The STAR Experiment

The STAR Experiment

Texas A&M University A. M. Hamed for the STAR collaboration

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Hot Quarks 2008, 18-23th August, Estes Park Colorado

One more ingredient

for energy loss quantification

Table of Contents:Table of Contents:

The Road Behind

Results

Analysis

Summary

Table of Contents and Disclaimer

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

D. d’Enterria and adapted from T. Schafer

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The multiple facets of QCD

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The Road Behind

J.D.Bjorken 1982

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The Road BehindHigh-pT: Nuclear modification factor RAA of light quarks,

heavy quarks and gluons at mid rapidity!

V~5 fm3 and ~10 fm/c

RAA is a measure of the deviation from the incoherent superposition of nucleon-nucleon collisions assumption.

Nuclear modification factor

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

Hard processes take place at early time of collisions (0.1 fm/c). pQCD

CTEQ6M

Proton Parton distribution functions.RHIC RHIC0.20.01

x2Pt/s

At mid rapidity at RHIC The ratio of quark structure functions

RHICSoft Hard

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RAA of light quarks is pt independent as expected by the radiative energy loss.

PRL. 96, 202301 (2006)

Direct photons follow the binary scaling.

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Unexpected level of suppression for the heavy quarks.

PRL 98 (2007)192301

According to QCD at zero temperature

Equark,m=0 Equark,m>0

Vacuum and medium radiation is suppressed due to quark massDokshitzer, kharzeev. PLB 519 (2001) 199

Dead cone effect

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STAR QM08

No sign for the color factor effect on energy loss.

According to QCD at zero temperature

Egluon Equark

Casimir factor (CF=4/3 “quarks” , CA=3 “gluons” ), i.e 2.25

Gluon should show stronger coupling to the medium.

E CR

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PRL 98 (2007)192301

STAR QM08

PRL. 96, 202301 (2006)

“But nature cannot realize contradictions. Paradoxes focus our attention, and we think harder”

F. Wilczek “Nobel Lecture 2004”

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?

The Road Behind

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F. Wilczek hep-ph/0003183v1

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

Weakly coupled or Strongly coupled medium!

The Road Behind

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Hep-lat/00010027v1

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

Lattice QCD

~20%

pT (GeV/c)

v2

Romatschke & Romatschke, arXiv:0706.1522

v2 of hadrons at RHIC data are in agreement with the ideal relativistic fluid dynamics predictions /s=0-0.8

/s ~ 1 pQCD calculations of a weakly coupled quark gluon plasma.

/s ~ 0.08 is reached in strongly coupled supersymmetric gauge theories.


The Road Behind

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“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

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


The Road Behind

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

Modeling the medium evolution/structure. Hierarchy of scales.

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Differences

On the jet quenching parameter q ^

Different assumptions in various models lead to similar descriptions of the light quark suppression with different model-dependent parameters.

Medium

qT

E

L

Energy loss

ASW and GLV: Similar models different ̂q

AMY and Higher twist: Different models same ̂q

“Static medium”<E> sCRqL2^

Scattering power of the medium ̂q

q k2=2/^ >>1

Pion gas

Ideal QGP

The Road Behind

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On the jet quenching parameter q ^

radiative energy loss If s(T) were weak… q1 GeV2/fm^ Baier Schiff

8-19 GeV2/fm

3 GeV2/fm 4-14 GeV2/fm

PHENIX; at 2, neglecting theoretical uncertainties

Zhang Owens Wang Wong Dainese Loizives Palc

q extracted via comparison with RHIC data is larger…^

q 8 GeV2/fm Armesto,Cauiari Hirono Salgado ^

Strong coupling calculation of q is required !^Nonperturbative calculation is needed !

The Baier plot

Cold nuclear matter

The Road Behind

All four major models utilize factorization:

Extracted from data, but evolution is

perturbative

Expansion in the coupling constant (LO,NLO,NNLO…)

The entire effect of energy loss in concentrated in the

modification of FF

Factorization is used without proof!

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On the pQCD framework

The characteristics time and length scale of the parton-parton interaction is short compared to the soft interactions between the bound partons in the initial

state and to those of the fragmentation process of the scattered partons in the final state.

Factorization validity

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.

Summary

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RAA saturates!

If the medium is black somewhere already, you can’t see it getting even blacker.

•at some point, large changes in q do not map into large changes of RAA, or:^

The Road BehindSingle particle spectra and di-hadron azimuthal correlations

Single jet

Dijet

No Glauber calculation is required for the suppression measurements.Different geometric bias and different fragmentation bias.

Di-hadron azimuthal correlations

PRL 98 (2007) 212301

Model dependent calculations show that IAA is more sensitive than RAA but both have diminished sensitivity at

high gluon density.

IAA is a quantity that measure the medium effect on the FF on dijet analysis.

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High-pT: di-hadron azimuthal correlations “conservation of p”

Away side In the near-side p+p, d+Au, and Au+Au are similar while in the away-side

“back-to-back” Au+Au is strongly suppressed relative to p+p and d+Au.

