Early Minimum Bias physics at Atlas

Prafulla K. Behera The University of Iowa

SLAC Experimental Seminar15th April 2010 1First Physics from LHC: ATLAS Measurements at s = 900 GeV 2MotivationTheoretical models and predictions for LHCDetector status and data takingObservables measured at ATLASAnalysis procedureSystematic uncertainties Results Summary and Conclusions15th Apr. 2010First Physics from LHC: ATLAS Measurements at s = 900 GeV - Prafulla K. BeheraOutline2 Definitions3Primary charged particles Nch per event Number of events NevPseudorapidity

Pseudorapidity density

DetectorBeamline

Multiplicity distribution15th Apr. 2010First Physics from LHC: ATLAS Measurements at s = 900 GeV - Prafulla K. BeheraNch Motivation4Modeling 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!

First Physics from LHC: ATLAS Measurements at s = 900 GeV - Prafulla K. Behera15th Apr. 2010

InelasticPhys. Rev. D79 (2009) 1120055 Previous measurements at hadron colliders, theoretical models and predictions for LHC

15th Apr. 2010First Physics from LHC: ATLAS Measurements at s = 900 GeV - Prafulla K. Behera Charged-Particle Multiplicity6

dNch/dhP(Nch)30 GeV 1.8 TeVFirst observable in collisions of hadronsImportant ingredient for the understanding of multi-particle productionLHC is in an energy realm where multiple parton interactions are in the bulk of the eventsConstrain, improve or reject modelsReview article arXiv:0912.002315th Apr. 2010First Physics from LHC: ATLAS Measurements at s = 900 GeV - Prafulla K. BeheraIn full phase space for NDSUnderlying production process can be described by uncorrelated emission, the multiplicity distribution is expected to be of Poisson form. Any deviation from this indicates correlations between the produced particles6 Feynman Scaling7

s (GeV)

Phenomenological arguments about the exchange of quantum numbersFeynman-x: x = 2pz/sFeynman scaling function f(pT, x) independent of CM energy (for large energies)Average Nch increases with ln sdNch/dh approx. constant (s)Violated in the large CM energy region PRL 23 1415 (1969)

First Physics from LHC: ATLAS Measurements at s = 900 GeV - Prafulla K. Behera15th Apr. 2010dNcn/d|=0 is approximately constant if Feynman scaling holds7 KNO Scaling8Koba, Nielsen, Olesen (1972)Based on Feynman scalingScaling variable z = Nch/Express P(Nch) as P(z) * Multiplicity distributions measured at different energies fall onto universal curveNucl. Phys. B40 317 (1972)

Successful for NSD eventsup 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) P(z)

Nch/ in |h| < 2.5First Physics from LHC: ATLAS Measurements at s = 900 GeV - Prafulla K. Behera15th Apr. 2010

KNO is violated for higher energy collision for full phase space but working for phase space (eta 500MeV.Correct back to the hadron level and measure distributions within || < 2.5 and pT > 500MeV.

First Physics from LHC: ATLAS Measurements at s = 900 GeV - Prafulla K. Behera15th Apr. 2010Primary charged particles: Charged particles with a mean lifetime > 10-10s directly produced in pp interactions or from subsequent decays of particles with a shorter lifetime

27455593 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. Behera15th Apr. 2010 Event Samples28Track Selection:pT > 500MeV|| < 2.5Number of Pixel Hits 1Number of SCT Hits 6d0PV < 1.5 mmz0PV sin(PV) < 1.5 mmEvent Selection:L1_MBTS_1 triggerPrimary vertex without beam-spot constraint and including three tracks.Number of selected tracks 1

First Physics from LHC: ATLAS Measurements at s = 900 GeV - Prafulla K. Behera15th Apr. 2010PV: Primary VertexBS: Beam Spot Offline Selection Data And MC comparison29First Physics from LHC: ATLAS Measurements at s = 900 GeV - Prafulla K. Behera15th Apr. 2010

Cuts comparison30

Data and MC agree well for each of the cutsFirst Physics from LHC: ATLAS Measurements at s = 900 GeV - Prafulla K. Behera15th Apr. 2010 Beam Background31Beam background before selectionBeam background after selectionThe background contamination is very small (10-4)First Physics from LHC: ATLAS Measurements at s = 900 GeV - Prafulla K. Behera15th Apr. 2010

Trigger Efficiency 32Measured 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.

