Pawan Kumar NetrakantiRHIC/AGS User Meeting 2006, BNL1 STAR identified particle measurements at high transverse momentum in p+p, d+Au and Au+Au collisions

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


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


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


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


STAR experiment - Detectors

Time-Of-Flight (–1.0 < < 0 and/30 in )

Time projection Chamber (||<1.8, full and 4.2 m long)


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


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


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


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


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


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.


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


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


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


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)


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


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


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


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


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


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


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


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


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


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.


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


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


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


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


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


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

Documents

Pawan Kumar NetrakantiRHIC/AGS User Meeting 2006, BNL1 STAR identified particle measurements at high transverse momentum in p+p, d+Au and Au+Au collisions