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Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 1
STAR identified particle measurements at high transverse momentum in p+p, d+Au and
Au+Au collisions at sNN = 200 GeV
Outline:
Pawan Kumar Netrakanti,VECC, Kolkata (for the STAR Collaboration)
Motivation STAR experiment Results p+p collisions d+Au collisions Au+Au collisions
Conclusions/Highlights
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 2
Low pT : Soft process, decides bulk properties of the matter and region where hydrodynamical models expected to work
Intermediate pT : Recombination /Coalescence seems to lead to unique difference between baryon-meson production
High pT : Hard process dominates, fragmentation is the mechanism of particle production, high pT suppression observed Jet energy loss
Understanding particle production at RHIC by studying pT spectra
Motivation
Focus of the talk
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 3
Brief overview of low pT results at RHIC
Success of hydrodynamical models at low pT : pT spectra and Azimuthal AnisotropyKinetic freeze-
out conditions : From pT spectra
Chemical freeze-out conditions : From particle ratios
peripheral
central
Tch ~ 165 MeV
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 4
Understanding high pT particle production at RHIC
Understanding the base line first : High pT spectra in p+p collisions ( PDF + Cross section + FF)
Understanding the d+Au collisions : Bridges the gap between p+p and Au+Au collisions (Addition of Nuclear effects……)
These shall provide solid grounds for models based on jet quenching and quark recombination
Understanding high pT particle production in Au+Au collisions by studying the Nuclear Modification Factor and particle ratios
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 5
STAR experiment - Detectors
Time-Of-Flight (–1.0 < < 0 and/30 in )
Time projection Chamber (||<1.8, full and 4.2 m long)
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 6
Log10(p)
Log
10(d
E/d
x)Particle Identification
High pT : Extend particle identification in TPC by exploiting the relativistic rise in ionization energy loss.
Low pT : Particle identification by Time-Of-Flight ( pT < 2.5 GeV/c)
Momentum: GeV/cdE/dx of K,p) separation: 2
10~
3 p
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 7
Transverse Momentum Spectra
pT spectra for pions,protons and anti-protonsupto 10 GeV/c in pp and dAu collisions
pT spectra for pions,protons and anti-protonsupto 12 GeV/c in AuAucollisions
STAR preliminary
PLB 637 (2006) 161
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 8
Understanding the p+p collisionsSensitivity to choice of fragmentation function
NLO pQCD calculations with Kretzer FF inconsistent with data at midrapidity
S. Kretzer, Phys. Rev. D 62 (2000) 054001
PLB 637 (2006) 161
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 9
Sensitivity to choice of fragmentation function
NLO pQCD calculations by W. Vogelsang
NLO pQCD calculations with KKP FF are consistent with pion data at high pT (> 2 GeV/c)
They are inconsistent with the proton+anti-proton data
KKP : B. A. Kniehl, G. Kraner and B. Potter, Nucl. Phys. B 597 (2001) 337
Difference between KKP and Kretzer FF is the way gg fragmentation is more in KKP
PLB 637 (2006) 161
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 10
Sensitivity to choice of fragmentation function
NLO pQCD calculations withAKK FF are consistent with pion data at high pT (> 2 GeV/c)
NLO pQCD calculations with AKK FF compares relatively better than KKP for the p+pbar data
AKK differ from KKP, in the way the light flavor FF are
obtainedfrom the light flavor separated
measurements in e+e- collisions by OPAL
AKK : S. Albino, B. A. Kniehl, and B. Potter, Nucl. Phys. B 725 (2005) 181
OPAL Collaboration : Eur. Phys. J. C 17 (2000) 207
PLB 637 (2006) 161
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 11
Gluon Jets Vs. Quark Jets
Protons dominated by gluon FF
& pions by quark FF at RHICMore Results : Mark Heinz talk later today
Gluon jet contribution to protons is significantly larger than to pions at high pT in p+p collisions at RHIC.
