp 2, ≥ chn = 7 TeVsATLAS   ]-2 [ GeVTpdη/dchN2) dTpπ 1/(2evN1/ [GeV]Tp-110 1 10Ratio0.511.52Data UncertaintiesMC / DataRatiochn/dev N d⋅ evN1/-610-510-410-310-210-1101Data 2010PYTHIA ATLAS AMBT1PYTHIA ATLAS MC09PYTHIA DWPYTHIA 8PHOJET | < 2.5η > 100 MeV, | Tp 2, ≥ chn = 7 TeVsATLAS  chn/dev N d⋅ evN1/chn20 40 60 80 100 120 140 160 180 200Ratio0.511.5 Data UncertaintiesMC / DataRatio [ GeV ]〉Tp〈0.20.40.60.811.21.41.6Data 2010PYTHIA ATLAS AMBT1PYTHIA ATLAS MC09PYTHIA DWPYTHIA 8PHOJET | < 2.5η > 100 MeV, | Tp 2, ≥ chn = 7 TeVsATLAS   [ GeV ]〉Tp〈chn20 40 60 80 100 120 140 160 180 200Ratio0.811.2Data UncertaintiesMC / DataRatioFig. 2. – The measured charged particle multiplicity vs. pseudorapidity (top left), the distributionof the transverse momentum (top right), the multiplicity distribution of charged particles in theevent (bottom left) and the average transverse momentum as a function of the number of chargedparticles (bottom right) compared to different MC predictions.– pT > 0.1GeV, nch ≥ 2 and |η| < 2.5 for the most inclusive;– pT > 0.5GeV, nch ≥ 6 and |η| < 2.5 for the diffraction suppressed (used to producethe new ATLAS Minimum Bias Tune (AMBT1) [8]);– pT > 0.5GeV, nch ≥ 1 and |η| < 2.5, studied also at√s = 2.76TeV.Figure 2 shows the pseudorapidity(1), pT , nch and the average transverse momentum as afunction of the number of charged particle distributions measured at√s = 7TeV for the(1) Defined as η = − ln(tan(θ/2)) where θ is the polar angle from the beam axis.4 R. DI NARDO[rad]wrt leadφΔ-3 -2 -1 0 1 2 3φΔ dη/dchN2 d0.20.30.40.50.60.70.80.9 = 900 GeVs > 1.0, 1.5, 2.0, 2.5 GeV, bottom to topleadTp| < 2.5,η> 0.5  GeV and |TpATLASTransverse TowardAway Transverse AwayData 2009PYTHIA ATLAS MC09[rad]wrt leadφΔ-3 -2 -1 0 1 2 3φΔ dη/dchN2 d0.511.522.5 = 7 TeVs > 1, 2, 3, 5 GeV, bottom to topleadTp| < 2.5,η> 0.5  GeV and |TpATLASTransverse TowardAway Transverse AwayData 2010PYTHIA ATLAS MC09Fig. 3. – Δφ distribution of charged particle densities for pT > 0.5GeV and |η| < 2.5 comparedto ATLAS Pythia MC09 predictions at√s = 900GeV (left) and√s = 7TeV (right) for differentpleadT thresholds. The error bars show the statistical uncertainty while the shaded areas showthe total errors.phase-space nch ≥ 2, pT > 0.1GeV and |η| < 2.5. Data are compared with various MCpredictions, including also AMBT1. The shape of the charged multiplicity has a flat shapewith a smooth dip in the central pseudorapidity and decreases at forward pseudorapidity.Even if the shape seen in data for dN/dη is well reproduced in MC predictions, all themodels shown underestimate the data. The AMBT1 tune agrees well with the pT dataspectra in the intermediate pT range between 0.5 and 3GeV and at high pT all MCmodels agree with the data within 20% (with the exception of the ATLAS MC09 tune [9]that reaches 70%) and within 35% at low pT (with the exception of the PYTHIA DWtune [10] that reaches 45%) where a larger contribution of diffractive events is expected.The charged particle multiplicity distribution per event is not described properly by MCmodels in the low multiplicity region while AMBT1 agrees within 10% with the datadistribution for nch ≤ 20. Finally, the models vary widely in the prediction on theaverage transverse momentum as a function of the number of charged particles with theAMBT1 that is the closest to the data (agrees within 10%).4. – Underlying Event studies with charged particlesAlthough the UE cannot be separated event-by-event from the hard scattering, sev-eral observables sensitive to the UE properties can be studied. The Δφ variable, thatrepresents the difference between charged particles and the leading particle azimuthal an-gle [11], is used to