1. Identified Particle Production at High Transverse Momentum at RHIC
Vlad Pantuev, Stony Brook University Moriond QCD 2005

2. Motivation – Based on the Questions:
What kind of medium produced at RHIC?
How do we measure properties of this medium?
What is the origin of baryon-meson anomaly: Is it medium effect? Modification of fragmentation function? Somehow, just mass effect?
Will try to answer some questions by measuring identified particle production at high pt:
Other hadrons. Wider momentum range.
In p+p collisions, establishes the elementary hadronic reference. From RCP to RAuAu .
In d+Au collisions, establishes the size of initial state nuclear matter effects. Detailed study of the Cronin effect for mesons and baryons.
In Au+Au collisions at lower energies, 62.4 GeV, absence or presence of the baryon anomaly ? What is the magnitude at lower energies ?
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3. Au+Au collisions at sNN = 200 GeV
Some history of the problem, jet quenching
Strong suppression of p0 yields above pT ~ 2 GeV/c at 200 GeV central Au+Au collisions at RHIC, but no suppression for proton and antiproton at intermediate pT ~ (2-5 GeV/c): “Baryon anomaly at RHIC”
“Anomalous” proton to pion ratios at 200 GeV central Au+Au collisions at RHIC.
PHENIX: PRL 91, (2003), PRC 69, (2004)
Au+Au collisions at sNN = 200 GeV
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4. Other hadrons:
In nuclear modification factor all baryons follow the same tendency
Mesons are suppressed regardless of its mass
PS. Please remember, R_AA and Rcp have different definition but close to each other
PHENIX, 200 GeV
GLV RAA
dNg/dy = 1100
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5. “Controlled” d+Au collisions
Old data: 38.8 GeV p+W vs. p+Be
Cronin effect
No suppression as in central AuAu.
Larger Cronin effect for baryons, but
Can’t explain factor 4-5 difference in AuAu
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6. Au+Au, lower energy, 62.4 GeV 0-10% chargedp0
charged
0-10%
10-30%
Still significant pion suppression, but less.
Proton/pion >1, this explains Raa for charged.
What about other observable, FLOW, at 62.4?
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7. Elliptic flow at 62 GeV vs 200 GeV Au+Au
Upper curves: v2 {2-particle}
Lower curves: v2 {4-particle}
Solid points: 62 GeV
Open points: 200 GeV
Preliminary
STAR
Charged hadrons.
v2(pT) for 62 GeV is close to 200 GeV
In many aspects 62.4 GeV and 200 GeV data look similar
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8. V2_meson(pt) = 2 V2_q(pt/2) V2_baryon(pt) = 3 V2_q(pt/3)
Description of baryon production at high pt is tough task for theorists.
Yet crude, but very interesting idea is used in quark recombination model.
One of the demonstrations how simple quark counting works is explanation of the elliptic flow for mesons and baryons by quark coalescence model, D.Molnar and S.Voloshin:
V2_meson(pt) = 2 V2_q(pt/2)
V2_baryon(pt) = 3 V2_q(pt/3)
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9. ede Two competing processes for hadron production: Fries et al.
So, recombination model:
Two competing processes for hadron production:
Jet-Fragmentation in the vacuum
Recombination of 3 quarks or a quark/anti-quark pair in a densely populated phase space
Fries et al.
In case of thermalized partons at RHIC, fragmentation wins over recombination only above pT = 5 GeV/c
This explains p/p ratio
Cronin effect could also be explained by recombination of Thermal and Shower partons in the final state.
ede
Fries, et al, nucl-th/
Also, Greco, Ko, Levai, nucl-th/
Also, R.C. Hwa and C.B.Yang, Phys.Rev.Lett. 93 (2004)
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10. Proton term will have: TTT+TTS+TSS+SSS
Predictive power of the model is at moderate to high pt through the TS and TT term for mesons.
Proton term will have: TTT+TTS+TSS+SSS
The result that recombination model can nicely describe Au+Au AND d+Au is puzzling for me personally. – I can imagine “quark bath” in AuAu, but how the same is true for cold dAu collisions, especially for “quark thermal part” ?
What about difference in Baryon- and
Meson-triggered jets?
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11. Difference in STAR/PHENIX yields due to various |η| windows.
STAR vs PHENIX  correlations in Au+Au. 2-particle correlation with identified hadron
PHENIX nucl-ex/
|η|<0.7
|η|<0.35
STAR preliminary
meson – π, K
baryon – p, p
Difference in STAR/PHENIX yields due to various |η| windows.
Meson and Baryon “jets” are very similar!
Moriond QCD, 2005

12. Conclusions
In central Au+Au collisions in the intermediate pt region, baryons are not suppressed, unlike mesons.
The nature of the difference is probably in the number of constituent quarks, not mass.
At high pt > 5 GeV/c, there is also visible baryon suppression.
In d+Au at midrapidity we see enhancement, even larger for baryons. Cronin effect.
At 62.4 GeV Au+Au collisions there is suppressions of pions and ratio proton/pion>1. Evidence of smooth sqrt(s) energy dependense.
Proton enhancement and elliptic flow exibits a pattern suggestive of the relevance of constituent quark degree of freedom. But is it proof of de-confinement?
Intermediate pt region baryons possess jet-like characteristics. Baryon/meson ratio in central Au+Au is still a puzzle!
There are many other observations that particle and jet production is modified in Dense Medium created in central heavy ion collisions at RHIC. Strong_QGP ?
Much more coming soon from the last very high statistics Au+Au run and just finished Cu+Cu run
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13. PHENIX, preliminary, h-h azimuthal correlations in Au+Au, background subtracted
STAR Preliminary, particle spectra in jets
Away
Near
Very soon it will be possible to draw identified hadrons in plots like that…
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