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Identified baryon and meson spectra at intermediate and high pT in 200 GeV Au+Au Collisions
Outline:• Motivation• Intermediate pT
particle production mechanism at mid-rapidity
• High pT charged & p spectra up to 12 GeV/c from STAR
RCP, particle ratios
• Summary and Outlook
Lijuan Ruan (for the STAR Collaboration)(Lawrence Berkeley National Laboratory)
03/14/2006 WWND2006 at La Jolla 2
Particle production at pT>2 GeV/c in p+p collisions
Convolution of PDFpQCDFF
PDF (parton distribution function): derived from e-h deep inelastic scattering.
pQCD (parton-parton interaction cross section from pQCD calculation)
FF (fragmentation function): derived from e+e- collisions.
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p+p collisions and pQCD (AKK)
NLO pQCD calculations withAKK FF are consistent with pion data at high pT (> 2 GeV/c)
NLO pQCD calculations onlywith AKK FF (Nucl.Phys.B725:181-206,2005)consistent with the p+pbar data
AKK differ from KKP, in the way the light flavor FF are
obtainedOPAL Collaboration : Eur. Phys. J. C 17
(2000) 207STAR Collaboration, nucl-ex/0601033
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from p+p Au+Au collisions
Soft particle production region: pT<2 GeV/c
Hard particle production region: pT>2 GeV/c, which can be described by NLO pQCD calculations.
pQCD approach p/ ratio <<1 due to D(q->p)/D(q->)<<1.
At low energy, pbar/p (pbar/) ratio< 0.1, consistent with very low anti-baryon production from quark jets.
Hot, dense, strongly interacting matter created
p(pbar)/>=1 at pT=2 GeV/c, which cannot be reproduced by pQCD approach.
Intrigue a lot of study for particle production mechanism: hydrodynamic, recombination/coalescence, gluon junction
The medium pushes the soft particle production region up to higher pT.
p + p collisions Au + Au collisions
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hydro+jet: Interplay between Soft and Hard
NSOFT~NHARD
ppTT
(1/
(1/ ppTT)
()(dNdN
// dpdpTT))
HydrodynamicHydrodynamicafterburnerafterburner
JetJetquenchingquenching
Interesting regionIntermediate pT
(2<pT<3.5 GeV/c)Pion hard, Proton soft
T.Hirano & Y.Nara(’03), PRC69(2004)034908
Crossing pCrossing pTT moves movestoward high ptoward high pTT
softhard
Au+Au at b=2 fm
PT, cross~ 1.8 GeV/c for
2.7 GeV/c for K3.7 GeV/c for p
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Consequence from hadron species dependent pT,cross
Hydro+JetT.Hirano & Y.Nara(’03), PRC69(2004)034908
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Recombination models
Recombination models predict that a scale according to the number of constituent quarks in a hadron seems to govern the particle production at intermediate pT.
R.J. Fries, et al., Phys. Rev. Lett. 90 202303 (2003).R. C. Hwa, et al., Phys. Rev. C 70, 024905 (2004).V. Greco, et al., Phys. Rev. C 68, 034904(2003).
Recombination models:
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Centrality Dependence: Rcp in Au+Au
Baryons (p, ) increase faster than mesons (, Ks, K) from peripheral to central collisions. consistent with the coalescence of constituent quarks at hadronization from a collective partonic system.
STAR: Phys. Rev. Lett. 92 (2004) 052302PHENIX: Phys. Rev. Lett. 91 (2003) 172301
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Centrality Dependence: Rcp in Au+Au
Two groups (2<pT<6GeV/c):
- , Ks, K, K*, Φ mesons
- p, Λ, Ξ, Ω baryons
STAR: Nucl. Phys. A 757 (2005) 102
X. Cai QM05, STAR Preliminary
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From intermediate pT high pT region
• The interplay between soft and hard plays an important role for particle production at intermediate pT.
• The RCP shows baryon/meson difference.
• No precise measurements of identified baryon and meson spectra at high pT (pT>6 GeV/c).
The high pT PID spectra will be presented to address
• The transition between coalescence and jet fragmentation.
• Possible color charge dependence of energy loss (g/q)
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g/q jet contribution to particle production
Gluon jet contribution to protons is significantly larger than to pions at high pT in p+p collisions at RHIC.
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Color charge dependence of energy loss
X.N. Wang: PRC58(2321)1998.
