Vlad Pantuev Stony Brook University Identified Particle Production

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

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 RAu. In d+Au collisions, establishes the size of initial state nuclear matter effects. Detailed study of the Cronin effect for mesons and baryons. Moriond QCD, 2005 In Au+Au collisions at lower energies, 62. 4 Ge. V, absence or presence of the baryon anomaly ? What is the magnitude at lower energies ? 2 2

Some history of the problem, jet quenching • Strong suppression of p 0 yields above p. T ~ 2 Ge. V/c at 200 Ge. V central Au+Au collisions at RHIC, but no suppression for proton and antiproton at intermediate p. T ~ (2 -5 Ge. V/c): “Baryon anomaly at RHIC” Moriond QCD, 2005 l“Anomalous” proton to pion ratios at 200 Ge. V central Au+Au collisions at RHIC. PHENIX: PRL 91, 172301 (2003), PRC 69, 034909 (2004) 3 Au+Au collisions at s. NN = 200 Ge. V 3

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, Au. Au @ 200 Ge. V GLV RAA d. Ng/dy = 1100 4 Moriond QCD, 205 4

“Controlled” d+Au collisions Old data: 38. 8 Ge. V p+W vs. p+Be Cronin effect No suppression as in central Au. Larger Cronin effect for baryons, but Moriond QCD, 2005 5 Can’t explain factor 4 -5 difference in Au. Au 5

Au+Au, lower energy, 62. 4 Ge. V charged 0 -10% p 0 charged Still significant pion suppression, but less. Proton/pion >1, this explains Raa for charged. 10 -30% p 0 What about other observable, FLOW, at 62. 4? 6 Moriond QCD, 2005 6

Elliptic flow at 62 Ge. V vs 200 Ge. V Au+Au Upper curves: v 2 {2 -particle} Preliminary STAR Lower curves: v 2 {4 -particle} Solid points: 62 Ge. V Open points: 200 Ge. V Charged hadrons. v 2(p. T) for 62 Ge. V is close to 200 Ge. V Moriond QCD, 2005 In many aspects 62. 4 Ge. V and 200 Ge. V data look similar 7 7

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: V 2_meson(pt) = 2 V 2_q(pt/2) V 2_baryon(pt) = 3 V 2_q(pt/3) 8 Moriond QCD, 2005 8

So, recombination model: • • ede • 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 p. T = 5 Ge. V/c • This explains p/p ratio • Cronin effect could also be explained by recombination of Thermal and Shower partons in the final state. Fries, et al, nucl-th/0301087 Also, Greco, Ko, Levai, nucl-th/0301093 Also, R. C. Hwa and C. B. Yang, Phys. Rev. Lett. 93 (2004) 082302 Moriond QCD, 2005 9 9

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 Au. Au, but how the same is true for cold d. Au collisions, especially for “quark thermal part” ? What about difference in Baryon- and Meson-triggered jets? 10 Moriond QCD, 2005 10

STAR vs PHENIX correlations in Au+Au. 2 -particle correlation with identified hadron PHENIX nucl-ex/0408007 |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! 11 Moriond QCD, 2005

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 Ge. V/c, there is also visible baryon suppression. • In d+Au at midrapidity we see enhancement, even larger for baryons. Cronin effect. • At 62. 4 Ge. V 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 ? Moriond QCD, 2005 Much more coming soon from the last very high statistics Au+Au run and just finished Cu+Cu run 12 12

STAR Preliminary, particle spectra in jets Near PHENIX, preliminary, h-h azimuthal correlations in Au+Au, background subtracted Away Very soon it will be possible to draw identified hadrons in plots like that… Moriond QCD, 2005 13 13

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