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Soft QCD in ATLAS

VI Workshop on Particle Correlations and Femtoscopy Bogolyubov Institute for Theoretical Physics, Kiev, Ukraine September 14, 2010. Soft QCD in ATLAS. Edward Sarkisyan-Grinbaum

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Soft QCD in ATLAS

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  1. VI Workshop on Particle Correlations and Femtoscopy Bogolyubov Institute for Theoretical Physics, Kiev, Ukraine September 14, 2010 Soft QCD in ATLAS • Edward Sarkisyan-Grinbaum • (University Texas – Arlington/CERN) on behalf of the ATLAS Collaboration

  2. The Large Hadron Collider • Proton-proton collider • 27 km circumference • At 4 interaction points • detectors measure the • outcome of the collisions • - ALICE, ATLAS, • CMS, LHCb

  3. ATLAS Collaboration ~3000 scientists from 174 Institutions and 38 Countries

  4. ATLAS Detector AToroidalLHCApparatuS Minimum Bias Trigger Scintillator (MBTS) Inner Detector (ID)

  5. Minimum Bias Motivation (I) PYTHIA 900 GeV/7TeV /(~14mb) /(~9mb) /(~49mb) σtot = σel + (σND + σSD +σDD) The processes are not well predicted, Monte Carlo models need to be tuned to the data Use these data to better understand tracking, detector, simulation, reconstruction Not consider elastic: no hits in central detectors

  6. Minimum Bias Motivation (II) Modeling Minimum Bias necessary for high pT physics Improves our understanding of QCD effects, total cross section, saturation, jets, mass reconstruction Minimum Bias measurements should be done early on, at low luminosity to remove effect of overlapping collisions 1.96 TeV ●7 TeV? Phys. Rev. D79 (2009) 112005

  7. Measurements for Min Bias Analysis Distributions of hadrons (primaries) within a phase-space accessible to the ATLAS Inner Detector Data collected at 900 GeV, 2.36 TeV in December 2009, and at 7 TeV starting in March 2010

  8. Minimum Bias Trigger Scintillator • 32 independent wedge-shaped plastic scintillators • (16 per side) read out by PMTs, 2.09<|η|<3.84 • Designed to for triggering on min bias events, >99% efficiency • MBTS timing used to veto halo and beam gas events • Also being used as gap trigger for various diffractive subjects

  9. ATLAS Inner Detector • Includes different tracking subdetectors such as Pixel detectors, silicon SemiConducter Tracker (SCT) and Transition Radiation Tracker (TRT), pseudorapidity coverage |η|<2.5 • Main detector to measure charged tracks • Well modeled by Monte Carlo

  10. Selection Criteria Events pass the data quality criteria (“good events”: all ID sub-systems nominal cond., stable beam, defined beam spot) ATLAS-CONF-2010-046 • 1 side MBTS hit (MBTS_1 trigger) • Have a vertex, no pile-up, no mis-measured tracks • At least 2 tracks with pT>100 MeV, |η|<2.5 • At least 1 first Pixel layer hit & 2, 4, or 6 SCT hits • for pT>100, 200, 300 MeV respectively • Cuts on the transverse (d0) and longitudinal (z0) impact • parameters w.r.t. the primary vertex 900 GeV: 357,523 events selected, total luminosity 7 µb-1 7 TeV: 10,066,072 events selected, 190 µb-1 2.36 TeV: 8,151 events selected • Correct distributions for detector effects: • where possible use the data to reduce the MC dependencies • Monte Carlo derived corrections for tracking

  11. Stable beam collision events √s= 900 GeV

  12. Monte Carlo generators/tunes used • PYTHIA 6, actually 6.4.21 (P6): pT-ordered parton shower, MRST LO* • p.d.f., multiple parton-parton scattering, string fragmentation • PYTHIA ATLAS AMBT1: P6 tuned by ATLAS to the low-multiplicity • data • PYTHIA ATLAS MC09 (reference) P6 tune: parameters tuned to • underlying events and minimum bias data from Tevatron at 630 GeV • to 1. 8 TeV (ATLAS optimization), used to determine ATLAS detector • acceptances and efficiencies, to correct the data • More PYTHIA (P6) tunes used in the following: PYTHIA MC09c, • Perugi0, DW – see back up slides for details • PYTHIA 8: includes new features such as hard scattering in diffractive • systems, up-to-date LO p.d.f. set, possibility to use one p.d.f. set for • hard scattering and another one for the rest • PHOJET: two-component Dual Parton Model with soft hadronic • processes by Pomeron exchange and semi-hard processes by • perturbative parton scattering

