hard probes 2008, june 8-14th, galicia, spain

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Hard Probes 2008, June 8-14th, Galicia,

Spain

A. Hamed Hard Probes 2008

The STAR Experiment

Direct -charged hadron azimuthal correlation measurements

Direct -charged hadron azimuthal correlation measurements

Texas A&M University

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Outline

Discussion on energy loss observables

Analysis technique outline

Direct results

Summary and outlook

Luminosity projection

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A. Hamed STAR Experiment HP2008 Spain 8-14th JuneA. Hamed STAR Experiment HP2008 Spain 8-14th June

= RAA of light quarks, heavy quarks and gluons!TAA . d2/dydpT

d2NAA/dydpT

But nature cannot realize contradictions. When our physical theories lead to paradox we mustfind a way out. Paradoxes focus our attention, and we think harder. F. Wilczek “Nobel Lecture 2004”

No sign for the color factor effect on energy loss.

Unexpected level of suppression for the heavy quarks.

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

PRL 98 (2007)192301

STAR QM08

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.

PRL. 96, 202301 (2006)

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IAA from di-hadron “charged” azimuthal correlations

Direct comparison to the fragmentation functions in vacuum and in medium

The ratio DAuAu/DdAu is independent of zT for zT > 0.4 and has similar magnitude to RAA.

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

,

Access to parton initial energy Direct photon + jet provides probe to measure the fragmentation function

PRL 98 (2007) 212301

PRL 97 (2006) 212301

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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


Analysis technique

0

2

Eπ ‹ E

parton

0


Analysis technique

BEMC

Beam

axis

TPC

No track with p > 3 GeV/c points to the trigger tower

STAR Detector180°

Eγ = Eparton

Associated charged particles

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Use Transverse Shower Profile to distinguish between /0.

0.1


STAR BEMC and BSMD

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

Cross section in

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A. Hamed STAR Experiment HP2008 Spain 8-14th juneA. Hamed STAR Experiment HP2008 Spain 8-14th june

On the Transverse Shower Profile Cut

0

7 RM

∑i i ri1.5

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

Wider 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

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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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oThe -rich sample has lower near-side yield than 0 but not zero.

Effect of Shower Shape Cut


Applying Shower Shape Cut

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Limitations of the shower shape cut

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.

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

PRL 97 162301 (2006).

The zT dependence of per trigger yields in near- and away-side correlation peaks of

0-charged and charged-charged correlations are very similar.

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IAA of the away-side in 0-trigger shows similar suppression for all pt associated bins (3-8GeV/c).


Centrality dependence of IAA of 0 triggers

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Extraction of direct away-side yields

R=Y-rich+h/Y0+h

near near

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

Assume no near-side yield for direct

then the away-side yields per trigger obey


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.

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"T. Renk and K. Eskola, Phys.Rev.

C75:054910,2007"

"X-N Wang & H. Zhang

et al (to be published)"


Centrality dependence IAA of direct triggers.

Within the current uncertainty in the scaling due to p+p measurements,

the IAA of direct and 0 are similar.

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Icp of -jet exhibits similar suppression of the away-side yield per trigger for associated particles of 3-8GeV/c.


Centrality dependence of ICP of direct triggers

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D(ZT) of direct triggers vs. 0 triggers

The away-side yield per trigger of direct triggers shows smaller value compared to 0 triggers.

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D(ZT) of direct triggers vs. 0 triggers

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

STAR Preliminary

ICP of direct triggers vs. theories

STAR Preliminary

Calculations for 7 < pT < 9 GeV/ctrig

Icp agrees with theoretical predictions.

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Luminosity Projections

-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.

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Summary and Outlook


Summary and Outlook

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.

Shower shape analysis is effective for /0 discrimination.

The IAA of -jet agrees with the theoretical predictions.

Within current systematic uncertainties, -triggered IAA is similar to RAA of inclusive hadrons and is similar

to 0’s IAA.Direct photon-hadron correlations are moving from the proof of

principle stage towards the stage of precision measurement.

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Thank you for your attention and

many thanks to all STAR collaborators

hard probes 2008, june 8-14th, galicia, spain

Documents

star experimentdirect

star qm08prl

diffuse shower

juneeffect of shower

single photons wider

trigger photons

juneapplying shower

pt independent