The CMS detector Two-particle correlation functions Results in m inimum bias collisions
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Transcript of The CMS detector Two-particle correlation functions Results in m inimum bias collisions
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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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 2Gábor Veres
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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 3Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 4Gábor Veres
Angular Correlation Functions
I. DefinitionCorrelation Functions:
II. Anatomy
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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 5Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 6Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 7Gábor Veres
Correlations in Min Bias pp
CMS pp Data
Pythia D6T
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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 8Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 9Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 10Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 11Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 12Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 13Gábor Veres
Event and Track Selection
Event-selection and analysis done with tracks pointing to primary vertex with O(100m) resolution
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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 14Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 15Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 16Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 17Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 18Gábor Veres
Correlations in PYTHIA8
No structure at large Same for Herwig++, madgraph, PYTHIA6
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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 19Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 20Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 21Gábor Veres
Like-Sign vs. Unlike-Sign Pairs
No dependence on relative charge sign
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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 22Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 23Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 24Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 25Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 26Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 27Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 28Gábor Veres
Detector: uniformity in
Ridge region shows no structure in vs 2
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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 29Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 30Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 31Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 32Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 33Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 34Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 35Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 36Gábor Veres
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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Zimányi 2010 Winter School on Heavy Ion Physics, 29 Nov 2010, RMKI/ELTE Budapest 37Gábor Veres
Reconstructed high multiplicity pp event