Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006 1
J/ψ measurements in Au+Au and p+p Collisions at 200 GeV
• Motivation• J/ψ in Au+Au • J/ψ in p+p• Conclusions
Outline
Johan Gonzalezfor the STAR Collaboration
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Motivation• Charm quarks are primarily produced in the initial
stages of the collision via hard scatterings Can be used to probe the initial conditions of the hot, dense matter produced
• Screening and scattering mechanisms predicted[1] to destroy the J/ψ in the Quark-Gluon Plasma
– This leads to a suppression of J/ψ production, which would imply deconfinement
• On the other hand, regeneration models[2,3,4] predict an enhanced production of hidden charm states for sufficiently high charm densities
– This would imply thermalization of charm quarks, and by extension, the light quarks that comprise the QGP
• Scrutiny of the centrality dependence and transverse momentum systematics of charmonium production can help distinguish between the two scenarios
Color Screening
cc
J/Ψ
regeneration[1] T. Matsui and H. Satz, Phys. Lett. B 178 (1986) 416.[2] Grandchamp, Rapp, Brown hep-ph/0403204
[3] Thews, Schroedter, Rafelski, Phys. Rev. C 63, 054905[4] P.B.M., Andronic, hep-ph/0402291
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Motivation
• Suppression mechanisms§, such as Debye screening or gluon scatterings, destroy the J/ψ. Magnitude of suppression depends on collision energy and centrality
• Regeneration mechanisms, such as Statistical Hadronization, are expected to contribute in conditions of high parton densities
• The two scenarios are not mutually exclusive – regeneration mechanisms may be at play even in the case where only a suppression is observed
An illustration of the competition of regeneration and suppression mechanisms
suppression
regeneration
L. Grandchamp and R. Rapp, Phys. Lett. B523 60 (2001)
§ These exclude “normal” suppression mechanisms, such as collisions with co-movers
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The STAR Detector
• For the Au+Au analysis in run 4, we only used the TPC
• For the p+p analysis, we used the TPC in conjunction with the BEMC.
Schematic side view of the STAR detector
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J/ψe+e- in Au+Au @ 200GeV
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Data & Analysis Cuts
• Dataset– Run IV Au+Au collisions– Trigger selection: Minbias
• Analysis Cuts– |vertexZ|<30cm– |η| < 1.0– p > 500 MeV/c– Selected electron band,
skipped hadron bands– Scanned 20M minbias events12M 0-80%
events after cuts
p (GeV/c)
dE
/dx
selected tracks
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Dielectron Invariant Mass Spectra
• The dielectron invariant mass spectra are generated using the event mixing technique
• Subtracted spectrum shows a peak of 3.5 sigma significance in the vicinity of the J/ψ mass
Subtracted spectrum
0-80% 12M events
STAR Preliminary
Zoom
Same event
Mixed event
STAR Preliminary
J/ψe+e- (BR = 5.93%)
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Dielectron Invariant Mass Spectra
• The invariant mass spectra in the 0-20, 20-40, 40-80, and 0-80% centrality classes are shown
• Number of J/ψ in each centrality class is determined by bin counting
• Gaussian fits (widths are held fixed to the value seen in minbias events) are used to estimate systematics
• Signal in the 0-20% bin is rather weak, so only an upper limit is quoted
STAR Preliminary
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STAR Preliminary
0-80%
Centrality Dependence of Scaled J/ψ yields
• The Nbin-scaled yields are plotted vs number of participants
• Bars indicate statistical uncertainties and the bands indicate the systematic uncertainties
• An upper limit is quoted for the most central bin
Binary Scaling (black line and grey band) Determined from PHENIX data (nucl-ex/0507032)
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Centrality Dependence of Scaled J/ψ yields
Statistical Hadronization Synopsis:Complete screening of primordial J/ψ’sJ/ψ’s regenerated at chemical freezout from thermalized c-cbars
[1] A. Andronic et al., Phys.Lett. B571 (2003) 36-44
[3] STAR, Phys. Rev. Lett. 94 (2005) 062301
pQCD x-section from:Cacciari, Nason, Vogt, PRL 95 (2005) 122001
[2] PHENIX, Phys.Rev.Lett. 96 (2006) 032001
• Statistical Hadronization[1] calculations are shown for various values of the differential c-cbar cross section
• The model appears to overpredict the scaled J/ψ yields for most values of the c-cbar cross section
• However, it should be noted that the uncertainties in the measured[2,3] c-cbar cross sections are rather large at this time.
