Transcript Document

Identified baryon and meson spectra at intermediate
and high pT in 200 GeV Au+Au Collisions
Lijuan Ruan (for the STAR Collaboration)
(Lawrence Berkeley National Laboratory)
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
03/14/2006
WWND2006 at La Jolla
1
Particle production at pT>2 GeV/c in p+p
collisions
Convolution of PDFpQCDFF
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.
03/14/2006
WWND2006 at La Jolla
2
p+p collisions and pQCD (AKK)
NLO pQCD calculations with
AKK FF are consistent with pion
data at high pT (> 2 GeV/c)
NLO pQCD calculations only
with 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 obtained
OPAL Collaboration : Eur. Phys. J. C 17 (2000) 207
STAR Collaboration, nucl-ex/0601033
03/14/2006
WWND2006 at La Jolla
3
from p+p  Au+Au collisions
p + p collisions
Au + Au collisions
Soft particle production region:
pT<2 GeV/c
Hot, dense, strongly interacting
matter created
Hard particle production region:
pT>2 GeV/c, which can be
described by NLO pQCD
calculations.
p(pbar)/>=1 at pT=2 GeV/c, which
cannot be reproduced by pQCD
approach.
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.
03/14/2006
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.
WWND2006 at La Jolla
4
hydro+jet: Interplay between Soft and Hard
Au+Au at b=2 fm
(1/pT)(dN/dpT)
T.Hirano & Y.Nara(’03), PRC69(2004)034908
NSOFT~NHARD
03/14/2006
PT, cross~
1.8 GeV/c for 
2.7 GeV/c for K
3.7 GeV/c for p
Hydrodynamic
afterburner
soft
hard
Crossing pT moves
toward high pT
Jet
quenching
pT
Interesting region
Intermediate pT
(2<pT<3.5 GeV/c)
Pion hard, Proton soft
WWND2006 at La Jolla
5
Consequence from hadron species
dependent pT,cross
Hydro+Jet
T.Hirano & Y.Nara(’03), PRC69(2004)034908
03/14/2006
WWND2006 at La Jolla
6
Recombination models
Recombination models:
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 predict that a scale according to the
number of constituent quarks in a hadron seems to govern the
particle production at intermediate pT.
03/14/2006
WWND2006 at La Jolla
7
Centrality Dependence: Rcp in Au+Au
STAR: Phys. Rev. Lett. 92 (2004) 052302
PHENIX: Phys. Rev. Lett. 91 (2003) 172301
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.
03/14/2006
WWND2006 at La Jolla
8
Centrality Dependence: Rcp in Au+Au
STAR: Nucl. Phys. A 757 (2005) 102
X. Cai QM05, STAR Preliminary
Two groups (2<pT<6GeV/c):
- , Ks, K, K*, Φ  mesons
- p, Λ, Ξ, Ω  baryons
03/14/2006
WWND2006 at La Jolla
9
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)
03/14/2006
WWND2006 at La Jolla
10
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.
03/14/2006
WWND2006 at La Jolla
11
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 lossTo further understand how
the gluon jet/quark jet interact with the medium created in Au+Au.
collisions.
03/14/2006
WWND2006 at La Jolla
12
STAR Detector: TPC & MRPC-TOF
• RHIC run4 (2004) 200
GeV Au+Au data
• ~14 M minbias events
after |z_vertex|<30cm
• ~7.8 M TOF matched
events
A new technology ---Multi-gap Resistive Plate Chamber (MRPC)
adopted from CERN-Alice.
A prototype detector of time-of-flight (TOF)
was installed in year4
Intrinsic timing resolution: 85 ps in STAR.
03/14/2006
WWND2006 at La Jolla
13
Spectra
STAR Preliminary
03/14/2006
WWND2006 at La Jolla
14
Nuclear Modification Factors RCP
STAR Preliminary
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.
03/14/2006
WWND2006 at La Jolla
15
Anti-particle to particle ratios
STAR Preliminary
1.
2.
-/+ are consistent with flat at unity in all pT, no significant centrality
dependence.
pbar/p ratio: no significant centrality dependence, not consistent with the
jet quenching prediction (X.N. Wang, PRC 58 (2321) 1998).
03/14/2006
WWND2006 at La Jolla
16
Proton over pion ratios
STAR Preliminary
1.
2.
The p(pbar)/ ratios in Au+Au collisions show strong centrality dependence.
In central Au+Au collisions, the p(pbar)/ ratios reach maximum value at pT~23 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).
03/14/2006
WWND2006 at La Jolla
17
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.
03/14/2006
WWND2006 at La Jolla
18
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.
03/14/2006
WWND2006 at La Jolla
19
Discussion: energy loss in QCD matter
S. Wicks et al., nucl-th/0512076
•
Color charge dependence: g vs q.
experimentally, p and pbar at
high pT.
STAR Preliminary
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) nonphotonic electrons have similar
energy loss.
03/14/2006
WWND2006 at La Jolla
20
Outlook: A Heavy-Flavor Tracker for STAR
03/14/2006
WWND2006 at La Jolla
21
Outlook: A Heavy-Flavor Tracker for STAR
The energy loss
of heavy flavor
will be measured
from open charm
directly!!!
03/14/2006
WWND2006 at La Jolla
22