4 < pT,trig < 6 GeV/c

2 < pT,assoc < pT,trig

PRL. 91, 072304 (2003)

Background is subtracted

The Road Behind

Away-side yield neither depend on zT nor broaden in .

Clear jet-like peaks seen on near and away side in Au+Au at high trigger pT and high associated pT

Away-side yield strongly suppressed to level of RAA

An access to the parton initial energy is required in order to better quantify the energy lost

Surface bias free probe is needed

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Jet-energy calibration “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 not a surface bias probe.

The Road Behind

Examples of higher order diagrams

Examples of Bremsstrahlung diagrams

Compton Annihilation

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Direct photon

The Road Behind

Direct photon-hadron correlationsDirect photon energy balances the outgoing parton.

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

Challengeable measurements!

Photon doesn’t couple to the medium.

Possible candidate for quark/gluon jet discrimination.

0 is suppressed at high pT by a factor of ~5 in central AuAu collisions.

O(ααs)

O(αs2α(1/αs+g))

O(ααs2)

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Analysis technique

Build correlation function for neutral “triggers” with “associated” charged particles

Use transverse shower profile to distinguish 2-photon from single-photon showers

Comparison of 0 – triggered yields with previously measured charged-hadrons- triggered yields.

Extract the yields associated with direct photon triggers

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Correlate photon candidate “triggers” with “associated tracks”

Use triggers to explore

fragmentation functions in p+p and Au+Au

0

2

Eπ ‹ E

parton

0

BEMC

Beam

axis

TPC

Analysis technique

pT,trig > 8 GeV/c

180°

Eγ = Eparton

Associated charged particles

“3 <pT,assoc < 8 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)

~2.2

m

Charged hadrons

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

Analysis: Shower Shape Analysis

i : strip energy

ri : distance relative to energy maxima

7 RM

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”

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STAR Preliminary

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

Trigger photons-charged particles azimuthal correlations

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Results: Effect of shower-shape cut

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.

-sample is not pure direct ! How about the 0 ?

Vacuum QCD

Centrality Centrality

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Comparison of 0-triggered yields to charged-hadron triggered yields

Completely different data set from different RHIC runs, different detectors were involved in the analysis too.

Associa

ted

yie

lds p

er

trig

ger

0-charged and charged-charged results are consistent.

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

Central Au+Au

?

Surface bias

0 sample is pure.

PRL 97 162301 (2006).

This analysis

Away side: Yields show big difference between p+p and central Au+Au

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0

Extraction of direct away-side yields

R=Y-rich+h/Y0+h

near near

Y+h = (Y-rich+h - RY0+h )/(1-R)away away

Assume no near-side yield for direct

then the away-side yields per trigger obey

Method of extract direct associated yield

This procedure removes correlations due to contamination (asymmetric decay photons+fragmentation photons) with assumption that correlation

is similar to 0 – triggered correlation at the same pT.

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Direct 0

Associa

ted

yie

lds p

er

trig

ger

Fragmentation function of direct triggers and 0 triggers

The away-side yield per trigger of direct triggers shows smaller value compared to 0 triggers which is consistent with

partons loose energy “dense medium” and then fragment.

Differences between and 0 triggers

0 -triggers are resulted from higher parton energy than

-triggers.

0 -triggers are surface biased.

Color factor effect.

What is the medium color charge density?

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Icp agrees with theoretical predictions.

Results: Medium effect on fragmentation function

Icp(zT) =D0-10% (zT)

D40-80% (zT)

STAR Preliminary

7 < pT < 9 GeV/ctrig

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

STAR Preliminary

Within the current uncertainty in the scaling the Icp of direct and 0 are similar.

If there is no medium effect

Icp(zT) = 1

Strong medium effect

IAA(zT) =DAA (zT)

Dpp (zT) 8 < pT < 16 GeV/ctrig

pT > 3 GeV/cassoc

Data points

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First result of -jet azimuthal correlations and fragmentationfunction D(zT) in AuAu at RHIC energy is reported.

All results of 0’s near and away-side associated particle yields shows consistency with that of charged hadron triggers.

Summary and Outlook

Large luminosity at RHIC enables these measurements. Expect reduced uncertainties from further analysis and future runs.

Away-side yield for direct photons is significantly suppressed in heavy ion events. Suppression level agrees with theoretical

expectations.

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Thank you for your attention and

thanks to all STAR Collaborators

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Shower Shape Cuts:Reject most of the 0’s.

highly asymmetric 0 decay.

But do not reject photons from:

’s - similar level of background as asymmetric 0

fragmentation photons

10% of all 0 with pT > 8 GeV/c

10% of inclusive at intermediate pT in p+p~30-40% of direct at PT > 8 GeV/c.

Limitations of the shower shape cut

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Breakdown of factorization claimed in dijets at N3LO Collins, Qiu ‘07

Measurement of the differential cross section for the production of an isolated photonwith associated jet in p¯p collisions at √s =1.96 TeV

arXiv:0804.1107v2

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The STAR Experiment