  13. ID Performance vs. MC: 7 TeV ATLAS-CONF-2010-046 • Excellent agreement between data and MC: Pixel • and SCT hits per track and impact parameters • Similar for 900 GeV

  14. Efficiencies: 7 TeV ATLAS-CONF-2010-046 Tracking reconstruction efficiency Trigger efficiency Vertex reconstruction efficiency Similar efficiencies for 900 GeV Summary of systematic uncertainties – see back-up slides

  15. Correction Procedure • The correction on event-level distribution (nch): • Track-by-track weights: fsec(pT): fraction of secondaries, fokr(pT,η): fraction of track for which the corresponding primary particles are outside the kinematic range, originate from resolution effects • A correction factor due to tracking inefficiency • (≥ 2 particles but ≤ 2 tracks): • Iterative Bayesian unfolding applied to both nch and pT • 2D matrix approach used to correct pT-nch

  16. Comparison of pT for different experiments: 900 GeV • First ATLAS publication: • Phys. Lett. B 688 (2010) 21 pT>500 MeV, nch≥1 • Measurements at high • pT threshold • Good agreement for • the same selection • criteria • Further on: new results at low-pT threshold • and higher energies

  17. Results on η spectrum: 900 GeV vs 7 TeV ATLAS-CONF-2010-046 7 TeV 900 GeV • Mid-rapidity density (η≈0), pT>100 MeV: • 3.486 ± 0.008(stat) ± 0.077(syst) at 900 GeV, 5.635 ± 0.002(stat) ± 0.049(syst) at 7 TeV • Measurements 5-20% higher than Monte Carlo predictions

  18. Results on nch : 900 GeV vs 7 TeV ATLAS-CONF-2010-046 900 GeV 7 TeV low nch full nch low nch full nch • At low nch data differs from MC and the models differ • between each other due to diffraction, see later discussion • At nch>40 data exceed PYTHIA tunes by 50% • PHOJET fails to describe data above nch~20

  19. Results on pT spectrum: 900 GeV vs 7 TeV ATLAS-CONF-2010-046 900 GeV • Monte Carlo predictions • agree with the data at • intermediate pT from • 0.5 to 3 GeV • All Monte Carlo differ • from the data by 10-50% 7 TeV

  20. ‹pT› vs. nch: 900 GeV vs. 7 TeV ATLAS-CONF-2010-046 900 GeV 7 TeV Change in MC description with energy increase See comparison of distributions in back-up slides

  21. ‹pT› vs nch: Monte Carlo vs Data ATLAS-CONF-2010-046 7 TeV • PYTHIA6 has no hard diffraction, no diffractive events with nch>27 • PYTHIA8 and PHOJET seem to be similar at large nch but differ for small nch

  22. Adding 2.36 TeV data: η spectrum ATLAS-CONF-2010-047 • Midrapidity density (η≈0), pT>100 MeV: • 1.739 ± 0.019(stat) ± 0.058(syst) • AMBT1 & MC09 slightly underestimate the data,other Monte Carlo • predictions lower by 10-20% than the measurements

  23. Results on mid-rapidity densities ATLAS-CONF-2010-047 • ATLAS AMBT1 and MC09(c) close to the pT>500 MeV data • No model describes the low-pT measurements: more tunes • needed • Data at √s = 900 GeV, pT > 500 MeV from: ATLAS Collab., • Phys. Lett. B688 (2010) 21

  24. Diffractive Enhanced Minimum Bias rapidity gap σtot = σel + (σND + σSD +σDD) Little information about the diffractive component of inelastic cross section, while its non trivial part Large uncertainty in the diffractive model makes difficult to tune Monte Carlo models , restricts phase space areas of studies Goals: look at kinematics, constrain the ratio Diff/ND

  25. Measurement Strategy ATLAS-CONF-2010-048 Select events with activity on only 1 side of MBTS: single-diffractive events Measurements of the minimum bias distributions, distribution in Δη=|ηMBTS(no hit)-η(track)| Find ratio of “single-sided” events with respect to inclusive MBTS triggered sample Rss No corrections for detector effects

  26. Selection Criteria and Systematics Events pass the data quality criteria (“good events”: all ID sub-systems nominal cond., stable beam, defined beam spot) • Only one-side MBTS hit • At least 1 track: pT>500 MeV, |η|<2.5 • At least 1 Pixel hit & 6 SCT hits • Cuts on the transverse (|d0|<1.5 mm) and large longitudinal • (|z0|<100mm) impact parameters w.r.t. the BS • No vertex requirement to avoid any bias at low track • multiplicity ATLAS-CONF-2010-048 7 TeV: ~7.69M events selected (2 first runs), 23 µb-1 Systematic Uncertainties: • Beam bkg (beam halo and beam gas interaction) • MBTS detector response, noise, material between • MBTS and interaction point • The gap on positive vs. negative side • Material in the ID

