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Probing the medium with photons

Outline:Outline:

oMotivation

oExperiment

oResults

oConclusion

oIntroduction

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Saskia MioduszewskiAhmed Hamed

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Probing the medium High-pT Spectra I – Light quarks and gluons Probing the medium High-pT Spectra I – Light quarks and gluons

The suppression of 0,s and ,s is

very similar.suppression occurs at the parton level.

The binary scaling of

direct photons is strong

evidence that suppression is not

an initial state effect.

Mid-rapidity

Statistical Method

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<E> sCxqL^ 2 “Static medium”

yT es

px 2

~ Gluons dominance at mid-rapidity at RHIC energy.

PhotonsPHENIX, QM05

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QCD is flavor independent, but

heavy quarks at same pT are moving

much slower than light quarks.

Expected “dead-cone” with no induced

gluon radiation.

Non photonic electrons-Charm and Beauty

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Probing the medium High-pT Spectra II– Heavy quarks and gluons Probing the medium High-pT Spectra II– Heavy quarks and gluons

osingle-particle suppression does not constrain the mechanism of energy loss.

osingle-particle suppression in AuAu is strong evidence for the hot and

dense medium formation.

nucl-ex/0607012

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Baryons and Mesons

Clear meson-baryon yield differences at intermediate pT .

Probing the medium High-pT Spectra III– quarks and gluons Probing the medium High-pT Spectra III– quarks and gluons

No reduction is observed in the baryon/meson ratio as expected in the gluon dominance picture.

Calibrated probe of the QGP is needed for better understanding of energy loss.

STAR QM05 and nucl-ex/0601042

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?

Near-side: p+p, d+Au, Au+Au

is similar.Back-to-back: Au+Au

strongly suppressed relative to p+p and d+Au.Suppression of the

back-to-back correlation in central Au+Au is a final-state

effect

Probing the medium Jet-like azimuthal correlationsProbing the medium Jet-like azimuthal correlations

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Surface bias for the trigger particle.

Trigger particle with no surface bias is required for better quantitative measurements of the away-side modifications.

Charged hadrons

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Introduction SummaryIntroduction Summary

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Better understanding for the energy loss mechanism!

Elliptic flow.

oDirect Photons:

Gamma-charged hadrons correlation.

Four multipurpose experiments (BRAHMS, PHENIX, PHOBOS, STAR) Empirical lines of evidence:

Energy density well beyond critical value.Large elliptic flow. Jet quenching. dAu control experiment.

Interpreted in terms of a strongly coupled QGP and a new QCD state (?) Color Glass Condensate

Required:

Challengeable measurements!

Doesn’t couple to the medium.QGP thermal photons.

Test for binary scaling for hard process.

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Motivation LO Direct photonsMotivation LO Direct photons

Bremsstrahlung

fragmentation component

direct component

Decay photons

nT

1

phard:

/ E Tethermal:

schematic view

Calibrated probe of the QGP – at LO.

No Surface Bias Hard process

single-particle suppression does not effectively constrain detailed energy-loss pictures.

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Possible candidate for quark/gluon jet discrimination at LO.

oGamma-charged hadrons correlation.

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Experiment STAR DetectorExperiment STAR Detector

Tracker detectors(slow), Trigger detectors(fast), and Calorimeters(fast).

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Measurements of hadrons production over a large

solid angle.

STAR BEMC can probe for further higher transverse

energy.

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Experiment STAR BEMCExperiment STAR BEMCC

ross-s

ecti

on

in

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o120 modules.

o4800 channels

oSMDs: 36000 channels

oPreShower: 4800 channels

oLead-scintillator detector.

oSampling calorimeter.

oProjective towers.

Cro

ss-s

ecti

on

in

BEMC face is

~2.2m away

from the point of interaction

at =0.

West side 0<<1

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Experiment Electromagnetic ShowerExperiment Electromagnetic Shower

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-plan

e

-plane

E M C M o d u le

= 0

= 1+

S h o w e r M a xd e te c to r

To w e r

w ire s

w ire s

P re S h o w e r

A . A . P. S u a id e

High energy core.Low energy halo.

Electromagnetic transverse shower characteristics

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Inclusive-jet in Au+Au at s=200GeV

Thomas DietelQuark Matter

2005

Results QM 2005Results QM 2005

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SIMULATION (pp)

The background is higher for central events.Away-side decreases with increasing centrality.Decrease in near-side due to the increased

fraction of prompt photons.Need /0 discrimination.

STAR Preliminary

Simulation shows no associated particles in -charged correlation.

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Results QM 2006Results QM 2006

S. ChattopadhyayQuark Matter

2006

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Reduction in near angle peak towards photon Bin.Effect is more prominent for larger Et

trigger .Away-side yield is reduced.

pp at s=200GeV

STAR Preliminary

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Results Transverse Shower ProfileResults Transverse Shower Profile

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Clear structure for the two showers in and 0

at moderate energy.

0 shower at high energy is still wider

than the single photon shower.

Could be used to distinguish 0/

Clear sensitivity to the halo region.

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cucu 0-10%

Et_trg>12GeV

Et_trg>6GeV

/c

/c /c

(rad)

(rad)

(rad)

(rad)

/c0

Results Raw correlation functionResults Raw correlation function

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Preliminary

Similar Away-side for 0 and

Reduction in the near-side for compared to 0.

Reduction is more noticeable at higher Et_trg

and also at higher centrality bins.

d

1

Ntrg

dN()

Y-axis:

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Conclusion Conclusion

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-charged hadrons correlation is very promising tool for better understanding of the medium.

Shower shape study is required for direct photons identification.

Promising study for transverse shower profile is undertaken.

Comparison with the previous study of transverse shower profile is necessary.

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Thank you all

Thanks to Texas A&M nuclear physics group.

Thanks to all STAR Collaborators