PHENIX measurements of PHENIX measurements of reaction plane dependence of reaction plane dependence of high phigh pTT photons and pions in photons and pions in

Au+Au collisionsAu+Au collisions

Vladislav Pantuev, University at Stony Brook

for PHENIX collaboration

Outline:

Part 1. Direct photons

• Motivation

• Method

• Results

Part 2. High pT neutral pion suppression and reaction plane angular dependence

• RAA and Partial energy loss

• Reaction plane dependence

• Possible explanation

Conclusion2

Part 1. Direct photons. Motivation

Why in this case Why in this case we are looking for we are looking for any angular any angular dependence?dependence?

High-High-ppTT direct photons direct photons

produced in initial hard produced in initial hard parton-parton scatteringsparton-parton scatterings

Low Low ppTT thermal photons thermal photons

expected to reflect the expected to reflect the initial temperature of the initial temperature of the fireballfireball

Photons leave the Photons leave the subsequently produced subsequently produced medium unalteredmedium unaltered

Leading Particle

Direct

Hadrons

gq

frag.

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There are theoretical predictions with a There are theoretical predictions with a sizable azimuthal parameter v2 :sizable azimuthal parameter v2 :

S.Turbide, C.Gale, R.J.Fries, PRL 96 032303 (2006)

R. Chatterjee et al., PRL 96, 202302 (2006)

Thermal photons are produced throughout the expansion history and reflect quark anisotropy

Jets lose more energy where the medium is thicker - more jet-photon conversions (v2<0), + photons from fragmentation of quenched jets (v2>0)

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How does PHENIX measure direct photons and v2?

Measure inclusive photon yield, Ninc

Measure hadron contribution components 0 and By Monte Carlo calculate and decay background in

inclusive sample, Nbg

Calculate direct photon excess over hadron decays, R Measure inclusive and hadron v2 by reaction plane

method Calculate direct photon v2 as

R * v2inc – v2

BG

R = N inc / NBG

v2dir =

R – 1

Use large statistics Run 4 Au+Au data set

See also poster 86 by Kentaro Miki 5

Step by step calculations:

PHENIX preliminary

PHENIX preliminary

PHENIX preliminary

PHENIX preliminary

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PHENIX direct photon v2 result

Within statistical and systematic errors v2 is consistent with

zero. v2=0 or cancellation of different contributions?

Systematic errors dominantly from R:

Enormous background of decay photons at low pT

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Part 2. Neutral pion suppression and azimuthal anisotropy

Suppression of high pt pions is in favor of energy loss of hard partons

On the other hand, this canonical view is in trouble to describe heavy quark suppression

Clearly, inclusive particle spectra are not sufficient to validate or exclude different models

Centrality dependence of RAA is an effective estimator of path length dependence for energy loss, but

It is more precise to vary path length keeping the same medium conditions: select different angles versus the reaction plane of the event

See PHENIX paper nucl-ex/0611007, submitted to PRC8

PHENIX Run 2 final 0 results.Level-2 trigger data – factor 3 more statistics

Nuclear modification factor:

Systematic error

Factor of ~5 0 suppression at high pT

Approximately constant with pT

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PHENIX, nucl-ex/0611007, submitted PRC

Suppression as spectrum shift: shift in momentum by fractional energy loss of primary parton

At high pT spectra are linear

and parallel in log-log scale

Power law shape, ~PT-n

More details in nucl-ex/0611007, submit PRC

Can calculate average fractional parton energy loss:

Original momentum of parton:

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In agreement with model estimations by:

Calculate Sloss from RAA :

In average, parton loses 15-20% of its original energy in most central Au+Au events

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Pion absorption versus angle w.r.t. reaction Pion absorption versus angle w.r.t. reaction plane – plane – other way to control thickness of the other way to control thickness of the mediummedium

Centrality 40-50%At fixed centrality change parton path length by varying and keeping the same :

•Initial conditions

• Longitudinal and transverse expansion

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The results:

3 < pT < 5 GeV/c 5 < pT < 8 GeV/c

-RAA in plane and out of plane changes by factor ~2

- For peripheral bins no suppression in plane, while a factor ~2 out of plane

PHENIX Run2, nucl-ex/0611007, submitted PRC

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We vary path length by centrality and angle , both results should agree

Colors represent different centralities

RAA is universal function of L

Sloss is universal and linear with L

RAA~1 and no energy loss for L < 2 fm

Variable 1 : simply L, distance

from the center of interaction region to the edge

60-70%

10-20%

Flow contribution up to 8 GeV/c?

Cronin effect?

Something else?

See D. Winter poster 47, Run4 data, preliminary 14

Surprising, variables like L, L2, Lxy (see

details in nucl-ex/061107) do not work so well:

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Why no absorption? Alternative explanation: Time matters!

Let jets fly in ANY direction:

Ncoll distribution in transverse plane, Glauber + Woods-Saxon

Stop jet after some time T.

T =2.3 fm/c to fit peripheral data

see my poster 32 and hep-ph/0506095

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The result:

Results of calculation

Describes inclusive RAA and dependence

v2=11% at high pT,

Simple explanation of lack of absorption in a layer < 2 fm,

some other features

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From V.P. hep-ph/0506095

Conclusions:Conclusions:

Within errors direct photon azimuthal asymmetry is consistent with zero

As in previous papers we observe a factor of ~5 0 suppression at high pT, approximately constant with pT

We see factor ~2 suppression for out of plane compared to in plane

RAA~1 or there is no jet absorption if the medium size is less than 2 fm

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Twice better reaction plane resolution in Twice better reaction plane resolution in upcoming Run 7 with new reaction plane upcoming Run 7 with new reaction plane

detectordetector

MC simulation

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backupbackup

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Green line is for Raa extracted with free expansion method.

Free streaming is automatically taken into account in the original assumptions

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22s

Can calculate elipticity parameter v2 as

jet surviving probability

in and out of plane

Data are for high pt pi0s, PHENIX,

blue cicles – 4.59 GeV/c,

green squares – 5-7 GeV/c, preliminary

No hydro/collective flow!

How to explain rising and falling down v_2 with momentum?

100%

0%

pt

core+hydro+exponent corona

Relative contribution

At low momentum hydro scenario produces most of particles and v2 increases with momentum.

At high pt, particles are produced from corona with smaller v2.

Corona contribution “dilutes” hydro v2 at mid pt to the value of geometry limit. Knowing corona contribution can correct for hydro 23v2

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Compton Annihilation

In p+p Hard photons:direct component

q + g +q q +q + g

Photons in A+A

Direct Photons Decay Photons

hard thermal hard+thermal

QGP Hadron gasdirect fragmentation

Preequilibriumphotons

jet--conv.

Medium induced bremsstr.

In A+A picture is much more complicated:

V2=0V2<0V2<0

V2>0V2>0V2>0