  27. Results on Rss ATLAS-CONF-2010-048 • Ratio of single-sided events to double-sided events in data: • Rss = 4.52 ± 0.02(stat) ± 0.61(syst) [%](uncorrected) • Using given acceptances, Monte Carlo predictions • vs data(all generators prefer ~30% diff events)

  28. Results on η spectrum ATLAS-CONF-2010-048 • Data flat, well modelled by PHOJET & PYTHIA8, underestimated by • PYTHIA6 • The MC sub-process separation clarifies the origin of the features

  29. Results on multiplicity spectrum ATLAS-CONF-2010-048 • Data much steeper than the inclusive sample, well modelled by • PHOJET & PYTHIA8, underestimated by PYTHIA6 • The MC breaking down by sub-processes indicates reasons of • observed differences

  30. Results on pT distribution ATLAS-CONF-2010-048 • PHOJET well describes the data, PYTHIA8 predicts softer spectrum, • PYTHIA6 is much softer than the data • PYTHIA6 suffers of lack of hard component to diffraction

  31. Results on Δη distribution ATLAS-CONF-2010-048 • As Δη increases the models start deviate from the measurements • PHOJET favours SD, while PYTHIA models show quite similar • contributions from SD and DD, need more ND ingredient

  32. Conclusions and Outlook • First ATLAS 900 GeV paper published: • Physics Letters B 688 (2010) 21-42 • - many supporting (internal) notes • Test of the tracking and detector successful • The Minimum Bias data at 2.36 and 7 TeV • results reveals new features, crucial in • understanding models • Soft diffraction analysis at 7 TeV provides new • information on diffraction processes • More soft QCD ATLAS studies available • - see talk by Camille Bélanger-Champagne • Many ATLAS analyses in pipeline, stay tuned!

  33. Back-up slides

  34. ATLAS Inner Detector Plan View • Includes different tracking subdetectors such as Pixel detectors, silicon SemiConducter Tracker and Transition Radiation Tracker, pseudorapidity coverage |η|<2.5

  35. Monte Carlo generators/tunes used • PYTHIA 6, actually 6.4.21 (P6): pT-ordered parton shower, MRST LO* p.d.f., • multiple parton-parton scattering, string fragmentation • PYTHIA ATLAS AMBT1: P6 tuned by ATLAS to the low-multiplicity data • PYTHIA ATLAS MC09 (reference) P6 tune: parameters tuned to underlying • events and minimum bias data from Tevatron at 630 GeV to 1. 8 TeV • (ATLAS optimization), used to dtermine ATLAS detector acceptances and • efficiencies, to correct the data • PYTHIA ATLAS MC09c tune: MC09 optimizing0 the strength of the colour • reconnection to describe pT dependence on nch in the CDF data at 1.96 TeV • PYTHIA Perugia0 P6 tune: soft QCD part is tuned using only minimum bias • data from Tevatron and CERN ppbar data • PYTHIA DW P6 tune: uses the virtuality-ordered showers and used to • describe the CDF II underlying events and Drell-Yan process data • PYTHIA 8: includes new features such as hard scattering in diffractive • systems, up-to-date LO p.d.f. set, possibility to use one p.d.f. set for hard • scattering and another one for the rest, more underlying-event processes • (J/ψ, DY,…) • PHOJET: two-component Dual Parton Model with soft hadronic processes by • Pomeron exchange and semi-hard processes by perturbative parton • scattering

  36. Triggering on Minimum Bias 14 TeV Simulation Dedicated minimum bias trigger Inner Detector Space Points Minimum Bias Trigger Scintillators Cerenkov Detector (LUCID) Select events with minimal bias using Level 1 and Level 2 trigger items Zero degree calorimeter (ZDC) 8.3 → ∞

  37. BeamBackground ATLAS-CONF-2010-046 Beam background before selection Beam background after selection The background contamination is very small

  38. Summary on Systematic Uncertainty ATLAS-CONF-2010-046

  39. Comparison of 900 GeV and 7 TeV ATLAS-CONF-2010-046

  40. Tracking efficiencies and summary on systematic unceratities: 2.6 TeV ATLAS-CONF-2010-047

  41. Comparison of 900 GeV, 2.6 and 7 TeV ATLAS-CONF-2010-047

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