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 12
Understanding the d+Au collisions
•pQCD calculations with Kretzer FF under predicts pion data
•NLO pQCD calculations with KKP and AKK FF and EPOS are consistent with pion data at high pT (> 4 GeV/c)
NLO pQCD calculations with only AKK fragmentation function consistent with the p+pbar data
Parton distribution functions : L. Frankfurt, et al., Phys. Rev. D 71 (2005) 054001
D. De Florian and R. Sassot, PRD 69 (2004) 074028
PLB 637 (2006) 161
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 13
Scaling of particle production in p+p collisions
n ~ 4 for basic scattering process
n ~ 5-8 depending on evolution of structure function and fragmentation function
Pions,protons and anti-protons show the xT (= 2pT/s) scaling for pT > 2 GeV/c at various CM energies
PLB 637 (2006) 161
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 14
Scaling of particle production in p+p and d+Au
More on mT scaling : Mark Heinz talk later today
mT scaling observed and absence of xT scaling for pT < 2 GeV/c Dominance of hard process starts around pT ~ 2 GeV/c
xT
At high pT : no scalingMass effect
Or baryon-meson effect
mT scaling observed for1 < mT < 2 GeV/c2
STAR preliminary
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 15
d+Au collisions : Rapidity asymmetry
To understand the relative contribution of various physics effects to particle production mechanism in d+Au collisions we study
Forward and backward rapidity Asymmetry ratio for identified particles
YAsym = Yield Backward rapidity (Au-side) / Yield Forward rapidity (d-side)
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 16
Comparison to Nuclear Shadowing Model
Nuclear shadowing Model (Various parameterization of shadowing)R. Vogt Phys.Rev.C70:064902,2004
Incorporation of extremes of gluon shadowing (FGS) at low x does not reproduce the measured YAsym. Thereby providing an upper limit on contribution of nuclear shadowing to the YAsym.
STAR preliminary
Cronin + Shadowing + Energy lossJ. Qiu, I. Vitev PLB 632:507-511,2006
Trend for Cronin Only is opposite to data. The model does a reasonable job for lower rapidity.Energy loss in cold nuclear matter for Vitev I < Vitev II
STAR preliminary
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 17
Model comparison – EPOS model
EPOS modelK. Werner, F. Liu, T. Pierog
hep-ph/0506232
Model does a reasonable job for Rapidity integrated spectra, but fails when we study forward and backward ratio
STAR preliminary
PLB 637 (2006) 161
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 18
Model comparison – Recombination
Recombination modelR. C. Hwa, C.B. Yang, R.J. Fries Phys.Rev.C71:024902,2005
Model does a reasonable job.It will be interesting to get the results from the model for pions and protons separately.
STAR preliminary
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 19
Particle - antiparticle ratios Vs. rapidity
At high pT and rapidities away from mid rapidity the role of valence quarks become important.We study this through -/+ and pbar/p for peripheral dAu and n-tag (pA) collisions. p(uud) may induce more +(u dbar) production then - (d ubar)
Within systematic errors difficult to see the effect. So we take double ratio (cancels most systematic errors)
Clear difference in -/+ ratio seen inThe forward and backward regions
STAR preliminary
STAR preliminary
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 20
Fragmentation of u-quark to charged pions
Particle ratio and double ratios can be used to get the relative fragmentation of u-quark to - and +
It can be shown u- / u+ = 1 – r/xq-jet
ris the double ratio-/+ and xq-jet is the fraction of pions originating from quark jets
Contrasting this with the NLO pQCD calculations using AKK FF we find the fraction u- / u+ ~ 0.3 – 0.6
STAR preliminary
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 21
d+Au : Nuclear Modification Factor
STAR preliminarySTAR preliminary
STAR preliminarySTAR preliminary
Phys. Rev. Lett. 91 (2003) 072304
evt 2AB AB T
AB 2AB pp T
1 N d N /dydpR
T d /dydp
RdAu (p + pbar) > RdAu ()
Similar dependence has been observed at lower energies. Phys. Rev D 19 (1979) 764
Qualitative agreement with recombination model for dAu collisions. Phys.Rev.Lett.93 (2004) 082302
At high pT, RdAu for > 1, Cronin effect Absence of high pT suppression in particle production
PLB 637 (2006) 161
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 22
Au+Au : Nuclear Modification Factor
RCP (p > RCP ( : 1.5 < pT < 7 GeV/cRCP (p+pbarshows a decreasing trend atintermediate pT
At high pT – particle production strongly suppressed. p+pbar and pion approach each other to a value of 0.3
STAR Preliminary
pT
Au+Au 200 GeV
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 23
Particle Ratios – p+p collisions
-/+ ~ 1 and pbar/p ~ 0.8p/ and pbar/ increases with pT ~ 2 GeV/c and then decreases to ~ 0.2 p/ agrees with lower energy results. pbar/ shows a distinct energy dependencePYTHIA predicts a moreprominent pT dependence
for -/+ and pbar/p and a flat dependence at high pT
for p/ and pbar/
PLB 637 (2006) 161
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 24
Particle Ratios – d+Au collisions
~ 1- independent of pT
pbar/p decreases with pT
p/ and pbar/ increases with pT up to 2 GeV/c and then decreases.
