Measurements of thermal photons in heavy ion collisions with PHENIX - Torsten Dahms - Stony Brook University February 8 th, 2008 Real photons at low p.

Measurements of thermal photons in heavy ion collisions

with PHENIX- Torsten Dahms -

Stony Brook University

February 8th, 2008

Real photons at low pT

• Production mechanisms• Traditional EMCal

measurement• Tagging• Beam pipe conversions

Real photons at low pT

• Production mechanisms• Traditional EMCal

measurement• Tagging• Beam pipe conversions

Virtual photons• Production mechanisms• Background• Au+Au and long awaited p+p results

Virtual photons• Production mechanisms• Background• Au+Au and long awaited p+p results

see poster by Y. Yamaguchi (P125)

High pT photons see talk by K. Miki (XV)

2008-02-08 2

Direct Photons• Direct photon sources:

– QCD Compton scattering

– Annihilation

– QCD Bremsstrahlung

• Hard photons from inelastic scattering of incoming partons

• Thermal photons are emitted via same processes but from thermalized medium carry information about the temperature of the medium

T

1np

hard:

/ E Tethermal:

Decay photons(0→, →, …)

γqqg

γgqq

2008-02-08 3

Thermal photons?

No significant excess at low pT

Conventional method:• Measure inclusive photons

γincl = γdecay + γdirect

• Calculate double ratio:(γincl/π0)measured / (γdecay/ π0)background = γincl/ γdecay =1+ γdirect/ γdecay

• If double ratio > 1 direct photons

• high pT excess consistent with pQCD• Run4: more statistics, but still no

conclusive measurement• Limited by detector resolution and

neutral hadron contamination

2008-02-08 4

Clean Photon Sample• Method I:

– Only use EMCal clusters which fulfill very strict PID cuts• Method II:

– Identify conversion photons in beam pipe using their orientation w.r.t. the magnetic field

– Additional advantage: • very good momentum resolution of charged tracks at low pT

• No detector artifacts– But statistics limited due to small X0

• Combining these photon with others measured in EMCAL with loose PID cut tag photons coming from π0 decays

• Correct for missing π0 decay partners• Subtract η, ω, η’ decay photons• Calculate ratio Nγ

incl/N γdecay

• Uses very pure photon sample• avoid explicit calculation of π0 spectrum reduce systematic uncertainties

no pair cutwith pair cut

Dal

itz

Conversions

Conversion pairs from π0 decaysConversion pairs from π0 decays

γe+e- tripletsγe+e- triplets

2008-02-08 5

Results in Au+Au

• Agreement of all three results within their errors

• There seems to be an excess above the decay photons at low pT

2008-02-08 6

Compton

q

g q

Compton

q

g q

e+

e-

phase space factorform factorinvariant mass of virtual photon

invariant mass of Dalitz pair

phase space factorform factorinvariant mass of Dalitz pair

invariant mass of virtual photon

32

222

2

2

2

2

)1()(1

)2

1(4

13

21

M

mmF

mm

m

m

m

dm

dN

Nee

eeeeee

e

ee

e

ee

ee

ee

ee

dm

dN

N

1

Virtual Photons

32

2

)1(M

meeeeee

e

ee

e

mm

m

m

m 1)

21(

41

3

22

2

2

2

22 )( eemF

• Start from Dalitz decay

• Calculate inv. mass distribution of Dalitz pairs

• Now direct photons

• Any source of real produces

virtual with very low mass

• Rate and mass distribution given by same formula

– No phase space factor for mee<< pT

photon

• Improved S/B by measuring direct photon signal in mass region in which π0 are suppressed

0

0

e+

e-

2008-02-08 7

The Data• 800M MinBias Au+Au events• 2.25pb-1 of triggered p+p data as

reference• Material conversion pairs

removed by analysis cut• Combinatorial background

removed by mixed events (0.25% syst. uncertainty in Au+Au)

• additional correlated background:– cross pairs from decays with four

electrons in the final state– particles in same jet (low mass)– or back-to-back jet (high mass)

• well understood from MC

π0

π0

e+

e-

e+

e-

γ

γ

π0

e-

γ

e+

p+p at √s = 200GeVp+p at √s = 200GeV

arXiv:0802.0050

2008-02-08 8

Cocktail comparison

QM2005

• Results from Au+Au

QM2008

• long awaited result from p+p

• important confirmation of method

p+p

• Agreement of p+p data and hadronic decay cocktail

• Small excess in p+p at large mee and high pT

Au+Au

• data agree for mee <50MeV

• Clear enhancement visible above for all pT

1 < pT < 2 GeV2 < pT < 3 GeV3 < pT < 4 GeV4 < pT < 5 GeV

p+p Au+Au (MB)

