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First Physics from LHC: ATLAS Measurements at √s = 900 GeV

First Physics from LHC: ATLAS Measurements at √s = 900 GeV. Prafulla K. Behera The University of Iowa SLAC Experimental Seminar 15 th April 2010 . Outline. Motivation Theoretical models and predictions for LHC Detector status and data taking Observables measured at ATLAS

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First Physics from LHC: ATLAS Measurements at √s = 900 GeV

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  1. First Physics from LHC: ATLAS Measurements at √s = 900 GeV Prafulla K. Behera The University of Iowa SLAC Experimental Seminar 15th April 2010

  2. Outline First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera • Motivation • Theoretical models and predictions for LHC • Detector status and data taking • Observables measured at ATLAS • Analysis procedure • Systematic uncertainties • Results • Summary and Conclusions

  3. Definitions • Primary charged particles Nch per event • Number of events Nev • Pseudorapidity • Pseudorapidity density θ Beam line Detector Multiplicity distribution Nch η First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  4. Motivation Modeling of minimum bias pile-up is a necessary tool for high pT physics! Physics: improve our understanding of QCD effects, total cross-section, saturation, jet cross-sections, mass reconstructions,… Minimum Bias measurements should be done early on, at low luminosity to remove effect of overlapping proton-proton collisions! Inelastic Phys. Rev. D79 (2009) 112005 First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  5. Previous measurements at hadron colliders, theoretical models and predictions for LHC First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  6. Charged-Particle Multiplicity • First observable in collisions of hadrons • Important ingredient for the understanding of multi-particle production • LHC is in an energy realm where multiple parton interactions are in the bulk of the events • Constrain, improve or reject models 30 GeV  1.8 TeV P(Nch) In full phase space for NDS dNch/dh Review article arXiv:0912.0023 η First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  7. <Nch> Ös (GeV) Feynman Scaling • Phenomenological arguments about the exchange of quantum numbers • Feynman-x: x = 2pz/Ös • Feynman scaling function f(pT, x) independent of CM energy (for large energies) • Average Nch increases with lns • dNch/dh approx. constant (√s) PRL 23 1415 (1969) Violated in the large CM energy region First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  8. KNO Scaling Nucl. Phys. B40 317 (1972) • Koba, Nielsen, Olesen (1972) • Based on Feynman scaling • Scaling variable z = Nch/<Nch> • Express P(Nch) as P(z) * <Nch> • Multiplicity distributions measured at different energies fall onto universal curve <Nch> P(z) • Successful for NSD events • up to 60 GeV for full phase space (ISR) • up to 900 GeV in central region (UA1, UA5) • remains only valid in |h| < 1 for soft events at 1.8 TeV (CDF) Nch/<Nch> in |h| < 2.5 First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  9. NegativeBinomialDistributions • Bernoulli experiment • Probability for n failures and k success in any order, but the last trial is a success • Physical interpretation • Cascade production (clan model,Giovannini, Van Hove, Z. Phys. C30 391 (1986)) • Ancestor particles are produced independently (Poisson) • Existing particle can produce additional one with log probability p P(Nch) Fit with single NBD • Successful for NSD events • up to 540 GeV in full phase space (ISR, UA5) • central intervals up to 1.8 TeV (UA5, CDF) Nch Fit with a combination of two NBDs P(Nch) Nch First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  10. NBD Two-Component Approach UA5:√s =900 GeV • Combination of 2 NBDs representing soft and semi-hard part of the collision (with and without minijets)(Giovannini and Ugoccioni ,PRD59 094020 (1999)) • Two classes of events, not two production mechanisms in the same event • Other data-driven approach identifies several KNO components (Alexopoulos, Phys. Lett. B435 453 (1998)) P(Nch) Normalized Residuals:1/e)(p(Nch)-fit) Successful for all regions up to 1.8 TeV Physical interpretation unclear UA5:√s =900 GeV Nch First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  11. Predictions for LHC Energies • Extrapolations of trends at lower Ös • Just for average multiplicities • For the multiplicity distribution • Gluon saturation models (Armesto et al, Kharzeev et al) • Dual Parton Model / Quark-Gluon String Model • Monte Carlo generators • Pythia (pQCD + soft phenomenology) with all its tunes • Phojet (based on DPM/QGSM) • Epos (allows QGP formation in p+p) dNch/dh @ 7 TeV predictions range from 4.3 to 6.4 First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  12. Predictions for Ös = 14 TeV Multiplicity distributions