PLB 637 (2006) 161
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 25
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.
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 26
Particle Ratios – Au+Au collisions
1. -/+ are consistently with flat at unity at all pT, no significant centrality dependence.
2. pbar/p ratio: no significant centrality dependence pT, parton energy loss underpredicts the ratios (X.N. Wang, PRC 58 (2321) 1998).
STAR preliminary
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 27
Particle Ratios – Au+Au collisions
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~2-3 GeV/c, approach the corresponding ratios in p+p, d+Au collisions at pT>5 GeV/c. In general, parton energy loss models underpredict p/ ratios. 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
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 28
Highlight – p+p collisions
NLO pQCD describes the p+pbar spectrum for the first time in p+p collisions
Importance of the significant improvement of FF for baryons from the light-flavor separated measurements in e+e- collisions (OPAL)
The observed xT scaling of ,p(pbar) at high pT also
show dominance of hard processes related to PDF and FF Provide solid reference for effects of jet quenching and recombination in Nucleus-Nucleus collisions
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 29
Highlight – d+Au collisions
Significant Cronin effect observed for pion and proton in d+Au collisions with RdAu () > RdAu (p) at high pT
mT scaling together with xT scaling in p+p collisions shows that the dominance of hard over soft process for minbias collisions starts at pT ~ 2 GeV/c
Rapidity Asymmetry of identified particles can provide important information on relative contribution of various physical process to particle production
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 30
Highlight – Au+Au collisions
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). These indicate that partonic sources of p(pbar) and have similar energy loss
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.
This implies jet fragmentation mechanism region starts around pT>6 GeV/c
Pawan Kumar Netrakanti RHIC/AGS User Meeting 2006, BNL 31
CollaboratorsThe STARSTAR Collaboration
U.S. Labs: Argonne, Lawrence Berkeley, and Brookhaven National Labs
U.S. Universities: UC Berkeley, UC Davis, UCLA, Caltech, Carnegie Mellon, Creighton, Indiana, Kent State, MIT, MSU, CCNY, Ohio State, Penn State, Purdue, Rice, Texas A&M, UT Austin, Washington, Wayne State, Valparaiso, Yale
Brazil: Universidade de Sao Paolo
China: IHEP - Beijing, IPP - Wuhan, USTC,Tsinghua, SINAP, IMP Lanzhou
Croatia: Zagreb University
Czech Republic: Nuclear Physics Institute
England:University of Birmingham
France: Institut de Recherches Subatomiques Strasbourg, SUBATECH - Nantes
Germany: Max Planck Institute – Munich University of Frankfurt
India:Bhubaneswar, Jammu, IIT-Mumbai, Panjab, Rajasthan, VECC
Netherlands:NIKHEF/Utrecht
Poland:Warsaw University of Technology
Russia: MEPHI – Moscow, LPP/LHE JINR – Dubna, IHEP – Protvino
South Korea:Pusan National University
Switzerland:University of Bern
STARSTAR