1 34th International Conference on High Energy Physics (ICHEP 2008) The STAR Experiment Texas A&M University A. Hamed for the STAR collaboration Direct

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34th International Conference on High Energy Physics (ICHEP 2008)

The STAR Experiment

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

Direct -charged hadron

azimuthal correlation

measurementsTable of Contents:

Introduction

Results

Analysis

Summary

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A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29th -August 5th.A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29th -August 5th.

Introduction: Degrees of freedom

The space-time evolution of a relativistic heavy-ion collision

One of the most important characteristics is the medium color charge density, which might lead to understand the medium dynamics.

Nuclear density 0 ~ 0.15nucleons/fm3

Specific volume ~ 6fm3

Typical hadronic volume ~ 1-3 fm3

The average inter-Nucleon distance in the nucleus ~ 1.8fm

One must expect in case of nuclear density greater than 30 the nucleons to overlap,

and their individuality to be lost. J.C. Collins, M.J. Perry, Phys. Rev. Lett. 34 1353(1975).

Ordinary nuclear matter

How to probe the color charge density?

Small size “ a few fermi in

diameter”

Very short life time

5-10x10-23 s.

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Central Au+Au

Gluon radiation is induced

by multiple scattering

A particle distribution in fractional energy is

softened in the medium


Introduction: Hard probes- I

Like QED the charge density of the medium can be probed by its effect on the propagation of a fast particle.

Hard processesTake place at early time of collisions a good probe of the

medium.

How the hard probes can be used to measure the modification on the FF?

p+p or peripheral

Au+Au

Hard Scattering in vacuum-QCD

Hard Scattering in the mediumCompare versus

.

Dm

ed(z

, P(

E,E

))

h/a

Fragmentation Function Study the particles distribution in fractional energy. D vac

h/a(z)

Very short life time

medium

4


Introduction: Hard probes - II

Jet-like azimuthal correlations “conservation of linear momentum”

p+p di-jet

Trigger

An access to the parton initial energy is required to quantify the energy lost

Associated particles

Near side 0

Away side

TriggerAssociated particles

Au+Au ?

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

Central Au+Au

?

Eparton

Etrigger hadron

How much energy is

lost in here?

conservation of linear momentum

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Introduction: Jet-energy calibration “Direct ”


0

“Mid-rapidity”

P

P

Fast Detector“Calorimeter”

Leading particle“trigger”

xP xP

Associated particles

Background

How much energy is lost in the medium?

FF is softened in the

medium

No access to the parton initial energy

Color charge density?

get the initial parton energy with a powerful alternative method:

“Direct -hadron azimuthal correlations”

How to measure direct -hadron azimuthal correlations?

Due to fragmentation full jet reconstruction is required to access the initial parton energy

OR

zero near-side yieldfor direct photons

Direct photons escape from the

medium without any further interactions

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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: 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

7

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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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

Background is not subtracted

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Results: 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.

Ass

ocia

ted

yie

lds

per

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


Results: 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.

O(αs2α(1/αs+g))

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Direct 0

Ass

ocia

ted

yie

lds

per

trig

ger


Results: 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 many thanks to

all STAR collaborators

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Backup slides

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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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-jet yieldAway-side hadrons

Phys. Rev. C74 (2006) 034906

ET > 15 GeV

More precision is required to nail down the medium density

PRL 98 (2007) 212301

Projection for statistical uncertainties in γ-hadron suppression

as the integrated luminosity increases. Projection is for ET γ> 15 GeV, associated particle pT from 4-

6 GeV/c.A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29th -August 5th.A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29th -August 5th.

Luminosity Projections

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0

7 RM

Two photons (0) produce a more diffuse shower than single photons ()

∑i i ri1.5

EtotalWider shower has small value of such quantity

i : strip energy

ri : distance relative to energy maxima

Two dimensional shower shape

Very pure sample of 0

Shower shape cut for 0

selection is not tight rich sample

Shower shape cut for

On the transverse shower profile cut


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Away-sideY-rich+h=1/N-rich(N-rich+h)=1/N-rich [N0+h+N+h]

=(N0+h/N-rich)+(N+h/N-rich)

Y+h=(N-rich/N )[Y-rich+h-(N0/N-rich)*Y0+h]

=(N0/N-rich)*N0+h/N0+(N/N-rich)*N+h/N

Y0+h Y+h

1

Near-side

Y-rich+h=(1/N-rich)*N-rich+h =(1/N-rich)*N0+h

N+h=0

=(N0/N-rich)*Y0+h

Solve for Y+h

3 unknowns N-rich, N0, and N.

N0/N-rich=Y-rich+h/Y0+h=R

Y+h=(N-rich/N)[Y-rich+h-R*Y0+h]

1-R=1-(N0/N-rich)=(N-rich-N0)/N-rich=N/N-rich 1/(1-R)=N-rich/N

Substitute in 1 2

Substitute in 2 Y+h=[Y-rich+h-R*Y0+h]/1-R 3(N-rich-N0)=N


Method of extract direct associated yield

Documents

1 34th International Conference on High Energy Physics (ICHEP 2008) The STAR Experiment Texas A&M University A. Hamed for the STAR collaboration Direct