the cms detector two-particle correlation functions results in m inimum bias collisions
DESCRIPTION
G ábor I. Veres CERN Geneva and ELTE Budapest CMS Collaboration. Based on: - JHEP 1009:091,2010 - My talk at ECT* Trento: QCD at the LHC, September 28, 2010 - CERN seminar talks by Gunther Roland and Guido Tonelli, Sept. 21, 2010. - PowerPoint PPT PresentationTRANSCRIPT
Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 1Gábor Veres
• The CMS detector
• Two-particle correlation functions
• Results in minimum bias collisions
• Results in high multiplicity collisions
• Cross-checks
Gábor I. VeresCERN Geneva and ELTE Budapest
CMS Collaboration
Based on:- JHEP 1009:091,2010- My talk at ECT* Trento: QCD at the LHC, September 28, 2010- CERN seminar talks by Gunther Roland and Guido Tonelli, Sept. 21, 2010
Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest
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The CMS Silicon Tracker
• Coverage up to ||<2.5; extremely high granularity, to keep low occupancy (~ a few%) also at LHC nominal luminosity.
• Largest Silicon Tracker ever built: Strips: 9.3M channels; Pixels: 66M channels. Operational fractions: strips 98.1%; pixel 98.3%
TOBTOB
TIDTIDTIBTIB
TECTEC
PDPD
PDPD
TIBTIB
TOBTOB
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Angular Correlation Functions
I. DefinitionCorrelation Functions:
II. Anatomy
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Correlation Function Definition
Background distribution:
BN (,) 1N 2
d 2N bkg
dd
R(,) (N 1)SN (,)BN (,)
1
N
pT-inclusive two-particle angular correlations in minimum bias collisions
1 2
1 2
SN (,) 1N (N 1)
d 2N signal
dd
Signal distribution:
Same event pairs Mixed event pairs
CMS pp 7TeV
Ratio Signal/Background
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Angular Correlation Functions
Short-range correlations ( < 2):Resonances, string fragmentation,“clusters”
Bose-Einstein correlations: ~ (0,0)
CMS 7TeV pp min bias
“Near-side” ~ 0) jet peak:Correlation of particles
within a single jet
“Away-side” ~ ) jet correlations:
Correlation of particles between back-to-back jets
Momentum conservation:~ -cos()
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Correlations in Min Bias pp
CMS pp Data
Pythia D6T
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Short-Range Correlations vs. s
1D “Projection” to axis
PYTHIA describes the energy dependence,
matches cluster width in data,but underestimates the cluster multiplicity Keff
Keff: Number of correlated particles: correlation width in
CMS
CMS
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High Multiplicity Events
268 reconstructed particles in the tracker in a single pp collision:the highest multiplicity event in ~70 billion inelastic events sampled (1/pb)
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Why study extreme multiplicities?
Our most recent correlation studies focus on the tail of the distribution, where several MC generators severely under-estimate the data(an exception: PYTHIA8).
Motivations: Trying to find (more) unexpected effects in this regime Learn more about (soft) QCD and particle production mechanisms with more differential measurements Highest multiplicities in pp begin to approach those in ion collisions; can we learn something about similarities or differences?
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High Multiplicity Trigger
Level-1 (hardware):Requires ET> 60 GeV in calorimeters
High-Level trigger (software):More than 70 (85) tracks with pT > 0.4 GeV/c, || < 2, within dz < 0.12 cm of a single vertex with z < 10 cm.~50% CPU usage of the HLT
Dedicated trigger was needed to record highest multiplicities
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High Multiplicity Event Statistics
Two different HLT thresholds: Nonline > 70 and Nonline > 85
HLT85 trigger range un-prescaledfor full 980nb-1
out of 5x1010 collisions
Multiplicity binning usespT > 0.4 GeV/c
|| < 2.4
1000 times morehigh multiplicity eventsrecorded with the triggercompared to Min. Bias
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Event and Track Selection
Event-selection and analysis done with tracks pointing to primary vertex with O(100m) resolution
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Results : data, inclusive pT
MinBias high multiplicity (N>110)
Jet peak/away-side correlations enhanced in high multiplicity eventsAbundant jet production in high multiplicity sample
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MinBias high multiplicity (N>110)
Results: data, inclusive pT
After cuting off the jet peak at (0,0) we can observe:Structure of away-side ridge (back-to-back jets)
Small change for large around ~ 0 ?
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Results: data, pT: 1-3 GeV/c
Pronounced new structure at large around ~ 0 !
MinBias high multiplicity (N>110)
CMS Collab., JHEP 1009:091,2010, arXiv:1009:4122
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Illustration of the effect
Particles surfacing in the same time zone, but far away in latitude, talk to each other…
…What mechanism is
the “telephone line”??!
p
p
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Correlations in PYTHIA8
No structure at large Same for Herwig++, madgraph, PYTHIA6
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Multiplicity- and pT -Dependence
“Ridge” maximal for highest multiplicity and 1 < pT < 3 GeV/c
Incr
easi
ng
mu
ltip
licit
yIncreasing pT
Project || > 2onto
!!! !!!
