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DOE Review (Arlington TX, November 9, 2010)

Soft QCD/Minimum Bias Physics at ATLAS

Edward Sarkisyan-Grinbaum(University of Texas, Arlington)

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Minimum Bias Studies

Soft (low pT) hadroproduction processes are not well predicted, Monte Carlo models need to be tuned to the dataUse these data to better understand tracking, detector, simulation, and reconstruction

σtot = σel + (σND + σSD +σDD)

PYTHIA900 GeV/7TeV

/(~49mb) /(~14mb) /(~9mb)

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Motivation

• Improves our understanding of different QCD effects, total cross section, jet production, mass reconstruction

• Multiplicity and kinematic variables are the first measured observables and provide fundamental information on the interaction dynamics

• Collision energy dependence is of great interest, asymptotic regime of QCD partonic (quark-gluon) picture

• Extremely important for tuning Monte Carlo models, for heavy-ion physics, high luminosity detector upgrades (many minimum bias events pile-up at high luminosity, so very sensitive to minimum bias modelling), new physics discoveries

● 7 TeV?

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• 32 independent wedge-shaped plastic scintillators

(16 per side) read out by PMTs, 2.09<|η|<3.84*

Minimum Bias Trigger Scintillator

• Designed 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

*Pseudorapidity is defined as η = -½ ln (tan (θ/2)), θ is the polar angle with respect to the beam

UTA took a major role in min bias trigger validation and MBTS commissioning due to the importance of this detector for commissioning ATLAS central detector and to help publish early physics

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•Includes different tracking subdetectors such as Pixel detectors, silicon SemiConducter Tracker (SCT) and Transition Radiation Tracker (TRT), |η|<2.5

ATLAS Inner Detector

• Main detector to measure charged tracks • Well modeled by Monte Carlo

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Stable beam

collision events

√s= 900 GeV

First ATLAS events in December 2009 used MBTS trigger

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First ATLAS Results• First ATLAS 900 GeV paper published: Physics Letters B 688 (2010) 21-42 Big efforts, 40+ people, meetings all times of day+night, lots of supporting notes, provides a reference for all following analyses

Followed by 7 TeV results (based on first 7 TeV collisions in March 2010): ATLAS-CONF-2010-024 (April 2010), ATLAS-CONF-2010-046 (July 2010); publication soon!

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Measurements for Minimum Bias

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

Pseudorapidity η = -½ ln (tan (θ/2))

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ID Performance vs. MC: 900 GeV

Good agreement between data and MC observed Validated tracking algorithms and detector

Phys. Lett. B 688 (2010) 21

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pT vs. Nch: 900 GeV vs. 7 TeV

MC does not describe the physics too well, more deviation with energy increase, leads to new MC tunes

Phys. Lett. B 688 (2010) 21 ATLAS-CONF-2010-024

Low nch is highly influenced by description of diffractive component

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Comparison of pT for different experiments: 900 GeV

• Good agreement for the same selection criteria

pT>500 MeV, nch≥1• First ATLAS publication: Phys. Lett. B 688 (2010) 21

• Today: many new results at low-pT threshold and higher energies

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Results on mid-rapidity densities

• ATLAS AMBT1 and MC09(c) close to the pT>500 MeV data• No model describes the low-pT measurements: more tunes needed

ATLAS-CONF-2010-046

• Mid-rapidity density (η≈0), pT>100 MeV: 3.486 ± 0.008(stat) ± 0.077(syst) at 900 GeV,

• Measurements 5-20% higher than Monte Carlo predictions

5.635 ± 0.002(stat) ± 0.049(syst) at 7 TeV

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Beyond Minimum Bias Analysis More detailed studies of multi-particle production (led by E S-G)

Many analyses undergo under ATLAS soft QCD Multiparticle Correlations and Fluctuations subgroup umbrella

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“Correlations” Papers• Study of forward-backward rapidity correlations or “b- correlations” study, slope b shows the effect strength

• Rapidity range dependence and average cluster energy dependence – test of modelsAnalysis with Editorial Board

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“Correlations” paper•Two-particle η-Φ correlations: cluster structure

• “Cluster” fit extracts size and range of cluster emission Analysis in progress, to be ready for winter confs.

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“Correlations” paper - IV• Multiplicity spectra fits and KNO scaling: energy independence of the multiplicity spectrum and its moments

Negative Binomial (clan production model) fit to the multiplicity distribution in different rapidity intervals (UA5 900 GeV):• fulfills in |y|<0.5 but deviates for larger intervals (2 parameters: ‹n›, k) • possible 2NBD fit: soft vs. semi-hard component, 5 parameters (back-up slides)• ALICE and CMS show KNO scaling and NBD to hold for small central η intervals

Analysis with EditorialBoard, for winter confs.

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“Correlations” paper• Correlations of soft particles in ΔΦ around the leading jet • Allows accurate measurements of QCD (partonic) angular correlations, sensitive to Monte Carlo

ATLAS-2010-082

Further analysis on two-particleazimuthal correlations is ongoing

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Bose-Einstein Correlations• Bose-Einstein (two- & three-particle) correlations: enhancement in identical particle (boson) correlation function at near momenta• Provides estimate on particle emission source size (radius), coherency vs. chaoticity in the hadroproduction process

• Main problems: reference sample of non-correlated particle pairs, suitable fit function (usually Gaussian)• Main interest: radius energy dependence, its particle mass dependence, multiplicity dependence• First results from ALICE, CMS available

dQdNdQdN

QRreference

signal

//

)(

Work in progress,next year paper(s)

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“Ridge” effect• Long-range (|Δη|>2) near-side (ΔΦ=0) angular two-particle correlations – unexpected in pp collisions, thought purely nuclear matter effect• Observed by CMS using high-multiplicity events at 7 TeV (09/2010)

Work in progress, expected for winter/spring conferences

2020

Dynamical Multiparticle Correlations• Search for dynamical (non-Poissonian) fluctuations in small phase space intervals (factorial multiplicity moments, intermittency, QCD fractality/self-similarity, phase transition)• Study of genuine multi-particle correlations (factorial cumulant moment method)

UA1 ppbar 630 GeVNon-statisticalfluctuations riseNA22, πp 22GeV: low-pT effect

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q=4

• Studied in all types of collisionsWork in progress, next year paper(s)

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High-Multiplicity Thermalization• Study of very high multiplicity (VHM) events to learn whether thermodynamic equilibrium is reached as predicted • Study of energy “correlators” with R3(n)=K3

2/3(n)/|K2(n)| << 1, higher-order correlators analyses as function of multiplicity n

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q=4

(a)“thermalized” states(b) hard and(c) soft production

Work in progress, next year paper(s)

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Outlook and Plans• Many ongoing interesting analyses• Many analyses expected to be ready for winter 2010-2011, spring 2011 conferences • Next year many papers expected

2323

Back-up slides

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Efficiencies: 7 TeV

Vertex reconstruction efficiencyTrigger efficiency

Tracking reconstruction efficiency

Detector and tracking well understood

ATLAS-CONF-2010-046

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‹pT› vs. nch: 900 GeV vs. 7 TeV

New ATLAS tune better describes the data. More studiesongoing.

ATLAS-CONF-2010-046900 GeV

7 TeV