PHENIX Preliminary

2008-02-08 9

Shape Comparison• At m=0 Dalitz and internal conversion

pairs have indistinguishable shape• Shape differs as soon as π0 is

suppressed due to phase space limitation

• Assume internal conversions of direct photons

– Fix absolute normalization of cocktail and direct photons by normalizing to data in mee<30MeV

– Fit paramater r is fraction of direct photons

– Two component fit in80 < mee < 300MeV gives: χ2/DOF=11.6/10

• It’s not the η:– Independent measurement of η in

Au+Au fixes π0/η ratio to: 0.48 ± 0.08– Fit with eta shape gives:

χ2/DOF = 21.1/10)(mrf)(mr)f()f(m eedirecteecocktailee 1

2008-02-08 10

Fraction of direct photons• Fraction of direct photons

• Compared to direct photons from pQCD

p+p

• Consistent with NLO pQCD

• favors small μ

Au+Au

• Clear excess above pQCD

μ = 0.5pT

μ = 1.0pT

μ = 2.0pT

μ = 0.5pT

μ = 1.0pT

μ = 2.0pT

p+p Au+Au (MB)

2008-02-08 11

Comparison

• Agreement of all three methods within their errors

• Internal conversion method observes clear excess above decay photons

• Extract direct photon spectrum by multiplying with measured inclusive photon spectrum: Nγdirect = r · Nγinclusive

2008-02-08 12

The spectrum• Compare spectra to NLO pQCDp+p• consistent with pQCDAu+Au• above binary scaled pQCD• If excess of thermal origin:

inverse slope is related to initial temperature

2008-02-08 13

Conclusion

• Various techniques employed to measure direct photons at low pT

• Excess of real photons above decay background observed at low pT

• Measured excess in dielectron spectra– Shape consistent with internal conversions of

virtual photons– p+p in agreement with pQCD– Au+Au above pQCD

Backup

2008-02-08 15

Relativistic Heavy Ion Collider

2008-02-08 16

The PHENIX Experiment• Charged particle tracking:

– DC, PC1, PC2, PC3• Electron ID:

– Cherenkov light RICH– shower EMCal

• Photon ID:– shower EMCal

• Lead scintillator calorimeter (PbSc)• Lead glass calorimeter (PbGl)

– charged particle veto• Central arm physics

(|y|<0.35, p ≥ 0.2 GeV/c):– charmonium J/ψ, ψ’→ e+e-

– vector meson ρ, ω, φ → e+e- – high pT π0, π+, π-

– direct photons– open charm – hadron physics

• Two muon arms at forward rapidity (1.2 < |η| < 2.4, p 2 GeV/c)

e+e

• Measure rare probes in heavy ion collisions (e.g. Au+Au) as well as in p+p (+spin program)

2008-02-08 17

Electron Identification• Charged particle tracking (δm: 1%)

DC, PC1, PC3• PHENIX optimized for Electron ID• Cherenkov light RICH + • shower EMCAL

• Emission and measurement of Cherenkov light in the Ring Imaging Cherenkov detector→ measure of min. velocity

• Production and of em. shower in the Electro-Magnetic Calorimeter measure of energy E

• Electrons: E ≈ p• Hadrons: E < p

Cerenkovphotons from e+ or e- are detected by array of PMTs

mirror

Most hadrons do not emit Cerenkov light

Electrons emit Cerenkovphotonsin RICH.

Central Magnet

ＲＩＣＨ

PMT arrayPMT array

Cerenkovphotons from e+ or e- are detected by array of PMTs

mirror

Most hadrons do not emit Cerenkov light

Electrons emit Cerenkovphotonsin RICH.

Central Magnet

ＲＩＣＨ

PMT arrayPMT array

RICH

Energy-Momentum

All charged tracks

Background

Net signal

RealRICH cut

2008-02-08 18

0 signal extraction

Real events

Mixed event

•combine conversion pairs with all photons in EMCal

•BG subtraction within pT bins

•Normalized outside the π0 peak

2008-02-08 19

In practice

0-30

• Material conversion pairs removed by analysis cut

• Combinatorial background removed by mixed events

• Calculate ratios of various mee bins to lowest one: Rdata

• If no direct photons: ratios correspond to Dalitz decays

• If excess: direct photons

• Fit of virtual photon shape to data in principle also possible(done for d+Au)

0

0

direct

data

incl.

direct

*

*

RR

RR

incl.

direct

÷

200-300 M

eV

÷

140-200

Rdata

÷

90-140

From conventional measurement

2008-02-08 20

2008-02-08 21

Low pT mass spectra