First Physics from LHC: ATLAS Measurements at s = 900 GeV - Prafulla K. Behera15th Apr. 2010mbSpTrk: Level2 Inner Detector trigger Trigger Efficiency 3319037 events passing the event selection were recorded using the mbSpTrk_BX0 and mbSpTrk_BX1Seeded from L1_BPTX0 and L1_BPTX1 respectivelyBPTX Electrostatic beam pickup, unbiased (located at 175m from interaction point.mbSpTrk:L2: SCT space points > 6 AND Pixel clusters > 6EF: 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. Behera15th Apr. 2010 L1_MBTS_1 Trigger Efficiency34Efficiency is estimated from dataThe efficiency is ~100% Systematic is very small.First Physics from LHC: ATLAS Measurements at s = 900 GeV - Prafulla K. Behera15th Apr. 2010

Number of track selected w.r.t beam spot Vertexing Efficiency 35Efficiency is ~100% (Nch >2)Pt < 500 MeV is used for efficiency as a function of selected charged track w.r.tbeam spot.Extracted from DataFirst Physics from LHC: ATLAS Measurements at s = 900 GeV - Prafulla K. Behera15th Apr. 2010

Number of track selected w.r.t beam spotValidation of Inner Detector Simulation36First Physics from LHC: ATLAS Measurements at s = 900 GeV - Prafulla K. Behera15th Apr. 2010

ATLASATLASATLASATLAS37

Tracking efficiency is extracted from MC after detailed validation. First Physics from LHC: ATLAS Measurements at s = 900 GeV - Prafulla K. Behera15th Apr. 2010

Tracking EfficiencyCone-matching in the - planeR = ()2 + ()2 < 0.05Efficiency drop in || > 1 is mainly due to more material in this regionpT cut at pattern-recognition levelComparison of Raw distributions38

---------------Data and MC agree well before correctionFirst Physics from LHC: ATLAS Measurements at s = 900 GeV - Prafulla K. Behera15th Apr. 2010

Correction procedure39We 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, ) is given by:First Physics from LHC: ATLAS Measurements at s = 900 GeV - Prafulla K. Behera15th Apr. 2010

fsec(pT): Fraction of secondaries, fokr(pT,): Fraction of track for which the corresponding primaryparticles are outside the kinematic range, originate from resolution effects MC Correction Test40Treat MC as data and apply offline selection.Correct back to the hadron level and compare with the input distribution.Comparison is excellentComparison is excellentFirst Physics from LHC: ATLAS Measurements at s = 900 GeV - Prafulla K. Behera15th Apr. 2010

Systematic Uncertainties41Dominant systematic uncertainty comes from tracking: SCT extension is a good indicator of systematic uncertainty. dependency of systematic is taken from difference data/MC simulation5.5% (1.6< < 2.3 ) and 8% ( || > 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. Behera15th Apr. 2010

ATLASATLAS Charged particle distribution42Average charged particle multiplicity in ||=0: 1.333 0.003 (stat.) 0.040 (syst.)5-15% higher than the MC models predictExcellent agreementFirst Physics from LHC: ATLAS Measurements at s = 900 GeV - Prafulla K. Behera15th Apr. 2010

43Data has sensitivity to exclude different tunings.First Physics from LHC: ATLAS Measurements at s = 900 GeV - Prafulla K. Behera15th Apr. 2010

Results

Comparison of Results

44

Good agreement between the experimentsFirst Physics from LHC: ATLAS Measurements at s = 900 GeV - Prafulla K. Behera15th Apr. 2010

To simplify comparison (Same phase space): 1.241 0.040 (NSD, ATLAS) (using DW tune) 1.202 0.043 (NSD, CMS)

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 Summary and Conclusions45The 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.First Physics from LHC: ATLAS Measurements at s = 900 GeV - Prafulla K. Behera15th Apr. 2010Thank you for your attention Open Experimental Issues46Discrepancy of multiplicity distributions of UA5 and E735 in full phase spaceExtrapolated from |h| < 5 (UA5) and|h| < 3.25 (E735) Restricted phase space?Data points not in electronic formatGo 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. Behera15th Apr. 2010