J measurements in au au and p p collisions at 200 gev
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J/ ψ measurements in Au+Au and p+p Collisions at 200 GeV. Johan Gonzalez for the STAR Collaboration. Motivation J/ ψ in Au+Au J/ ψ in p+p Conclusions. Outline. Motivation. J/ Ψ.

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

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J measurements in au au and p p collisions at 200 gev

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

Johan Gonzalez

for the STAR Collaboration

  • Motivation

  • J/ψ in Au+Au

  • J/ψ in p+p

  • Conclusions

Outline

Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006


Motivation

Motivation

J/Ψ

  • 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

regeneration

[3] Thews, Schroedter, Rafelski, Phys. Rev. C 63, 054905

[4] P.B.M., Andronic, hep-ph/0402291

[1] T. Matsui and H. Satz, Phys. Lett. B 178 (1986) 416.

[2] Grandchamp, Rapp, Brown hep-ph/0403204

Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006


Motivation1

Motivation

An illustration of the competition of regeneration and suppression mechanisms

  • 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

L. Grandchamp and R. Rapp, Phys. Lett. B523 60 (2001)

regeneration

suppression

§ These exclude “normal” suppression mechanisms, such as collisions with co-movers

Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006


The star detector

The STAR Detector

Schematic side view of 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.

Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006


J measurements in au au and p p collisions at 200 gev

J/ψe+e- in Au+Au @ 200GeV

Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006


Data analysis cuts

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 events12M 0-80% events after cuts

selected tracks

dE/dx

p (GeV/c)

Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006


Dielectron invariant mass spectra

Dielectron Invariant Mass Spectra

J/ψe+e- (BR = 5.93%)

STAR Preliminary

Zoom

Same event

Mixed event

  • 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

0-80% 12M events

STAR Preliminary

Subtracted spectrum

Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006


Dielectron invariant mass spectra1

Dielectron Invariant Mass Spectra

STAR Preliminary

  • 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

Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006


Centrality dependence of scaled j yields

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

STAR Preliminary

0-80%

Binary Scaling (black line and grey band) Determined from PHENIX

data (nucl-ex/0507032)

Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006


Centrality dependence of scaled j yields1

Centrality Dependence of Scaled J/ψ yields

STAR Preliminary

  • 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.

  • Statistical Hadronization Synopsis:

  • Complete screening of primordial J/ψ’s

  • J/ψ’s regenerated at chemical freezout from

    thermalized c-cbars

[1] A. Andronic et al., Phys.Lett. B571 (2003) 36-44

[2] PHENIX, Phys.Rev.Lett. 96 (2006) 032001

[3] STAR, Phys. Rev. Lett. 94 (2005) 062301

pQCD x-section from:Cacciari, Nason, Vogt, PRL 95 (2005) 122001

Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006


J measurements in au au and p p collisions at 200 gev

J/ψe+e- in p+p @ 200GeV

Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006


Data analysis cuts1

Data & Analysis Cuts

A typical triggered event

  • 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

Track selection

Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006


Dielectron invariant mass spectrum

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/ψe+e- in p+p @ 200 GeV

More statistics are expected from Run 6

Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006


Summary conclusions

Summary & Conclusions

  • We have investigated the production of J/ψe+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

Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006


Extra slides

Extra slides

Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006


Efficiency corrections 1 3

Efficiency Corrections (1/3)

TPC+TOF

  • Embedded simulated J/ψ’s into “Run 4-like” events:

    • Full field

    • |vertexZ|<30cm

    • J/ψe+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

Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006


Efficiency corrections 2 3

Efficiency Corrections (2/3)

20-40%

0-20%

40-80%

0-80%

Centrality dependence of efficiency corrections

  • 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

Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006


Efficiency corrections 3 3

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:

N

dN

dy

Σ

(

)

1

ε(pT)

=

Where N is the number of candidates

in a given centrality bin and ε is the

efficiency

Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006


Systematic uncertainties

Systematic Uncertainties

Error Budget

  • 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

Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006


Comparison with phenix results

Comparison with PHENIX Results

Comparison with PHENIX Run 2

Comparison with PHENIX Run 4

  • 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!

Johan Gonzalez - Strange Quark Matter 2006, UCLA - March 2006


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