categorized the particles in every event. The region 60◦ < |Δφ| < 120◦(transverse region) contains particles from the UE while the regions |Δφ| < 60◦ (towardregion) and |Δφ| > 120◦ (away region) contains mainly particles from the hard scattering.In order to correct back data to particle level, the same correction for trigger, vertex andtracking efficiency as in the Minimum Bias measurements are applied. Figure 3 showsthe Δφ distribution of charged particle densities (d2N/dη dΔφ) for pT > 0.5GeV and|η| < 2.5 for different values of the transverse momentum of the leading particle for dataat√s = 900GeV (left) and√s = 7TeV (right). A significant difference in the shape ofthe Δφ distributions between data and ATLAS Pythia MC09 predictions is seen. Withthe increase of the leading charged particle pT , the development of jet-like structure canbe observed, as well as the corresponding sharper rise in transverse regions comparedto the MC. Figure 4 shows the charged particle density in the transverse region as aMINIMUM BIAS AND SOFT QCD ATLAS RESULTS 5 [GeV]leadTp1 2 3 4 5 6 7 8 9 10MC/Data0.60.811.21.4MC/Data>φdη/dchN2<d0.10.20.30.40.50.60.70.8Transverse Region = 900 GeVs| < 2.5η> 0.5  GeV and |TpATLASData 2009PYTHIA ATLAS MC09HERWIG+JIMMY ATLAS MC09PYTHIA DWPYTHIA Perugia0PHOJET [GeV]leadTp2 4 6 8 10 12 14 16 18 20MC/Data0.60.811.21.4MC/Data>φdη/dchN2<d0.20.40.60.811.21.41.6Transverse Region = 7 TeVs| < 2.5η> 0.5  GeV and |TpATLASData 2010PYTHIA ATLAS MC09HERWIG+JIMMY ATLAS MC09PYTHIA DWPYTHIA Perugia0PHOJETFig. 4. – Charged particle densities in the transverse region as a function of the pT of the leadingcharged particle in p-p collisions at 900GeV (left) and 7TeV (right).function of the pT of the leading particle for 900GeV and 7TeV data. The density risesup to 4–6GeV due to the increasing probability to have one hard collision and reachesa plateau when the UE activity stops increasing with the transverse momentum of theleading particle. The density value at the plateau is a factor two larger with respect to themultiplicity measured in minimum bias events and this is due to the fact that the high-pTtrack selection required for the leading track implies more momentum exchange and alack of diffractive contribution in the plateau region. Moreover the UE activity is seento increase by a factor of approximately two between the 900GeV and 7TeV data. Thisis roughly consistent with the rate of increase predicted by MC models tuned to Teva-tron data. All the models taken into account show at least 10–15% lower activity in theplateau region with respect to data. The larger difference between data and MC is seenfor the PHOJET generator [12] while the PYTHIA DW tune is the closest model to datafor the transverse region. Since the JIMMY [13] model requires at least one hard scatter-ing, the strong deviation of HERWIG+JIMMY from the data for low-pleadT is expectedbecause it is not applicable in this region. An UE analysis has also been performed bythe ATLAS experiment using calorimetric information [4]. This incorporates the resultsobtained by the track based analysis and is also sensitive to the neutral component.φΔ0 0.5 1 1.5 2 2.5 3/50)π) / (  opp  T - N same T(NΣ)/  opp  T - N same T(N-0.0200.020.040.060.080.10.120.14|<1η>500 MeV, |Tp=7 TeVsData, Pythia Tune APythia Tune P0Pythia GAL PreliminaryATLASφΔ0 0.5 1 1.5 2 2.5 3/50)π) / (  opp  T - N same T(NΣ)/  opp  T - N same T(N-0.0200.020.040.060.080.10.120.14|<2η>500 MeV, |Tp=7 TeVsData, Pythia Tune APythia Tune P0Pythia GAL PreliminaryATLASφΔ0 0.5 1 1.5 2 2.5 3/50)π) / (  opp  T - N same T(NΣ)/  opp  T - N same T(N -0.0200.020.040.060.080.10.120.14|<2.5η>500 MeV, |Tp=7 TeVsData, Pythia Tune APythia Tune P0Pythia GAL PreliminaryATLASFig. 