The Quest to Detect Thermal Photons
Axel Drees, Stony Brook University, Lectures at Trento June 16-20, 2008
Photons from A+A
Direct photons
Non-thermal
Thermal
Pre-equilibrium
Hadron gas
“Prompt”
hard scattering
Quark-Gluon Plasma
1
10
Photons from hadron decays
10 7
log t (fm/c)
Measuring Photons is not Hard!
Axel Drees
Photons from Hadron Decays
 Example RHIC Au-Au




Main contribution p0 (~85%)
2nd largest contribution
h meson (~12%)
3rd largest contribution
w meson (~3%)
All other contributions
negligible
 These are high pT values; some
variation at lower pT
Axel Drees
Direct Contributions
 Direct photons

From initial hard scattering
“prompt”
g
q
q

decays

thermal
e
 E /T
prompt
1
pTn
From medium: “thermal”, “preequilibrium”, other effects
hadron gas:
p
p
r

QGP:
Direct contributions small (<10%) compared to
hadron decay contribution  measurement
limited by systematic uncertainties
g
q
q

Axel Drees
Theoretical Expectation for RHIC
Turbide, Rapp & Gale PRC (2004)
thermal
hard
Window for thermal radiation: 0.5 to 2.5 GeV
Axel Drees
Search for Direct “Thermal” Photons at the SPS
 1st and 2nd generation experiments gave upper limits


Experime
nt
With oxygen and sulfur beams
Measurement limited by systematic errors on data analysis
& h production
published
HELIOS 2 Z.Phys. C46 (90)
y
pT (GeV/c)
system
Upper
limit
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 production
from central O and S beams
Axel Drees
Measurement of Direct Photons
 Measure pT spectrum of p0 and
h mesons with high accuracy
 Calculate number of decay
photon per p0


Usually with Monte-Carlo
mT scaling for (h), h’, w, …
Handy formula:
d
  p Tn
dpT

 decay
p0
p0

2
 0.28 at RHIC
n1
 Get clean inclusive photon
sample

Charged background
subtraction
 Finally:
" direct =  inclusive -  decay"
Subtract decay background
from inclusive photon spectrum
Axel Drees
Why this is Difficult?
Reduce systematic uncertainties:
(e.g. energy scale non-linearity)
partially cancel in this ratio
 measured p 0  measured
R=
=
 decay
p 0  decay
Signal !
 direct
1
= (1 - ) × measured
R
“Subtraction method”
Axel Drees
WA98 Result

20% direct photon excess at
high pT in central Pb+Pb
collisions at CERN SPS

No signal within errors in
peripheral collisions
Axel Drees
WA98 Result and Interpretation
Data: WA98, PRL 85 (2000) 3595
Theory: Turbide, Rapp & Gale PRC (2004)
 WA98 data from Pb-Pb collisions


WA98 Pb-Pb
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
First hint of direct photons from Pb-Pb
Axel Drees
Direct Photon Search in the RHIC Era
 Significant progress with PHENIX:

Better input to decay cocktail
p0 and h measured more accurately
Axel Drees
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
Axel Drees
Comparison with Other Experiment
PHENIX Preliminary
Systematic errors are not shown
proton-proton collisions
proton-antiproton collisions
Axel Drees
Perturbative QCD: xT Scaling
 Excepted from QCD, if


Q2-Scaling of PDF,FF
No running coupling constant(s)



 s
n
 F xT 
n=constant xT=2pT/s
Can be express as two terms
Interaction
Structure
If leading order n=4
Next-to-leading order: n=4+
PHENIX data preliminary
All data consistent with
xT-Scaling n=~5
Axel Drees
d+Au Collisions
 Analysis method: p0 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.
Consistent with no “cold”
nuclear matter effects
Axel Drees
Direct Photons from Au-Au Collisions
Blue line: Ncoll scaled p+p cross-section
PRL 94, 232301 (2005) + preliminary data at high pt
Au-Au data consistent
with pQCD calculation
scaled by Ncoll
Axel Drees
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
1 d 2 AA
RAA  pT  
Ncoll dpT dh
d 2 pp
dpT dh
Axel Drees
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?
1 d 2 AA
RAA  pT  
Ncoll dpT dh
Shadowing?
Isospin effects?
Data wrong?
d 2 pp
dpT dh
Axel Drees
But what about thermal photons?
 Go back to quantity actually measured!

Present systematic error prohibit detection of thermal component
Search for Thermal
Photons ongoing:
(i) reduce systematic
(ii) use e+e
down to 500 MeV/c
Axel Drees
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
Axel Drees
Dileptons at low mass but high pT ?
Au+Au
p+p
0<pT<8.0 GeV/c
0<pT<0.7 GeV/c
m<<pT
0.7<pT<1.5 GeV/c
1.5<pT<8 GeV/c
Can we distil thermal photons from dileptons??
Axel Drees
The idea
 Start from Dalitz decay
 Calculate inv. mass distribution of Dalitz pairs‘
N.M.Kroll and W.Wada, Phys. Rev. 98 (1955) 1355
Compton
q
*

g
e+
e-
q
2
2
4me2
2me2 1
m
1 dN ee 2
2

1  2 (1  2 )
F (mee
 pTee2) )3
(m) ee(1<<
N  dm ee 3p
mee
mee mee
M
invariant mass of
Dalitz pair
invariant mass of
virtual photon
form factor
phase space factor
 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
Axel Drees
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

arXiv:0804.4168v1, 25 April 2008
Large exces at
all pT
Axel Drees
How to Extract a Direct Yield:
 Example: one pT bin for Au+Au collisions
f c (mee ) and f dir (mee )
normalized to data
for mee  30 MeV
f (mee )  (1  r ) fc (mee )  r f dir (mee )
fit: r  0.128  0.015
arXiv:0804.4168v1, 25 April 2008
Axel Drees
Fraction of Direct Photons
*
 dir
 dir
fraction or direct photons: r  * 
 incl  incl
arXiv:0804.4168v1, 25 April 2008
Axel Drees
First Measurement of Thermal Radiation at RHIC
 Slope analysis of data:

pQCD + exp.
Ae



pT
T

pT 2 
 B  N coll 1 

b


n
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:





arXiv:0804.4168v1, 25 April 2008
0.15 fm/c,
0.2 fm/c,
0.5 fm/c,
0.17 fm/c,
0.33 fm/c,
590 MeV
(d’Enterria et al.)
450-660 MeV (Srivastava et al.)
300 MeV
(Alam et al.)
580 MeV
(Rasanen et al.)
370 MeV
(Turbide et al.)
From data:
Tini > 220 MeV > TC
From models: Tini = 300 to 600 MeV
Axelfm/c
Drees
t0 = 0.15 to 0.5