Post on 01-Jan-2016
description
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DOE Review (Arlington TX, November 9, 2010)
Soft QCD/Minimum Bias Physics at ATLAS
Edward Sarkisyan-Grinbaum
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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 data• Use these data to better understand tracking, detector, simulation, and reconstruction
σtot = σel + (σND + σSD +σDD)
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 modeling), 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 (similar agreement at all beam energies) • 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
• Now have many new results with 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:at 900 GeV: 3.486 ± 0.008(stat) ± 0.077(syst)
• Measurements 5-20% higher than Monte Carlo predictions
at 7 TeV: 5.635 ± 0.002(stat) ± 0.049(syst)
• In our with A.Sakharov model of constituent quarks+Landau’s relativistic hydro the 2.36 data are reproduced and predictions
made for 7 TeV, agree with the data: arXiv:1004.4390 (in press)
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Further Studies • Single-particle distribution studies limited to global characteristics of particle production, difficult to distinguish among models with different different particle-production mechanisms
• Next step needed: to study multi-particle distributions, correlations and dynamical fluctuations providing insight into soft multi-hadron dynamics
• E S-G proposed to organize a group with different but related analyses on multi-particle distributions and correlations to concentrate on the subject and to share techniques and methodology
• Rich field of investigation, applied in many earlier experiments, aided in understanding and model development of underlying mechanisms • Requires more expertise, needs experienced people to be involved as ATLAS is short of soft QCD experts compared to ALICE and CMS
The Fluctuations and Correlations group established in March led by myself, with weekly meetings and a few analyses started. Now with many analyses ongoing, looking into readiness for publications next year
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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
4-7 papers expected next year and many conference notes
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Forward-Backward Correlations• 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 models
Analysis with Editorial Board
b-slope
‹nF›=a+b·nB
MC/data
|η|-bin
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Two-Particle Azimuthal Correlations•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.
7 TeV
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Multiplicity Spectra Scaling and Modeling• 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:• Reasonable agreement for ||<0.5 but deviates for larger || (2 parameters: ‹n›, k) • Possible 2NBD fit: soft vs. semi-hard component, 5 parameters
• 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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Two-particle Azimuthal Distributions• 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 (R) & coherency strength (λ) energy dependence, their particle mass and multiplicity dependences• First results from ALICE, CMS available
dQdN
dQdNQR
reference
signal
/
/)(
Work in progress,next year paper(s)
7 TeV
2020
“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 confs.
Fq
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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 collisions, different variables and dimensions
Work in progress, next year paper(s)
data 7 TeV
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High-Multiplicity Thermalization
• Study of energy “correlators” with R3(n)=K32/3(n)/ |K2(n)|
<< 1, higher-order correlators analyses as function of multiplicity n
Work in progress, next year paper
(a)“thermalized” states(b) hard and(c) soft production
• Study of very igh multplicity events to learn whether thermodynamic equilibrium reached as theory predicts
R3
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Outlook and Plans • UTA has had a major role in minimum bias trigger studies and validation of central detector• UTA has made essential contributions to minimum bias paper publication• Under leadership of UTA a new group has been formed and a rich program of analyses in correlations and fluctuations has been set up• We have presented results at topical workshops/conferences, and had convenorship roles in these workshops/conferences • Many conference notes in pipeline for winter 2010-2011 conferences, several papers under preparation for publication in 2011