Photon Measurements in Heavy Ion Collisions

Yorito YamaguchiCNS, University of Tokyo

Direct photons1/11

S. Turbide et al., PRL77,024909(2008)

Definition: Photons NOT from hadron decays. Reason why measure direct photons

1. Leave the medium without a strong interaction

2. Generated in every stage of the collision→ Their pT are characterized by their origins.

Direct photons

q

qg

High pT : Initial hard scatterings

1/11

S. Turbide et al., PRL77,024909(2008)

Definition: Photons NOT from hadron decays. Reason why measure direct photons

1. Leave the medium without a strong interaction

2. Generated in every stage of the collision→ Their pT are characterized by their origins.

Direct photons

q

qg

High pT : Initial hard scatterings Mid pT : Jet-Medium interactions

Jet-QGP photons

1/11

S. Turbide et al., PRL77,024909(2008)

Definition: Photons NOT from hadron decays. Reason why measure direct photons

1. Leave the medium without a strong interaction

2. Generated in every stage of the collision→ Their pT are characterized by their origins.

Direct photons

q

qg

High pT : Initial hard scatterings Mid pT : Jet-Medium interactions

Jet-QGP photons

Low pT : Thermal radiations from QGP and Hadron Gas

Thermal photons

1/11

S. Turbide et al., PRL77,024909(2008)

Definition: Photons NOT from hadron decays. Reason why measure direct photons

1. Leave the medium without a strong interaction

2. Generated in every stage of the collision→ Their pT are characterized by their origins.

Direct photons

q

qg

High pT : Initial hard scatterings Mid pT : Jet-Medium interactions

Jet-QGP photons

Low pT : Thermal radiations from QGP and Hadron Gas

Thermal photons

Thermal photons are of keen interest→ Provide key inputs (Tinit & 0) to describe evolution of the created matter

1/11

S. Turbide et al., PRL77,024909(2008)

Definition: Photons NOT from hadron decays. Reason why measure direct photons

1. Leave the medium without a strong interaction

2. Generated in every stage of the collision→ Their pT are characterized by their origins.

Direct photons

q

qg

High pT : Initial hard scatterings Mid pT : Jet-Medium interactions

Jet-QGP photons

Low pT : Thermal radiations from QGP and Hadron Gas

Thermal photons

Hadron decayphotons

Thermal photon measurement is very challenging due to a large background from hadron decays.

Thermal photons are of keen interest→ Provide key inputs (Tinit & 0) to describe evolution of the created matter

1/11

S. Turbide et al., PRL77,024909(2008)

Definition: Photons NOT from hadron decays. Reason why measure direct photons

1. Leave the medium without a strong interaction

2. Generated in every stage of the collision→ Their pT are characterized by their origins.

Measurement methods2/11

Successful direct photon measurement with two different methods at RHIC 1. Statistical subtraction method by EMCal Subtract hadron decay from inclusive

→ Remainder = direct Large uncertainty on hadron decay at

low pT

Suitable for pT>5GeV/c

2. Virtual photon method Measure low mass e+e- from * Selecting e+e- in mee>135MeV

→ Dramatically improvement of S/B ratio for direct *→e+e-

Suitable for pT<5GeV/c

A. Adare et al., PRL104,132301(2010)

Direct fractions

T

hadron

T

NLO

T

NLO

dp

d

dp

d

dp

d

μ = 0.5pT

μ = 1.0pT

μ = 2.0pT

μ = 0.5pT

μ = 1.0pT

μ = 2.0pT

NLO pQCD expectations are calculated as :

p+p & d+Au : Almost consistent with NLO pQCD expectations Cu+Cu & Au+Au : System size dependence is likely to be seen.

3/11

Direct photon spectra

p+p vs d+Au : Consistent → Little nuclear effects p+p vs Au+Au : Significant excess over scaled p+p result in pT<3GeV/c → Exponential fit gives inverse slope of T = 221±19stat±19systMeV (Central).

p+p vs d+Au

p+p vs Au+Au

4/11

A. Adare et al., PRL104,132301(2010)

Tinit & 0

TC from Lattice QCD ~ 170 MeV

TAuAu(fit) ~ 220 MeV

Hydrodynamic models agree with the data within a factor of 2. Uncertainty on Tinit (300-600MeV) is still large.

Depending on thermalization time 0 (0.1-0.6fm/c)→ Need sensitive observable to further constrain Tinit

5/11

A. Adare et al., PRC81,034911(2010)

Elliptic flow: v2

React

ion

plan

e

X

Z

Y

Px

Py Pz

Pressure gradient

6/11

Collective motion from conversion of spatial eccentricity into momentum anisotropy through pressure gradient. Expected v2 varies depending on a production process:

Initial hard scattering → v2=0 Thermal radiation → v2>0

Elliptic flow: v2

React

ion

plan

e

X

Z

Y

Px

Py Pz

Pressure gradient

6/11

Direct photon v2 is sensitive to thermalization time 0.

