The STAR Experiment
Texas A&M University A. M. Hamed for the STAR collaboration
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High-pT physics at LHC 2009, High-pT physics at LHC 2009, 4-7th February, Prague, Czech Republic4-7th February, Prague, Czech Republic
Table of Contents:Table of Contents:The Road Behind
Results
Analysis
Summary
Table of Contents and DisclaimerTable of Contents and Disclaimer
Disclaimer: The road behind is personal view, so Disclaimer: The road behind is personal view, so biases and mistakes are expected. biases and mistakes are expected.
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P
P
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Hard Scattering:
Heavy-ion collisions
e-6pT
Power law
At mid-rapidity
p+p
High-pT particles are produced from the hard scattering processes.
Take place at early time of collisions (~0.01 fm/c). V~ 5 fm3 and ~ 10 fm/c
xP xP
High momentum
transfer
High-pt Particle“biasedevent”
Their rates are calculable via pQCD.
The Road Behind: Why study high-pt particles?
Factorization
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
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1. Jet reconstruction
Glauber model uncertainties, Parton distribution functions , CGC.
“ An interesting signature may be events in which the hard collision occurs near the edge of theoverlap region, with one jet escaping without absorption and the other fully absorbed”
J.D.Bjorken 1982
2. Inclusive single high-pt particle spectra- Leading particle method “RAA”
Detection efficiency, quark- versus gluon-jet properties, Jet-mass effects. The jet modifies the medium as well as the medium quench the jet.
“elastic scattering?”
dAu control experiment
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: Methods for high-pt particles
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.
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!
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
No sign for the color factor effect on energy loss. E CR Egluon Equark
Unexpected level of suppression for the heavy quarks. Equark,m=0 Equark,m>0
The Road Behind: Exp. results of high-pt particles “RAA, IAA”
IAA is zT independent and there is no broadening in the associated correlation peak .
The four major models use pQCD framework to estimate energy loss.
Modeling the medium evolution/structure.Hierarchy of scales: E, Q,
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Differences
Different assumptions in various models lead to similar descriptions of the light quark suppression with different model-dependent parameters.
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: Theoretical models “radiative energy loss”
LPM-effect based approaches: BDMPS & AMYOpacity expansion: GLV & ASWMedium enhanced higher twist effectsMedium modified MLLA
ASW and GLV: Similar models different q̂
AMY and Higher twist: Different models same q̂
Ways of extrapolation of pQCD in non-pQCD regime, just make s small Data doesn’t allow to distinguish between q 5 or 15 GeV2/fm^
radiative energy loss If s(T) were weak… q1 GeV2/fm^
8 GeV2/fm Armesto,Cauiari Hirono Salgado
8-19 GeV2/fm PHENIX; at 2, neglecting theoretical uncertainties
3 GeV2/fm Zhang Owens Wang Wong
4-14 GeV2/fm Dainese Loizives Palc
q extracted via comparison with RHIC data is larger…^
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).
What happens to empty space, if you keep adding heat?What happens to empty space, if you keep adding heat?
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In particular, chiral symmetry is restored, and confinement comes completely undone.
Hep-lat/00010027v1
F. Karsch, E. Laermann, A. Peikert, CH. Schmidt, S. Stickan
Lattice QCD
~20%
The Road Behind: Time-ordered
“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”F. Wilczek
Energy loss
Scattering power of the medium q̂ q2=2/
Independent successive scattering centers
Medium
q
E
L
At T >> Tc : gT >> QCD
: dynamical scale of the medium, color screening scale “mass”, 1/ color screening length
form << Bethe-Heitler limit, E is fixed and
form >> LPM limit, E and >> 1
dErad/dz CR s E <q2>
dErad/dz CR s ln(E) <q2>
Relative phase:“Static medium, Energy dependence”
Where is the q ?^
1/ <<
The parton propagation is “time-ordered” and time-oredered perturbation theory is the natural framework to calculate the radiation amplitude.
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.
IMHO
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Jet-energy is calibrated by “Direct ”
“Mid-rapidity”
P
P
Fast Detector“Calorimeter”
Leading particle“trigger”
xP xP
Associated particles
Background
Due to fragmentation full jet reconstruction is required to access the initial parton energy
0
OR
xP xP
P
P
Direct photon“trigger”
Fast Detector“Calorimeter”zero near-side yield
for direct photons
get the initial parton energy with a powerful alternative method:
“Direct -hadron azimuthal correlations”
Direct photon is a surface bias free probe.Heavy ion collision
The Road Behind: Direct -jet azimuthal correlation
Compton Annihilation
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Direct photon: photons unaccompanied by additional hadrons
O(αs)
Examples of Bremsstrahlung diagrams
O(αs2)
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.
Photon Bremsstrahlung
LO are the dominant processes:
The Road Behind: Direct production mechanisms
10% of inclusive at intermediate pT in p+p “PHENIX”!~30-40% of direct at PT > 8 GeV/c in p+p “NLO pQCD Vogelsang”,
contribution is suppressed by a factor of with respect to single-0 rate. This suppression is offset somewhat: q fragmentation is flatter than q0.
takes only a fraction of the proton's momentum.
Sources of suppression and enhancement of direct photons yield.R
AA follows binary scaling RAA saturates “pt-independent” LPM effect
Heavy ion collisions
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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:
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)
The Road Behind: Direct -jet production mechanisms
D0: NLO pQCD is unable to describe the shape of the pT dependence of the across four
Different kinematic regions simultaneously . arXiv:hep-ex/0804.1107Effects due to intrinsic motionEnhancement of single-particle and jet cross sections due to the parton transverse momentum at moderate pT.