differ by a factor 2 up to an order of magnitude full phase space |h| < 1 |h| < 1 First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  13. AToroidal LHC ApparatuS Detector Overview First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  14. The Large Hadron Collider • Proton-proton collider • 27 km circumference • At 4 interaction points, • detectors measure the outcome • of the collisions: • ATLAS, CMS • Alice, LHCb First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  15. AToroidalLHCApparatuS • Multi-purpose particle detector to cover large range of physics measurements • Mass ~ 7000 tons • ~100 million electronic channels (mostly in the pixel and ID) First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  16. Pixel Detector • Innermost detector of ATLAS • Provides precision tracking, impact parameter measurements • and secondary vertices for long lived particles) • Radiation-hard silicon sensors and front-end electronics • Three layers in the central region •  < 2.0 • hermetic in azimuthal direction with shingling-angle • overlap (20°) • 5 cm, 9 cm, 12 cm from beam axis • Two endcaps of three disks each • 2.0 <  < 2.5 • 1744 modules • 46080 channels per module • Divided among 16 front-end chips per module • 80 million channels 96% of Pixel operational First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  17. Pixel Detector performance • Cluster size agrees well with MC • Tracks from collision events span over • larger incident angle. • Significant improvement of track • based alignment. • Tracks from collision events allowed to • align regions of the detector that were • not accessible with cosmic ray data • Close to hit residuals predicted by MC • Higher momentum tracks required to • minimize the contribution of multiple • scattering to the residuals. First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  18. Silicon Strip Detector (SCT) 98% of the SCT operational • Hit assignment probability higher than 99.5 % in the SCT • Confirmed by hole rate on track studies. • Lorentz angle measurements • Confirmed result from • 2009 cosmic ray data First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  19. Transition Radiation Tracker (TRT) • The TRT provides particle identification capability • Transition radiation of particles traversing • materials with strongly differing dielectric • constants proportional to Lorentz-gamma factor. • High threshold hit probability increased: e/pi • separation • TRT residuals close to expected value • from MC 98% of the TRT operational First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  20. Triggering on Minimum Bias 14 TeV Simulation 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 ∞ First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  21. Minimum Bias Trigger Scintillators Minimum Bias Trigger Scintillators (MBTS) 2.12 < |η| < 2.83 2.83 < |η| < 3.8 First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  22. Minimum Bias Trigger Scintillators First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  23. Data sets at √s = 900 GeV [ 20 μb-1 ] [ 12 μb-1 ] First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  24. Stable Beam Collision Event First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  25. Observables Measured at ATLAS First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  26. Measurement Overview • Select inelastic events with Minimum Bias trigger. • Select primary tracks using impact parameter with respect to a primary vertex. • Require 1 track within |η| < 2.5 and pT > 500MeV. • Correct back to the hadron level and measure distributions within |η| < 2.5 and pT > 500MeV. Primary charged particles: Charged particles with a mean lifetime τ > 10-10s directly produced in pp interactions or from subsequent decays of particles with a shorter lifetime First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  27. Event Samples • 455593 events were selected using the L1_MBTS_1 trigger. • Requires at least one hit in either A or C side in the MBTS, where there are 16 counters on side A and side C. • Data quality selection requiring fully operational Inner Detector, trigger and solenoid B-field. First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  28. Offline Selection • Track Selection: • pT > 500MeV • |η| < 2.5 • Number of Pixel Hits ≥ 1 • Number of SCT Hits ≥ 6 • d0PV < 1.5 mm • z0PV sin(θPV) < 1.5 mm • Event Selection: • L1_MBTS_1 trigger • Primary vertex without beam-spot constraint and including three tracks. • Number of selected tracks ≥ 1 PV: Primary Vertex BS: Beam Spot First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  29. Data And MC comparison First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  30. Cuts comparison Data and MC agree well for each of the cuts First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  31. Beam Background Beam background before selection Beam background after selection The