Early thermalization → Small thermal photon v2

Late thermalization → Large thermal photon v2

→ Possible to further constrain 0

R. Chatterjee & D. K. Srivastava, PRC 79, 021901 (2009)

Hydro after 0

Collective motion from conversion of spatial eccentricity into momentum anisotropy through pressure gradient. Expected v2 varies depending on a production process:

Initial hard scattering → v2=0 Thermal radiation → v2>0

Direct photon v2

7/11

Surprisingly, a large direct photon v2 is observed in pT<3GeV/c

Direct photon v2 →0 indicates prompt photons from initial hard scatterings are dominant in pT>5GeV/c

Au+Au@200 GeVminimum bias

Direct photon v2

preliminary

A. Adare et al., arXiv:1105.4126

8/11

charged0

direct

Centrality:0-20% Centrality:20-40%

Centrality:10-40%

Centrality dependence

Low pT (pT<5GeV/c) Inconclusive centrality dependence due to large errors

High pT (pT>5GeV/c) Consistent between different experiment results→ v2~0 independently of centrality

Comparison with models9/11

Model 1: H. Holopainen et al., arXiv:1104.5371Au+Au@200 GeV0-20%

Some hydrodynamic models predict direct photon v2 as well as pT spectrum.1. Tinit = 580MeV, 0 = 0.17fm/c

Shape is similar to predicted shape, but a magnitude is under-predicted.

Comparison with models9/11

Model 1: H. Holopainen et al., arXiv:1104.5371

2. Tinit = 450-520MeV, 0 = 0.4-0.6fm/c

Model can describe the data for pT>2GeV/c.

Model 2: R. Chatterrjee & D.K. Srivastava, PRC 79, 021901 (2009)

1. Tinit = 580MeV, 0 = 0.17fm/c

Shape is similar to predicted shape, but a magnitude is under-predicted.

Some hydrodynamic models predict direct photon v2 as well as pT spectrum.

Future prospects10/11

1. RHIC energy scan program Low energy Au+Au collision runs have

been done in 2010 & 2011: √sNN = 62.4, 39, 27, 19.6, 11.5, 7.7GeV

Help to search for Critical Point2. Measurement at LHC Direct photon measurement for wide pT range

Statistical subtraction & virtual photon methods Positive indication from single electron measurement at ALICE

Excess at low pT increases towards more central collisions.p+p 40-50% 0-10%

Summary11/11

Direct photons have been successfully measured in low pT region for different collision systems at RHIC.

pT spectrum Enhanced yield is observed in Au+Au→ Exponential fit : T = 221±19stat±19syst MeV for Central collisionsElliptic flow v2

Large v2 at low pT → Tinit = 450-520MeV, 0 = 0.4-0.6fm/c? v2 ~ 0 at high pT → Dominant source : Initial hard scatterings→ More theoretical models are needed to understand the data

Direct photons are still one of “hot” probes Low energy scan at RHIC & measurements at LHC→ Systematic study in wide collision energy

Backup

Process dependent factor

qg q

e+

e-

How to measure low pT photons Hard to measure by EMCals due to a finite energy resolution→ Alternative method has been developed : “Virtual photon method”Virtual photon method Basic idea : Any source of can emit *, convert to low mass e+e-

How to identify direct *→e+e- : Relation between and associated *→e+e- emission rates

Direct * : If pT2»mee

2, S(mee)~1 Dalitz decay :

→ Extraction of direct *→e+e- can be made by utilizing mee shape difference between direct * and hadrons.

Determination of direct fractionr : direct /inclusive eedireeceedata mfrmfrmf 1

Hadrons Direct *

A. Adare et al., PRL104,132301(2010)

Determination of direct fractions in 0.1-0.3GeV/c2 for pT>1GeV/c Negligible contribution of 0→e+e-

Satisfy an important assumption of pT2»mee

2 → S(mee)~1 No contribution of +-→e+e-

Enhanced e+e- yield over known hadron contributions is clearly seen due to direct *→e+e-.

Extended fit result can also describe the data well in mee>0.3GeV/c2.

How to obtain direct photon v2

122

2

r

vvrv

hadroninclusivedirect

Statistical subtraction method

preliminary

inclusive photon v2

Au+Au@200 GeVminimum bias

A. Adare et al., arXiv:1105.4126

1. Measure inclusive v2 using EMCals

hadron

inclusive

N

Nr

How to obtain direct photon v2

122

2

r

vvrv

hadroninclusivedirect

Statistical subtraction method

0 v2

preliminary

inclusive photon v2

Au+Au@200 GeVminimum bias

A. Adare et al., arXiv:1105.4126

1. Measure inclusive v2 using EMCals

2. Measure 0 v2, and then evaluate other hadron v2 (, , …) using MC calculation

hadron

inclusive

N

Nr

How to obtain direct photon v2

122

2

r

vvrv

hadroninclusivedirect

Statistical subtraction method

0 v2

preliminary

inclusive photon v2

Au+Au@200 GeVminimum bias

A. Adare et al., arXiv:1105.4126

1. Measure inclusive v2 using EMCals

2. Measure 0 v2, and then evaluate other hadron v2 (, , …) using MC calculation

3. Subtract hadron v2 from inclusive v2 a. pT<5GeV/c : r from virtual

photon methodb. pT>5GeV/c : r from statistical

subtraction method

hadron

inclusive

N

Nr

How to obtain direct photon v2

inclusive photon v2

Au+Au@200 GeVminimum biaspreliminary

122

2

r

vvrv

hadroninclusivedirect

Statistical subtraction methodA. Adare et al., arXiv:1105.4126

1. Measure inclusive v2 using EMCals

Confirm NO charged hadron contamination by external conversion method