PID spectra, centrality dependence of pbar/p and pbar/ ratios, address the color charge dependence of energy lossTo further understand how the gluon jet/quark jet interact with the medium created in Au+Au. collisions.
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STAR Detector: TPC & MRPC-TOF
A new technology ----Multi-gap Resistive Plate Chamber (MRPC)
adopted from CERN-Alice.
A prototype detector of time-of-flight (TOF) was installed in year4Intrinsic timing resolution: 85 ps in STAR.
• RHIC run4 (2004) 200 GeV Au+Au data
• ~14 M minbias events
after |z_vertex|<30cm
• ~7.8 M TOF matched events
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Spectra
STAR Preliminary
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Nuclear Modification Factors RCP
In central Au+Au collisions:
• At 4<pT<12 GeV/c, both and p are strongly suppressed in RCP.
• At 1.5<pT<7 GeV/c, RCP(p+pbar) > RCP() , RCP(p+pbar) shows obvious decreasing trend, not consistent with the binary scaling at intermediate pT.
I. Vitev, nucl-ex/0603010.
STAR Preliminary
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Anti-particle to particle ratios
1. -/+ are consistent with flat at unity in all pT, no significant centrality dependence.
2. pbar/p ratio: no significant centrality dependence, not consistent with the jet quenching prediction (X.N. Wang, PRC 58 (2321) 1998).
STAR Preliminary
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Proton over pion ratios
1. The p(pbar)/ ratios in Au+Au collisions show strong centrality dependence. 2. In central Au+Au collisions, the p(pbar)/ ratios reach maximum value at pT~2-
3 GeV/c, approach the corresponding ratios in p+p, d+Au collisions at pT>5 GeV/c.
R.J. Fries, et al., Phys. Rev. Lett. 90 202303 (2003); R. C. Hwa, et al., Phys. Rev. C 70, 024905 (2004); DELPHI Collaboration, Eur. Phy. J. C 5, 585 (1998), Eur. Phy. J. C 17, 207 (2000).
STAR Preliminary
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Discussion
• In Au+Au collisions, at pT>6 GeV/c: RCP(p+pbar) ~ RCP();
p(pbar)/ ~ p(pbar)/ (d+Au,p+p); pbar/p ~ pbar/p (d+Au). Indicate that partonic sources of p(pbar) and have similar energy loss.
• Gluon contribution to p(pbar) is significantly larger than to (p); Gluons lose twice as much energy as quarks; RCP(p+pbar)<RCP(); p(pbar)/ < p(pbar)/ (d+Au,p+p); pbar/p < pbar/p (d+Au).
• With the addition of collisional energy loss in the recent calculation, the gluon jet quenching is a factor of 3 higher than quark jet quenching (S. Wicks et al., nucl-ex/0512076).
• There is no model to readily explain these measurements. Direct comparisons to the measurements require proper fragmentation functions.
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Summary• The spectra of and p (pbar) from 200 GeV Au+Au collisions are
analyzed up to pT=12 GeV/c.
• At central Au+Au collisions, at intermediate pT, RCP(p+pbar) > RCP() , RCP(p+pbar) decreasing trend with pT, not consistent with binary scaling; the p(pbar)/ ratio~1 at pT 2~3 GeV/c.
• In central Au+Au collisions, at high pT, RCP(p+pbar) ~ RCP() the p(pbar)/ ratios approach the ratios in p+p and d+Au collisions. In Au+Au collisions, jet fragmentation mechanism region: pT>6 GeV/c.
• The -/ + and pbar/p ratios, no significant centrality or pT dependence.
Indicate that the partonic sources of protons and pions have similar
energy loss.
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Discussion: energy loss in QCD matter
• Color charge dependence: g vs q. experimentally, p and pbar at high pT.
In order to systematically study this effect measure p(pbar)/, pbar/p ratio in different energy by changing the g/q relative contribution to particle production.
• Flavor dependence: u, d quarks vs c, b quarks. go to charm.
• The partonic sources of , protons, and (D,B to) non-photonic electrons have similar energy loss.
S. Wicks et al., nucl-th/0512076
STAR Preliminary
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Outlook: A Heavy-Flavor Tracker for STAR
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Outlook: A Heavy-Flavor Tracker for STAR
The energy loss
of heavy flavor
will be measured
from open charm directly!!!