Axel Drees, Stony Brook University, Lectures at Trento June 16-20, 2008

1 10 107 log t (fm/c)

The Quest to Detect Thermal Photons

Photons from A+A

Direct photons

Photons from hadron decays“Prompt” hard scattering

Pre-equilibrium

Quark-Gluon Plasma

Hadron gas

ThermalNon-thermal

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Measuring Photons is not Hard!

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Photons from Hadron Decays

Example RHIC Au-Au Main contribution (~85%) 2nd largest contribution

meson (~12%) 3rd largest contribution

w meson (~3%) All other contributions

negligible

These are high pT values; some variation at lower pT

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hadron gas:

QGP:

Direct Contributions

Direct photons From initial hard scattering

“prompt”

From medium: “thermal”, “pre-equilibrium”, other effects

promptthermal

decays

Direct contributions small (<10%) compared to hadron decay contribution measurement

limited by systematic uncertainties

/ E TeT

1np

q

qg

q

qg

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Theoretical Expectation for RHIC

Window for thermal radiation: 0.5 to 2.5 GeV

thermal

hard

Turbide, Rapp & Gale PRC (2004)

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Search for Direct “Thermal” Photons at the SPS

1st and 2nd generation experiments gave upper limits With oxygen and sulfur beams Measurement limited by systematic errors on data analysis

& production

Experiment

published y pT (GeV/c) system Upper limit

HELIOS 2 Z.Phys. C46 (90) 1.0-1.9 0.1 – 1.5 p-W, O-W, S-W 13%

WA80 Z.Phys. C51 (91) 1.5-2.1 0.4 – 2.8 O-Au 15%

WA98 PRL (96) 2.1-2.9 0.5 – 2.5 S-Au 12.5%

CERES Z.Phys. C71 (96) 2.1–2.65 0.4 – 2.0 S-Au 14%

~13% upper limits on direct photon productionfrom central O and S beams

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Measurement of Direct Photons

Measure pT spectrum of and mesons with high accuracy

Calculate number of decay photon per

Usually with Monte-Carlo mT scaling for (), ’, , …

Get clean inclusive photon sample

Charged background subtraction

Finally:Subtract decay background from inclusive photon spectrum

T

T

decay

d

d

20.28 at RHIC

1

npp

n

Handy formula:

direct inclusive decay" = - "

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Why this is Difficult?

Signal !

measured measured

decay decay

= =R

Reduce systematic uncertainties: (e.g. energy scale non-linearity)partially cancel in this ratio

direct measured

1= (1- )

R×

“Subtraction method”

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WA98 Result

20% direct photon excess at high pT in central Pb+Pb collisions at CERN SPS

No signal within errors in peripheral collisions

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WA98 Result and Interpretation

WA98 data from Pb-Pb collisions Published 2000 14 years after start of SPS program

Clear signal above 2 GeV/c Access beyond prompt component Consistent Tinit~200-270 MeV Remains ambiguous

Upper limits below 1.5 GeV/c Systematic errors at low pT remain

prohibitive

Data: WA98, PRL 85 (2000) 3595Theory: Turbide, Rapp & Gale PRC (2004)

WA98 Pb-Pb

First hint of direct photons from Pb-Pb

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Direct Photon Search in the RHIC Era Significant progress with PHENIX:

Better input to decay cocktail0 and measured more accurately

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Reference Data from p+p

PHENIX preliminary result. NLO-pQCD calculation

Private communication with W.Vogelsang

CTEQ6M PDF. Sum of direct photon

bremsstrhlung photon 3 scales (1/2pT,1pT,2 pT)

For renormalization scale factorization scale

pQCD calculation consistent with PHENIX data

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Comparison with Other Experiment

Systematic errors are not shown

PHENIX Preliminary

proton-proton collisions proton-antiproton collisions

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Excepted from QCD, if Q2-Scaling of PDF,FF No running coupling constant(s)

n=constant xT=2pT/s Can be express as two terms

InteractionStructure

If leading order n=4Next-to-leading order: n=4+

Perturbative QCD: xT Scaling

Tn

xFs

All data consistent with xT-Scaling n=~5

PHENIX data preliminary

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Analysis method: 0 tagging method as used in p+p

