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: Why study high-pt particles?
PDF: Extracted from data but
evolution is perturbative “DGLAP"
Hard scattering: Expansion in coupling
constant (LO, NLO, NNLO, ..)
NB: Factorization used in many
context without proof
Breakdown of factorization claimed in
dijets at N3LO, Collins, Qiu ,07
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”
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
The Road Behind: Theoretical models “radiative energy loss”
If s(T) were weak…
q extracted via comparison with RHIC data is larger…
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
What happens to empty space, if you keep adding heat?
“Static medium, Energy dependence”
dErad/dz CR s E <q2>
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) <q2>
Where is the q ?
At T >> Tc : gT >> QCD
: dynamical scale of the medium, color screening scale “mass”, 1/ color screening length
In particular, chiral symmetry is restored, and confinement comes completely undone.
Independent successive scattering centers
Scattering power of the medium
The parton propagation is “time-ordered” and time-oredered
perturbation theory is the natural framework to calculate the
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 Road Behind: Direct -jet azimuthal correlation
zero near-side yield
for direct photons
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:
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 q0.
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”
RAA 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
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
0 is suppressed at high pT by a factor of ~5 in central
Analysis: Analysis technique
with “associated” charged particles
from single-photon showers “rich”
h triggered yields.
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
in p+p and Au+Au
particles “3 <pT< 16 GeV/c”
pT,trig > 8 GeV/c
Charged hadrons 3 <pT < 16 GeV/c
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)
Etower > 6 GeV,
Ecluster > 8GeV,
Esmd > 0.5 GeV,
Cluster is away from
the tower edge
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
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
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
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
Completely different data set from different RHIC runs, and different detectors were
involved in the analysis.
Associated yields per trigger
Results: Method of extraction away-side yield of direct
Background is dominated
by 0symmetric decay
Ydir+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.
IAA , IAA, IAA, RAA and RAA suppress to the same level.
Results: Associated yield of direct vs. 0 triggers results
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.
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.
More to come soon:
Summary and Outlook
function D(zT) in Au+Au at RHIC energy is reported.
Thank you for your