Search for Thermal Photons in PHENIX

Search for Thermal Photonsin PHENIX

- Torsten Dahms -Stony Brook University

23rd Winter Workshop On Nuclear DynamicsFebruary 13, 2007

Torsten Dahms - Stony Brook University 2

Outline

• Motivation: signature of QGP• Conventional direct photon measurements• New attempts at low pT:

– Tagging of EMCal photons– External conversions in detector material– Low mass internal conversions

• Summary


The “Little Bang” in the lab

space

time

Hard Scattering

AuAu

Exp

ansi

on

Hadronization

Freeze-out

QGPThermalization

electro-magnetic radiation: γ, e+e-, +-

rare, emitted “any time”; reach detector unperturbed by strong final state interaction

Final state probes

Soft probes

Penetrating probes

e- e+


Thermal photons?

T

1np

hard:

/ E Tethermal:

Decay photons(0→, →, …)

No significant excess at low pT


Direct photons in Au+AuConventional 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

• Run4: more statistics, but still no conclusive measurement


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

e+e


New Idea

• Measure inclusive photon spectrum, but a very clean sample

• Tag all photons, which combined with a photon from a second (less clean) sample can be identified as pion decay product

)(p γεaε)(pN

)(pγaε)(pN

Tπ

γ'T tagπ

TTinclγ

00

incl

f

DA

TA

a = acceptance

= efficiency

’= loose efficiency

f = conditional probability of having a loose photon (’) in the acceptance, once you already have a clean the acceptance

)(p γa)(pN

)(pγa)(pN

Tπ

T tagπ

TThadronγ

00

hadr

f

SIM

UL

AT

ION

sim

tagπγTγ

data

tagπγTγ

γ'hadr

incl0

hadr

0incl

N)(pN

N)(pNε

γ

γ

DOUBLE

RATIO

everything cancels out except for ’minimal systematics


Clean Photon Sample

• Method I:– Only use EMCal clusters which

fulfill very strict PID cuts

• Method II:– Identify conversion photons in

beam pipe– Additional advantage:

• very good momentum resolution of charged tracks at low pT

• No detector artifacts

no pair cutwith pair cut

Dal

itz

Conversions

r~mass

PHENIX Beam Pipe


0 signal extraction

Real eventsMixed event

•BG subtraction within pT bins•Normalized outside the π0 peak

• Clean EMCal sample has better S/B ratio

Clean EMCal γ sample

Beam pipe conversions


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


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

Another Idea

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

0

0

e+

e-


In practice

0-3

0

• 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)

From conventional measurement

0

0

direct

data

incl.

direct

*

*

RR

RR

incl.

direct

÷

200-3

00 M

eV

÷

140-2

00

Rdata

÷

90-1

40


Comparison

• Agreement of all three methods within their errors• Also internal conversions favor an excess above decay photons


The spectrum• Compare to NLO pQCD

– L.E. Gordon and W. Vogelsang

– Phys. Rev. D48, 3136 (1993)

• Above (questionable) pQCD

• Compare to thermal model– D. d’Enterria, D. Peressounko

– nucl-th/0503054

• Data above thermal at high pT

• Data consistent with thermal+pQCD

• Needs confirmation from p+p measurement

2+1 hydro

T0ave=360 MeV (T0

max=570 MeV)

0=0.15 fm/c


Internal Conversions: d+Au

• Internal conversion method provides smaller systematic errors • But not as small as in the case of Au+Au collisions in Run-4

(Large background of external conversion on MVD detector in Run-3)


Internal Conversion: d+Au

• Internal conversion extends range of significant points to pt > 2 GeV

• Data agrees with pQCD predictions in full pt range

No indication for nuclear effects


Comparison: d+Au & Au+Au

Direct photon spectrum measured in d+Au collisions and scaled with <Ncoll> agrees pretty well with spectrum measured in Au+Au at high pT

• There is room for thermal photons

• Systematic errors are still too big to decide whether excess seen in Au+Au can be assign to thermal source


Summary• Photons are penetrating hard and soft probes for relativistic

heavy ion collisions• Conventional Calorimeter measurement

– Systematic uncertainties at low pT too large to make definite statement about thermal photon contribution

• New methods show excess above decay photons• Consistent with each other• Internal conversions

– Promising new technique to measure direct photons– Thermal photon scenario consistent for pT<3GeV/c– Because of the large systematic errors comparison of binary scaled

d+Au spectrum with Au+Au does not allow to make a statement on the origin of the excess above pQCD observed in Au+Au

– Same analysis of p+p is needed as confirmation


Backup


The PHENIX experiment

Beam Pipe

West Arm East Arm

γ

e+

e-

e+

e-

γ

Collision Vertexe+

e-

γ

•electrons:•momentum reconstruction (1% resolution) •particle ID: RICH (loose cuts because clean signature of conversion peak)

•same or opposite arms: different pT acceptance

•photons: EMCal (loose cuts high efficiency ~ 98%)

track reconstruction assumes vertex in the interaction point conversion at radius r≠0: e+e- pairs ‘acquire’ an opening angle

they acquire an invariant mass m = B dl ~ r > 0if r=4 cm (beam pipe) m =20 MeV


Invariant e+e- mass spectrum of

Run 4 Au+Au:Dalitz decays

beampipeconversions

e eγ

e e γ γγπ0

e e γπ0

air conversions & combinatorial background

GeV 002sNN

•Conversion pairs are created off-vertex•Track reconstruction produces apparent opening angle•Leads to apparent mass ~20MeV/c2


• Dalitz decays have a real opening angle due to the π0 mass

• Conversion pairs have small intrinsic opening angle– magnetic field produces opening of the pair in azimuth

direction

– orientation perpendicular to the magnetic field

Pair properties

0ee Δ 0-

00

z

y

x e+

e-

BConversion pair

z

y

x

e+

e-

BDalitz decay

MC Simulationall pairsdalitz decaybeam pipe conversions

MC Simulationall pairsdalitz decaybeam pipe conversions


Simulations: Nγhadr(pT) and Nγ

π0

tag(pT)• Inclusive photon spectrum

– π0, η → γe+e-

• π0 parameterization from measured data

• η from mT scaling, yield normalized at high pT (0.45 from measurement)

– Use Dalitz decay (π0→ γ γ ~ π0 → γ γ* → γe+e- for pT > 0.8 GeV/c)

• All e+e- (from π0, η) in the acceptance pT spectrum of e+e-

• If γ from π0 is also in acceptancepT spectrum of e+e- from π0 all e+e- pairs

e+e- pairs from π0 1

/Nev

t dN

/dp

T [c

/Ge

V]


Cocktail ingredients (pp): 0

• most important: get the 0 right (>80 %), assumption: 0 = (+ + -)/2

• parameterize PHENIX pion data:n

TTT p

pbpap

c

pd

dE

0

23

3

)exp(

• most relevant: the meson (Dalitz & conversion)• also considered: ’• use mT scaling for the spectral shape, i.e.

• normalization from meson/0 at high pT as measured (e.g. 0 = 0.45±0.10)

222mmpp mesonTT


•Gale QM05

Search for Thermal Photons in PHENIX

Documents

decay photons

virtual g

loose photon g

clean g

conversion photons

g efficiencyeg

g acceptanceeg

incl decay