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The STAR Experiment. Direct -charged hadrons measurements in STAR. High- pT physics at LHC 2009, 4-7th February, Prague, Czech Republic. Texas A&M University A. M. Hamed for the STAR collaboration. Table of Contents and Disclaimer. Table of Contents:. The Road Behind. Analysis.

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The STAR Experiment

Direct -charged hadrons

measurements in STAR

High-pT physics at LHC 2009,

4-7th February, Prague, Czech Republic

Texas A&M University

A. M. Hamed for the STAR collaboration

Table of Contents and Disclaimer

Table of Contents:

  • The Road Behind

  • Analysis

  • Results

  • Summary

  • Disclaimer: The road behind is personal view, so biases and mistakes are expected.

The Road Behind: Why study high-pt particles?

  • High-pT particles are produced from the hard scattering processes.

At mid-rapidity


Hard Scattering:













PDF: Extracted from data but

evolution is perturbative “DGLAP"


Power law

Hard scattering: Expansion in coupling

constant (LO, NLO, NNLO, ..)

NB: Factorization used in many

context without proof

Heavy-ion collisions

Breakdown of factorization claimed in

dijets at N3LO, Collins, Qiu ,07

  • Their rates are calculable via pQCD.

  • Take place at early time of collisions (~0.01 fm/c). V~ 5 fm3 and  ~ 10 fm/c

The Road Behind: Methods for high-pt particles

“ An interesting signature may be events in which the hard collision occurs near the edge of the

overlap region, with one jet escaping without absorption and the other fully absorbed”

J.D.Bjorken 1982

“elastic scattering?”

1. Jet reconstruction

Detection efficiency, quark- versus gluon-jet properties, Jet-mass effects.

The jet modifies the medium as well as the medium quench the jet.

2. Inclusive single high-pt particle spectra- Leading particle method “RAA”

dAu control experiment

Glauber model uncertainties, Parton distribution functions , CGC.

RAA is a measure of the deviation

from the incoherent superposition

of nucleon-nucleon collisions assumption.

In DIS Q2 values are of orders of

magnitude greater than the typical

energies and momenta in nuclear physics.

3. Associated yield of high-pt particle , Fragmentation Function, dijet events “IAA”.

Better to shed insight on the underlying physics , no Glauber model, modified FF,

yield normalized per tigger.

The Road Behind: Exp. results of high-pt particles “RAA, IAA”

PRL 98 (2007)192301

  • Hadron RAA is pt independent as expected by the radiative energy loss .

  • Direct photons follow the binary scaling. Number of binary scaling works!

Equark,m=0  Equark,m>0

  • Unexpected level of suppression for the heavy quarks.

Egluon  Equark


  • No sign for the color factor effect on energy loss.

  • Similar near-side and strongly suppressed away-side in Au+Au relative to p+p and d+Au.

  • Away-side yield strongly suppressed to the level of RAA

  • IAA is zT independent and there is no broadening in the associated correlation peak .

The Road Behind: Theoretical models “radiative energy loss”

  • The four major models use pQCD framework to estimate energy loss.

  • Different assumptions in various models lead to similar descriptions of the

  • light quark suppression with different model-dependent parameters.





  • LPM-effect based approaches: BDMPS & AMY

  • Opacity expansion: GLV & ASW


q1 GeV2/fm

  • radiative energy loss

  • Medium enhanced higher twist effects

If s(T) were weak…


q extracted via comparison with RHIC data is larger…

  • Medium modified MLLA


  • Modeling the medium evolution/structure.

  • Hierarchy of scales: E, Q, 

Ways of extrapolation of pQCD in non-pQCD regime, just make s small

ASW and GLV: Similar models different

AMY and Higher twist: Different models same


Data doesn’t allow to distinguish between q 5 or 15 GeV2/fm

There is no single commonly accepted calculation of the underlying physics to describe in-medium energy loss for different quark generations as well as for the gluon.

The Road Behind: Time-ordered

Energy loss

What happens to empty space, if you keep adding heat?

“Static medium, Energy dependence”

dErad/dz CR s E <q2>

Relative phase:

form<<  Bethe-Heitler limit, E is fixed and 


The fundamental theoretical result regarding the asymptotic high temperature phase is that it becomes quasi-free. That is, one can describe major features of this phase quantitatively by modeling it as a plasma of weakly interacting quarks and gluons. In this sense the fundamental degrees of freedom of the microscopic Lagrangian, ordinarily only indirectly and very fleetingly visible, become manifest (or at least, somewhat less fleetingly visible).


“We will not have done justice to the concept of weakly interacting plasma of quarks and gluons until some of the predictions are confirmed by experiment”

form >> LPM limit, E and  >> 1

dErad/dz CR sln(E) <q2>


Where is the q ?

At T >> Tc :   gT >> QCD

: dynamical scale of the medium, color screening scale “mass”, 1/ color screening length

F. Wilczek



In particular, chiral symmetry is restored, and confinement comes completely undone.

