J/ ψ measurements in Au+Au and p+p Collisions at 200 GeV

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


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


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


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


J/ψe+e- in Au+Au @ 200GeV


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


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%)


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


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)


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


J/ψe+e- in p+p @ 200GeV


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


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


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


Extra slides


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


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


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


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


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

J/ ψ measurements in Au+Au and p+p Collisions at 200 GeV

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