NLO pQCD Calculation p+p collisions Calculated by W.Vogelsang CTEQ6M Scale(renormalization and

factorization scale) 0.5,1.0,2.0 pT

Binary scaling to d+Au Averaged number of collisions

(8.42) from the Glauber model was multiplied to the calculation.

d+Au Collisions

Consistent with no “cold” nuclear matter effects

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Blue line: Ncoll scaled p+p cross-section

Direct Photons from Au-Au CollisionsPRL 94, 232301 (2005) + preliminary data at high pt

Au-Au data consistent with pQCD calculation

scaled by Ncoll

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Direct Photons are a Key Calibration for Jet Production

Jet quenching in Au-Au collisions Direct photons follow binary collision scaling Pions are suppressed by factor of 5

ppAAAA T

Tcoll T

ddR p

dp dN dp d

221

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Most Recent Data out to 20 GeV Use pp data as reference rather than pQCD Use most recent data analysis

RAA for direct photons drops?

ppAAAA T

Tcoll T

ddR p

dp dN dp d

221

Shadowing?Isospin effects?Data wrong?

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But what about thermal photons?

Go back to quantity actually measured! Present systematic error prohibit detection of thermal component

Search for ThermalPhotons ongoing:

(i) reduce systematic(ii) use →e+e down to 500 MeV/c

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Alternative Approaches with Real Photons

Tagging method (explain of black board) Photons detected by calorimeter Photons detected by conversions, i.e. e+e pairs

Needs still more work and more statistics to get conclusive result

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Dileptons at low mass but high pT ?

0<pT<0.7 GeV/c

0.7<pT<1.5 GeV/c 1.5<pT<8 GeV/c

0<pT<8.0 GeV/c

p+pAu+Au

m<<pT

Can we distil thermal photons from dileptons??

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phase space factorform factorinvariant mass of virtual photon

invariant mass of Dalitz pair

form 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

32

2

)1(M

mee

Compton

q

g q

Compton

q

g q

e+

e-

ee

ee

dm

dN

N

1

The idea

eeee

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 equation Form factor * phase space factor

converge towards unity for mee<< pT or mee 0

phase space factor

N.M.Kroll and W.Wada, Phys. Rev. 98 (1955) 1355

(mee << pT )

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A Closer Look at Mass Region 150 to 300 MeV

p+p Well described

for pT<2 GeV Small excess at

higher pT

Au+Au Large exces at

all pT

arXiv:0804.4168v1, 25 April 2008

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How to Extract a Direct Yield: Example: one pT bin for Au+Au collisions

arXiv:0804.4168v1, 25 April 2008

(( )( ) (1 )

fit: 0.12 0.015

)

8dir ec ee eeef f m f mm r r

r

and

normalized to da

( )

ta

(

f

)

or 30

dir eec

e

e

e

ef

m

m

V

f m

Me

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Fraction of Direct Photons

arXiv:0804.4168v1, 25 April 2008

fraction or direct photons: dir dir

incl incl

r

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First Measurement of Thermal Radiation at RHIC

arXiv:0804.4168v1, 25 April 2008

Slope analysis of data: pQCD + exp.

Fix B, b, and n from p+p Inverse Slope: (min. bias Au-Au)

T = 224 16 (stat) 18 (sys)

Initial temperatures and times from theoretical model fits to data:

0.15 fm/c, 590 MeV (d’Enterria et al.) 0.2 fm/c, 450-660 MeV (Srivastava et al.) 0.5 fm/c, 300 MeV (Alam et al.) 0.17 fm/c, 580 MeV (Rasanen et al.) 0.33 fm/c, 370 MeV (Turbide et al.)

2

1T

npTT

coll

pAe B N

b

From data: Tini > 220 MeV > TC From models: Tini = 300 to 600 MeV = 0.15 to 0.5 fm/c