1/ << 

Independent successive scattering centers

Scattering power of the medium

Lattice QCD

The parton propagation is “time-ordered” and time-oredered

perturbation theory is the natural framework to calculate the

radiation amplitude.



F. Karsch, E. Laermann, A. Peikert, CH. Schmidt, S. Stickan



In DIS Q2 values are of orders of

magnitude greater than the typical

energies and momenta in nuclear physics but nuclear environment effect is significant and not understood yet!



  • The applicability of pQCD in describing the parton-matter interaction has been increasingly challenged by the “speculated” strongly coupled nature of the produced matter at RHIC.

The Road Behind: Direct -jet azimuthal correlation

Fast Detector


Leading particle















zero near-side yield

for direct photons

Fast Detector


Direct photon


  • Due to fragmentation full jet reconstruction

    is required to access the initial parton energy


get the initial parton energy

with a powerful alternative method:

“Direct -hadron azimuthal correlations”

Heavy ion collision

Direct photon is a surface bias free probe.

Jet-energy is calibrated by “Direct ”

The Road Behind: Direct  production mechanisms

Direct photon: photons unaccompanied by additional hadrons

Direct photon production provides an insight into the dynamics of hadronic constituents which is not obscured by their fragmentation.

High-pt direct photons are produced at a rate comparable to that of single particles: perform high-statistics measurements with practical facilities.


LO are the dominant processes:

Photon Bremsstrahlung


takes only a fraction of the proton's momentum.

contribution is suppressed by a factor of  with respect to

single-0 rate. This suppression is offset somewhat: q

fragmentation is flatter than q0.



10% of inclusive  at intermediate pT in p+p “PHENIX”!

~30-40% of direct  at PT > 8 GeV/c in p+p “NLO pQCDVogelsang”,

Examples of Bremsstrahlung


Heavy ion collisions

Sources of suppression and enhancement of direct photons yield.

RAA saturates “pt-independent”

LPM effect

RAA follows binary scaling

The Road Behind: Direct -jet production mechanisms

Both mechanisms yield associated photons recoiling against a gluon or quark jet depending on the value of xT .

In the approximation that the colliding partons are collinear in the CMS frame:

D0: NLO pQCD is unable to describe the shape of the pT dependence of the  across four

Different kinematic regions simultaneously . arXiv:hep-ex/0804.1107

Effects due to intrinsic motion

Enhancement of single-particle and jet cross sections due to the parton transverse momentum at

moderate pT.

The effect of parton kT is greatly reduced in the case of direct gamma- jet compared to single photon cross section. J. F. Owens, Phys. Rev. D 20, 221 (1979)

Direct photon-hadron correlations

Direct photon energy balances the outgoing parton, module negligible correction from initial state radiation.

Calibrated probe of the QGP – at LO. No Surface Bias.

Hard process. Possible discrimination power for q/g

  • Challengeable measurements!

0 is suppressed at high pT by a factor of ~5 in central

AuAu collisions.

Analysis: Analysis technique

  • Build correlation function for neutral “triggers”

    with “associated” charged particles

  • Use transverse shower profile to distinguish 2-photon “0”

    from single-photon showers “rich”

  • Comparison of 0 – triggered yields with previously measured

    h triggered yields.



  • A method of statistical subtraction of yield

  • associated per direct trigger using thefactthatdirect

  • photon has no near side yieldandassumingall sources of

  • background have similar correlations

  • to that of symmetric decay 0.

Analysis: -jet azimuthal correlation in STAR

One tower out of 4800

towers (0.05 x 0.05)

No track with

p > 3 GeV/c points

to the trigger tower


Eγ= Eparton


Beam axis



Use  triggers

to explore

fragmentation functions

in p+p and Au+Au

Associated charged

particles “3 <pT< 16 GeV/c”

Eπ‹ Eparton


  • Correlate photon candidate “triggers” with “associated tracks”

pT,trig > 8 GeV/c

Charged hadrons 3 <pT < 16 GeV/c

Online trigger:

Etower > 5.76 GeV,

Ecluster > 7.44 GeV “cluster =2 towers out of 3x3 towers”

Au+Au 506 ub-1 (p+p 19.6 pb-1)



Offline trigger:

Etower > 6 GeV,

Ecluster > 8GeV,

Esmd > 0.5 GeV,

Cluster is away from

the tower edge

BEMC: Barrel

Electro-Magnetic Calorimeter

Track quality, eff.

TPC: Time Projection Chamber

Event general QA

Full azimuthal coverage

|TPC| < 1.5, |TPC selected| < 1

|bemc| < 1, |bemcselected| < 0.9

How to distinguish between 0/ ?