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Associated yield grows with increasing multiplicity
Associated yield: correlated multiplicity per particle
Zero Yield At Minimum (ZYAM) • Data- PYTHIA8
N>1102.0<||<4.81GeV/c<pT<2GeV/c
Minimum of R
2.0<||<4.8
Quantifying the associated yield
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Like-Sign vs. Unlike-Sign Pairs
No dependence on relative charge sign
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Systematic Uncertainties, Checks
• Statistical uncertainty negligibly small
• However, the signal is subtle and unexpected
• Estimate systematic uncertainties
• Is there a way to fake the signal qualitatively?
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Systematic Uncertainties
Analysis code
Reconstruction
Trigger
Detector
CMS Event
Collision
Physics
+ pile-up, beam backgrounds
+ detector noise, acceptance, efficiency
+ trigger efficiency, bias
+ efficiency, fakes
+ bugs?
Test the complete chain with data-driven checks!
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Analysis Code
Independent codeSame definition of R
Same input file (skim)
Ridge is seen with three independent analysis codes
Standard analysis
Control analysis I
Control analysis II
Independent codeDifferent definition of R
Different input file (skim)
N>1101<pT<3GeV/c||>2
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Reconstruction Code
“HighPurity” tracksPixel + Silicon Strip tracker
(Largely) independent codeIndependent detectors
Also: variation of tracking +vertexing parameters
Pixel-only tracks3 hits in pixel detector
N>1101<pT<3GeV/c
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Trigger bias
Ridge is seen using min bias trigger + offline selection
Min bias trigger
Min-bias trigger vs. high mult trigger HLT 70 vs. HLT 85 for N > 110
No trigger bias seen from comparison of
trigger paths
N>1101<pT<2 GeV/c||>2
1<pT<2GeV/c||>2
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Detector: -symmetry
Ridge is invariant under rotation
Ridge is not caused by rare events with large #
of pairs
Constrain one track to one -octant
• Signal• Background
Pair multiplicity distribution
for ||>2 and ||<1
N>1101<pT<2 GeV/c||>2
N>1101<pT<2 GeV/c||>2
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Detector: uniformity in
Ridge region shows no structure in vs 2
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Event Backgrounds
Ridge region shows no sensitivity to beam backgroundNote: Analysis is done on HighPurity tracks
Enrich the sample with beam-gas and beam-scraping events
Reject beam background by vetoon fraction of low quality tracks
Standard event selection
Increased beam scraping eventsIncreased beam halo
N>1101<pT<2GeV/c||>2
N>1101<pT<2GeV/c||>2No eff correction
CMS preliminary
CMS preliminary
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Pileup collision events
No background or noise effects seen in cross-collision correlations
Correlate tracks from high multiplicity vertex with tracksfrom different collision (vertex) in same bunch crossing
N>110; 1 GeV/c<pT<3 GeV/c
2.0<||<4.8
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31
Fra
ctio
n o
f pile
up
Pileup of collision events
Compare different run periods (fraction of pileup
varies by x4-5)
Change in pileup fraction by factor 2-4
has almost no effect on ridge signal
Compare different vertex regions
(fraction of pile-up ~ dN/dvtxz)
CMS preliminary
1<pT<2GeV/c||>2
CMS preliminary
CMS preliminary
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Event Pileup
Pileup effects are suppressed due to excellent resolution
Track counting done with dz, dxy of O(100m)
Single-event track dz distribution
Track longitudinal and transverse impact
parameter (pT > 0.4 GeV/c)
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Track-Photon Correlations
CMS preliminary
Note: photons reconstructed using “particle flow” event reconstruction technique
Use ECAL “photon” signalMostly single photons from 0’s
No efficiency, and pT, smearing corrections
N>1101<pT<3GeV/c||>2
N>1101<pT<3GeV/c
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Photon-Photon Correlations
Use ECAL “photon” signalMostly single photons from 0’s
No efficiency, and pT, smearing corrections
Qualitative confirmationIndependent detector, independent reconstruction
CMS preliminary
N>1101<pT<3GeV/c||>2
N>1101<pT<3GeV/c
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Systematic Uncertainties
Analysis code
Reconstruction
Trigger
Detector
CMS Event
Collision
Each step tested with data-based checks
No indication of effect that would fake ridge signal
Conservative estimates of uncertainties on ridge associated yield
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Summary
• Study of short-range and long-range angular
correlations in pp collisions with CMS at LHC
• Observation of long-range, near-side correlations in
high multiplicity events
– Signal grows with event multiplicity
– Effect is maximal in the 1 < pT < 3 GeV/c range
– Not observed in low multiplicity events
– Not observed in MC generators
• This is a subtle effect in a complex environment –
careful work is needed to establish physical origin
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Reconstructed high multiplicity pp event