Thermal Photon Measurement using the HBD in PHENIX :Status and Update

Sky Rolnick UC Riverside APS April Meeting 2011

Thermal Photon Motivation

Photons are an ideal probe, carry information about the earliest, hottest, densest stages of the expanding medium as well as freeze out and other stages.

The observation of thermal photons will allow the determination of the initial temperature of the medium, the thermalization time and help theorists understand the EoS.

Sky Rolnick UC Riverside APS April Meeting 2011

Hydrodynamic models explain pT spectrum of thermal photons and imply high initial T and rapid τ0.

Hydrodynamic models also predict a non-zero elliptic flow for thermal photons.

A measurement of thermal photon v2 will allow us to estimate η/S by matching data to models of viscous hydro.

Some Theoretical Predictions

Sky Rolnick UC Riverside APS April Meeting 2011

D.d’Enterria, Eur.Phys.J.C 46 (2006)

Chatterjee, Srivastava & Heinz PhysRevC79, 021901

Tini = 300 to 600 MeV t0 = 0.15 to 0.5 fm/c

1

4

The PHENIX Detector at RHIC

Drift and Pad Chambers to measure charged particle momentum.

RICH for electron identification.

Electromagnetic calorimeters for measuring photon energy and ID.

Provides 3 Techniques for Photon Measurement Directly measured in EmCal. Internal Conversions using

dielectrons. External Conversions using

converter method.

Sky Rolnick UC Riverside APS April Meeting 2011

π0 e+ e-

π0 e+ e-comb. backg.pair

e+

e+e-

e-

HBD Detector Concept

The HBD is a Windowless Cherenkov detector with CF4 as radiator gas.

Utilizes small opening angle of Dalitz decays and conversions to remove background.

Total radiation length: ~2.4% = 0.82% (vessel) + 0.56% (CF4) + 1.03% (backplane)

Sky Rolnick UC Riverside APS April Meeting 2011

Z(cm)

(rad)

HBD Performance

Estimate from Run-9 p+p:

Step Bckg. reduction factor

1 matching to HBD 7.1 2.2

2double hit cutclose hit cut 6.5

3 single pad cluster cut

2Sky Rolnick UC Riverside APS April Meeting 2011

Pairs in Central Arms

Pairs matched to HBD

Pairs after HBD reject.

Inclusive Photon v2 using External Conversions from HBD

Utilizes external conversions …

Sky Rolnick UC Riverside APS April Meeting 2011

v2 v2 v2

Direct Photons from Internal Conversions

Internal conversion should be produced with a mass distribution that follows a “Dalitz” form.

eedirecteecocktaileedata MfrMfrMf 1

Sky Rolnick UC Riverside APS April Meeting 2011

Data can be described through a two component fit.

PRL 104, 132301 (2010)

r*dir(m 0.15)*inc(m 0.15)

*dir(m 0)*inc(m 0)

dirinc

rγ Direct over Inclusive

p+p result with * method agrees with NLO pQCD predictions.

For Au+Au there is a significant low pT excess above p+p expectationsInterpreted as thermal emission -> Initial T

Error Bars expected with HBD

Statistical Errors will be reduced significantly with HBD.Systematic errors, dominated by cocktail subtraction, will need to be reduced.

Sky Rolnick UC Riverside APS April Meeting 2011

Direct Photon v2 using EmCal

Sky Rolnick UC Riverside APS April Meeting 2011

Source 1-3GeV/c

V2 Inc. gamma hadron subtracted

~2-3%

V2 from Hadron contamination

~6%

Rgamma ~35%

Reaction Plane ~6-7%

Systematic Uncertainties on v2

v2 measurement using EmCalR

NN

NN

incl

decay

d irect

decay 1

v2,dir

R(pT )v2,inc v2

,bg

R(pT ) 1

Expected error bars on v2 using HBD

Sky Rolnick UC Riverside APS April Meeting 2011

Expected errors using HBDv2 measurement using EmCal

Using our results of rγ from before, and leaving everything else the same, we seesignificant improvement on direct photon v2 using the HBD over EmCal.

Summary

Photons are a great way to understand the medium.

The Signal to Background is significantly reduced using the HBD.

The HBD will provide improved measurement of thermal photons and allow for better measurement of v2.

Sky Rolnick UC Riverside APS April Meeting 2011

Collaboration Brookhaven National Lab:

B. Azmoun, A.Milov, R. Pisani, T. Sakaguchi,

A. Sickles, C. Woody

Columbia University: C.-Y. Chi

Stony Brook University: W. Anderson, Z. Citron, J. M. Durham,

T.Hemmick, J. Kamin, V. Pantuyev

Weizmann Institute of Science: A. Dubey, Z. Fraenkel, A. Kozlov, M.

Naglis, I. Ravinovich, D. Sharma, I. Tserruya,

Mihael Makek

UC Riverside:Rich Seto, Sky D. Rolnick

The End

Expected Stat. Errors from Run10/Run4

Au+Au collisions at 200 GeV:

5x data than in Run-4

Use S/B from Run4 and then assume 5x more signal and 5/20 more background since HBD will reduce B by 20 but stats will increase B by 5.

S/B factor for Run10/Run4

A2 1B

S

5 x BS

The factor of 5 comes from 5x more statistics, and the value of x used was X=1/20 for the background reduction.

Quick and dirty

Expected error bars on v2 using HBD

Sky Rolnick UC Riverside APS April Meeting 2011

Source 1-3GeV/c

V2 Inc. gamma hadron subtracted

~2-3%

V2 from Hadron contamination

~6%

Rgamma ~17%

Reaction Plane ~6-7%

Systematic Uncertainties on v2

RNN

NN

incl

decay

d irect

decay 1

v2,dir

R(pT )v2,inc v2

,bg

R(pT ) 1Use analysis note 518Extract data for each chart 0-92%

Propagate statistical error assuming same factor deduced earlier for each pT bin.