Analysis: Transverse shower shape analysis in STAR

 7 RM


The two photons originated from 0 hit the same tower at pT>8GeV/c

STAR Shower Maximum Detector is embedded at ~ 5x0 between the lead-scintillator layers “BEMC”

i : strip energy

The tower energy asymmetry cut to purify the rich sample

in case of 0 decay across the module in 

ri : distance relative

to energy maxima

Use the shower-shape analysis to separate the two close photons

shower from one photon shower.

Frag. Photons, asymmetric decay of pi0, and eta?

Results: inclusive -charged hadrons azimuthal correlation

Data Set: L=535 ub-1 of Au+Au and L=11 pb-1 of p+p

STAR Preliminary

STAR Preliminary

Clear dijet structure is seen for inclusive  – charged hadron azimuthal correlation in STAR

Background level increases with centrality as expected

Both near side yield and away side increase with trigger energy as the initial parton energy


Near side is suppressed with centrality which might due to the increase of /0 ratio .

Near and away side yields decrease with associated pt: the jet cross section falls more steeply than the Fragmentation Function does.

Results: rich and 0-charged hadronazimuthal correlation



Vacuum QCD

Medium effect

  • The away-side correlation strength is suppressed compared to pp and peripheral Au+Au.

  • The -rich sample has lower near-side yield than 0but not zero.

Results: Associated yield with 0 triggerresults

The near-side associated yield of 0trigger is

consistent with that of previous measurements of

ch-ch correlations over different collision systems.

No significant medium effect on the near-side yield.

The away-side yield is continuously suppressed with


The away-side yield of 0trigger is pt-independent at

the same centrality and trigger.

The IAA of 0 triggers and charged hadron triggers are


Results: Associated yield with 0 trigger vs. h trigger results

Surface bias


Central Au+Au

  • Away side: Yields show siginificant medium effect

Completely different data set from different RHIC runs, and different detectors were

involved in the analysis.

This analysis

PRL 97

162301 (2006).

Associated yields per trigger

  • Near side: Yields are similar for p+p, d+Au and central Au+Au

  • 0-charged and charged-charged results are consistent.

Results: Method of extraction away-side yield of direct


Background is dominated

by 0symmetric decay

Ydir+h = 0


Way to estimate systematic errors

Use Pythia to estimate the contribution from other sources and propagate it in Au+Au

Results: Associated yield with direct  trigger results

The associated yield with direct gamma trigger:

agrees with theoretical model predictions.

Shows no associated pt-dependence within the current scaling uncertainty.

Has a value similar to RAA of charged hadrons.

Has a value similar to IAA of 0 triggers and charged hadron triggers.

No sign for the color factor effect.




h  h

h 

IAA , IAA, IAA, RAA and RAA suppress to the same level.

Results: Associated yield of direct  vs. 0 triggers results

Differences between 

and 0 triggers

0 -triggers are resulted from

higher parton energy than


0 -triggers are surface


Color factor effect.

The associated yield with direct  trigger:

Shows smaller yield compared to 0 trigger at the same centrality and zT.

Supporting evidence for direct  production.

The difference between the associated yields with direct  and 0indicates the absence

/less dominant of color factor effect.

Results: Medium effect on the associated yield of direct

Eliminate the effect of the difference in the parton initial energy.

= associated yield per trigger in Au+Au 0-10% / associated yield per trigger in Au+Au 40-80%

The medium effect on the associated yield with direct  agrees with the theoretical prediction.

  • More precision is needed for the measurements to distinguish between different color charge densities.

Within the current uncertainty the medium have similar effect on the away-side of direct  and 0.

Results: Future of direct -jet measurements at STAR

Projection is for ET γ> 15 GeV,

associated particle pT from 4-6 GeV/c.

Luminosity Projections

More to come soon:

  • Improving correlated/uncorrelated systematics.

  • Check for kT effect on direct -jet azimuthal correlation.

  • More statistics with d+Au (Run 8) results to reduce scaling uncertainties.

  • Out-of-plane and in-plane of direct -jet azimuthal correlation.

  • Study the low zT region, low pT associated .

  • Search for the glasma.

Summary and Outlook

  • All results of 0’s near and away-side associated particle yields show reasonable consistency with that of charged hadron triggers within the measured pT trigger and pT associated.

  • Latest result of -jet azimuthal correlations, IAA (pT assoc.) and fragmentation

    function D(zT) in Au+Au at RHIC energy is reported.

  • Associated yield for direct photons is significantly suppressed

  • compared to that of 0 as a reflection of the difference in the parton

  • initial energy. This suppression level agrees with theoretical predictions.

  • IAA and Icp of direct  show neither pT nor zT dependence within the current

  • uncertainties and the measured pT trigger and pT associated.

  • Path length-dependence of energy loss make no significant difference between 0’s and direct -results within the current uncertainty and the measured pT trigger and pT associated.

  • No sign for the color factor effect within the current study.

  • Large luminosity at RHIC enables these measurements. Expect reduced uncertainties from further analysis and future runs, more precise study to come soon.