STAR Preliminary
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J/ψe+e- in p+p @ 200GeV
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Data & Analysis Cuts
• Dataset– Run V p+p collisions– Trigger selection: J/ψ
• Level-0 portion require 2 BEMC hits above 1.2 GeV with Δφ> 60o
• Level-2 portion acts as a photon veto and also makes an online invariant mass cut of 2<m<15 GeV (m2~2*E1*E2*(1-cos(θ)))
– Scanned 1.6M events
• Analysis Cuts– |vertexZ|<100cm– TPC tracks were required to
match with BEMC towers– 0.15 < p/Etower < 2.0– 3.2 < dE/dx < 5.0 keV/cm
A typical triggered event
Track selection
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Dielectron Invariant Mass Spectrum
• The (raw) background subtracted dielectron invariant mass spectrum is shown
• A signal of 5 sigma significance is clearly visible
• The trigger has been successfully commissioned and is currently running as we speak
STAR Preliminary
J/ψe+e- in p+p @ 200 GeV
More statistics are expected from Run 6
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Summary & Conclusions• We have investigated the production of J/ψe+e- in
Au+Au and p+p collisions at 200 GeV
• Taking the STAR measured c-cbar cross section, our Au+Au results rule out a model of J/ψ production via Statistical Hadronization
• A J/ψ trigger in p+p collisions was successfully tested and is now being implemented in this year’s run
• With the future inclusion of the Barrel Time of Flight detector, electron identification—and by extension J/ψ reconstruction—will be substantially augmented in STAR
• In future endeavors, we will carry out a detailed analysis of the centrality and transverse momentum dependence of J/ψ in order to fully disentangle regeneration and suppression scenarios
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Extra slides
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Efficiency Corrections (1/3)
• Embedded simulated J/ψ’s into “Run 4-like” events:– Full field– |vertexZ|<30cm– J/ψe+e- only
– Flat in pT and rapidity
• Quantities having to do with dE/dx are not well simulated must use data itself to correct spectra:– Isolated electron band using TOF
data in order to determine number of pairs lost due to nSigma cuts
– Used the correlation* between nHits and nHitsDedx to assess number of pairs lost due to nHitsDedx cuts
• Folded data-determined corrections into embedding efficiency analysis
TPC+TOF
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Efficiency Corrections (2/3)
• Efficiency corrections are shown as a function of transverse momentum
• Strong centrality dependence is observed
• Sag in the vicinity of 1.5 GeV/c induced by nSigma cuts
0-20%
40-80% 0-80%
20-40%
Centrality dependence of efficiency corrections
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Efficiency Corrections (3/3)• Correction factors were
obtained by embedding J/ψ’s into real data
• Due to lack of statistics, we had to resort to numerical integration
• Randomly sampled J/ψ candidates and carried out correction:
ΣdNdy =
N
( 1 ε(pT)
)Where N is the number of candidatesin a given centrality bin and ε is the efficiency
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Systematic Uncertainties
• Varied kinematic and track quality cuts in embedding. Variation of nHitsDedx cuts done in parallel with nHits variation
• Assessed uncertainty in correction the correction technique
• Uncertainty in background subtraction determined to contribute about 10% to overall errors, assumed equal across all centralities
• The uncertainty in the yield due to nSigma cuts is the dominant contributor to systematic errors, accuracy limited by TOF statistics
0-80 0-20 20-40 40-80
nHits>24 1.8 1.5 1.5 1.5
nHits>26 2.1 1.8 1.3 1.9
|etamax|<0.99 0.6 0.4 0.8 0.5
|etamax|<1.01 0.5 0.3 0.5 0.5
|y_jpsi|<0.49 0.3 0.2 0.1 0.4
|y_jpsi|<0.51 0.3 0.6 0.1 0.3
corr. Technique 0.5 0.9 1.0 1.0
bkg. Subtraction 10 10 10 10
nSigma cuts 11.7 8.6 7.9 15.2
quadratic sum 15.7 13.4 13.0 18.4
Error Budget
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Comparison with PHENIX Results
• Our data is similar to PHENIX Run 2 data• Our results are higher than PHENIX Run 4
data, but error bars should be taken seriously!
Comparison with PHENIX Run 2 Comparison with PHENIX Run 4
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