background contamination is very small (10-4) First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  32. Trigger Efficiency • Measured from data using Inner Detector HLT trigger (mbSpTrk) sample. • Efficiency of the L1_MBTS trigger for inelastic events with a track within ID acceptance and offline selection. mbSpTrk: Level2 Inner Detector trigger First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  33. Trigger Efficiency… • 19037 events passing the event selection were recorded using the mbSpTrk_BX0 and mbSpTrk_BX1 • Seeded from L1_BPTX0 and L1_BPTX1 respectively • BPTX – Electrostatic beam pickup, unbiased (located at ± 175m from interaction point. • mbSpTrk: • L2: SCT space points > 6 AND Pixel clusters > 6 • EF: One online silicon track pT > 200MeV • Data quality selection requiring fully operational Inner Detector and HLT (High Level Trigger). First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  34. L1_MBTS_1 Trigger Efficiency Efficiency is estimated from data The efficiency is ~100% Systematic is very small. Number of track selected w.r.t beam spot First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  35. Vertexing Efficiency Extracted from Data Efficiency is ~100% (Nch >2) Pt < 500 MeV is used for efficiency as a function of selected charged track w.r.t beam spot. Number of track selected w.r.t beam spot First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  36. Validation of Inner Detector Simulation ATLAS ATLAS ATLAS ATLAS First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  37. Tracking Efficiency Cone-matching in the η-ϕ plane ΔR = √(Δϕ)2 + (Δη)2 < 0.05 Efficiency drop in |η| > 1 is mainly due to more material in this region pT cut at pattern-recognition level Tracking efficiency is extracted from MC after detailed validation. First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  38. Comparison of Raw distributions Data and MC agree well before correction --------------- First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  39. Correction procedure • We correct our measured distributions to those for primary tracks • The correction for event level distributions (NCh) is given by: • The correction for track level distributions (PT, η) is given by: 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 First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  40. MC Correction Test Comparison is excellent Comparison is excellent • Treat MC as data and apply offline selection. • Correct back to the hadron level and compare with the input distribution. First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  41. Systematic Uncertainties ATLAS ATLAS • Dominant systematic uncertainty comes from tracking: SCT extension is a • good indicator of systematic uncertainty. • η dependency of systematic is taken from difference data/MC simulation • 5.5% (1.6< η < 2.3 ) and 8% ( |η| > 2.3 ). • Other major sources: Knowledge of material inside ID volume • and due to selection cuts (~3% each) First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  42. Charged particle distribution Average charged particle multiplicity in |η|=0: 1.333± 0.003 (stat.) ± 0.040 (syst.) 5-15% higher than the MC models predict Excellent agreement First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  43. Results Data has sensitivity to exclude different tunings. First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  44. Comparison of Results Good agreement between the experiments CMS data is consistently lower than the ATLAS: The offset is expected from the CMS measurement definition of NDS events where events with nch = 0 enter the normalization and SD component subtraction To simplify comparison (Same phase space): 1.241 ± 0.040 (NSD, ATLAS) (using DW tune) 1.202 ± 0.043 (NSD, CMS) First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  45. Summary and Conclusions • The first ATLAS physics paper is being published in Physics Letters B. (arXiv:1003.3124). • It is an excellent demonstration of understanding of our detector and the software. • The charged track density measurement is consistent with CMS measurement (NSD). • Our measurement (independent of trigger) has sensitivity to exclude some of the tunes. • We will have results for 7 TeV soon….. with new interesting physics measurements. Thank you for your attention First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

  46. Open Experimental Issues • Discrepancy of multiplicity distributions of UA5 and E735 in full phase space • Extrapolated from |h| < 5 (UA5) and|h| < 3.25 (E735) • Restricted phase space? • Data points not in electronic format • Go to the publication: plot quality? Lessons from this? Try to reduce extrapolations (and thus model dependence) Provide your numbers in a digital form (otherwise they are lost) First Physics from LHC: ATLAS Measurements at √s = 900 GeV - Prafulla K. Behera

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