Direct photon-hadron correlationsDirect 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 AuAu collisions.
Challengeable measurements!
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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 the fact that direct
photon has no near side yield and assuming all sources of background have similar correlations
to that of symmetric decay 0.
0rich
Analysis: Analysis technique
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Correlate photon candidate “triggers” with “associated tracks”
Use triggers
to explore fragmentation functions
in p+p and Au+Au0
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Eπ ‹ E
parton
0
BEMC
Beam
axis
TPC
pT,trig > 8 GeV/c
180°
Eγ = Eparton
Associated charged particles “3 <pT< 16 GeV/c”
How to distinguish between 0/ ?
BEMC: Barrel Electro-Magnetic Calorimeter
TPC: Time Projection Chamber
Full azimuthal coverage
No track with p > 3 GeV/c points
to the trigger tower
One tower out of 4800 towers (0.05 x 0.05)
~2.2m
Charged hadrons 3 <pT < 16 GeV/c
|bemc| < 1, |bemc selected| < 0.9 |TPC| < 1.5, |TPC selected| < 1
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)
Track quality, eff.
Offline trigger:Etower > 6 GeV, Ecluster > 8GeV, Esmd > 0.5 GeV, Cluster is away from the tower edge
Event general QA
Analysis: -jet azimuthal correlation in STAR
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The two photons originated from 0 hit the same tower at pT>8GeV/c
i : strip energyri : distance
relative to energy maxima
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0
Use the shower-shape analysis to separate the two close photons shower from one photon shower.
STAR Shower Maximum Detector is embedded at ~ 5x0 between the lead-scintillator layers “BEMC”
Frag. Photons, asymmetric decay of pi0, and eta?
The tower energy asymmetry cut to purify the rich sample in case of 0 decay across the module in
Analysis: Transverse shower shape analysis in STAR
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STAR Preliminary
Near side is suppressed with centrality which might due to the increase of /0 ratio .
Data Set: L=535 ub-1 of Au+Au and L=11 pb-1 of p+p
Both near side yield and away side increase with trigger energy as the initial parton energy increases.
Background level increases with centrality as expected
Clear dijet structure is seen for inclusive – charged hadron azimuthal correlation in STAR
Near and away side yields decrease with associated pt: the jet cross section falls more steeply than the Fragmentation Function does.
STAR Preliminary
Results: inclusive -charged hadrons azimuthal correlation
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oThe away-side correlation strength is suppressed compared to pp and peripheral Au+Au.
Medium effect
oThe -rich sample has lower near-side yield than 0 but not zero.
Vacuum QCD
Centrality Centrality
Results: rich and 0-charged hadron azimuthal correlation
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The near-side associated yield of 0 trigger is consistent with that of previous measurements of ch-ch correlations over different collision systems.
The IAA of 0 triggers and charged hadron triggers are similar.
No significant medium effect on the near-side yield.
The away-side yield is continuously suppressed with centrality.
The away-side yield of 0 trigger is pt-independent at the same centrality and trigger.
Results: Associated yield with 0 trigger results
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Completely different data set from different RHIC runs, and different detectors were involved in the analysis.
Asso
ciat
ed y
ield
s pe
r trig
ger
0-charged and charged-charged results are consistent.
Near side: Yields are similar for p+p, d+Au and central Au+Au
Central Au+Au
?
Surface bias
PRL 97 162301 (2006).
This analysis
Away side: Yields show siginificant medium effect
Results: Associated yield with 0 trigger vs. h trigger results
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0
Background is dominated by 0 symmetric decay
Ydir+h = 0NS
Use Pythia to estimate the contribution from other sources and propagate it in Au+Au
Results: Method of extraction away-side yield of direct
Way to estimate systematic errors
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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.0hh h h h 0
Results: Associated yield with direct trigger results
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Differences between and 0 triggers
0 -triggers are resulted from higher parton energy than
-triggers.0 -triggers are surface
biased.
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 0 indicates the absence /less dominant of color factor effect.
Results: Associated yield of direct vs. 0 triggers results
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= 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.
Eliminate the effect of the difference in the parton initial energy.
Within the current uncertainty the medium have similar effect on the away-side of direct and 0.
More precision is needed for the measurements to distinguish between different color charge densities.
Results: Medium effect on the associated yield of direct
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Out-of-plane and in-plane of direct -jet azimuthal correlation.
Results: Future of direct -jet measurements at STARLuminosity Projections
Projection is for ET γ> 15 GeV, associated particle pT from 4-6 GeV/c.
Study the low zT region, low pT associated .Search for the glasma.
More to come soon:
Improving correlated/uncorrelated systematics.
More statistics with d+Au (Run 8) results to reduce scaling uncertainties.Check for kT effect on direct -jet azimuthal correlation.
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Latest result of -jet azimuthal correlations, IAA (pT assoc.) and fragmentation
function D(zT) in Au+Au at RHIC energy is reported.
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.
Large luminosity at RHIC enables these measurements. Expect reduced uncertainties from further analysis and future runs, more precise study to come
soon.
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.
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.
IAA and Icp of direct show neither pT nor zT dependence within the current uncertainties and the measured pT trigger and pT associated.
Summary and Outlook
No sign for the color factor effect within the current study.
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Thank you for your attention