5. – The “same minus opposite” distribution for√s = 7TeV for |η| < 1.0 (left), |η| < 2.0(center) and |η| < 2.5 (right).6 R. DI NARDO5. – Angular correlations between charged particlesThe study of angular correlation in toward and away-side regions with respect tothe leading particle can be used for further understanding of soft QCD processes sincedifferent models provide significantly different predictions of the width and height of thetoward and away-side peaks of the Δφ (angles difference in the transverse plane betweenthe leading particle and all the other particles) distribution, making the characteristicsof these peaks (i.e. width and height) an interesting object of study. In particular thefollowing two distributions have been studied:– the Δφ crest shape, that is obtained from the Δφ distribution subtracting theminimum of the distribution (obtained with a second-order ploynomial fit);– the Δφ “same minus opposite” observable, obtained subtracting the oppositeside distribution (defined according to if pseudorapidity has the same sign or theopposite sign as the leading track) from the same side distribution and normalizingto unity.Figure 5 shows the “same minus opposite” distribution for√s = 7TeV for different η re-gions. The data (black dots) are compared to the predictions of Pythia tune: tune A [14],tune Perugia0 [15] and GAL [16]. All models describe the distributions poorly, with abetter agreement only if the central region is selected (|η| < 1). These variables can beused as further input for MC tuning.6. – ConclusionsCharged particle distributions, underlying event distributions and charged particlecorrelations in p-p collisions at 900GeV and 7TeV measured by the ATLAS detector atLHC have been studied. Since most of the pre-LHC models do not show a satisfactoryagreement with data, these measurements represent important inputs for MC tuningpurposes. In particular, the new AMBT1 tune represents the first improvement for thedescription of minimum bias results.REFERENCES[1] The ATLAS Collaboration, New J. Phys., 13 (2011) 053033, arXiv:1012.5104v2.[2] The ATLAS Collaboration, Phys. Lett. B, 688 (2010) 21, arXiv:1003.3124v2.[3] The ATLAS Collaboration, Phys. Rev. D, 83 (2011) 112001, arXiv:1012.0791v2.[4] The ATLAS Collaboration, EPJC, 71 (2011) 1636.[5] The ATLAS Collaboration, Angular correlations between charged particles from proton-proton collisions at√s = 900GeV and√s = 7TeV measured with ATLAS detector,ATLAS-CONF-2010-08.[6] The ATLAS Collaboration, JINST, 3 (2008) S08003.[7] The ATLAS Collaboration, Performance of the Minimum Bias Trigger in p-pCollisions at√s = 7TeV, ATL-CONF-2010-068.[8] The ATLAS Collaboration, Charged particle multiplicities in pp interactions at√s =0.9 and 7TeV in a diffractive limited phase-space measured with the ATLAS detector atthe LHC and new PYTHIA6 tune, ATLAS-CONF-2010-031.[9] The ATLAS Collaboration, ATLAS Monte Carlo tunes for MC09, ATL-PHYS-PUB-2010-002.[10] Albrow M. G. et al., Tevatron-for-LHC Report of the QCD Working Group, arXiv:hep-ph/0610012.MINIMUM BIAS AND SOFT QCD ATLAS RESULTS 7[11] The CDF Collaboration, Phys. Rev. D, 70 (2004) 072002.[12] Engel R., Z. Phys. C, 66 (1995) 203.[13] Butterworth J. M., Forshaw J. R. and Seymour M. H., Z. Phys. C, 72 (1996) 637.[14] Field R. D., The underlying event in hard scattering processes, arXiv:hep-ph/0201192.[15] Skands P. Z., The Perugia Tunes, arXiv:0905.3418.[16] Rathsman J., Phys. Lett. B, 452 (1999) 364.

Minimum bias and soft QCD ATLAS results

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Minimum bias and soft QCD ATLAS results

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