1. A Future for Measuring Thermal Radiation in Heavy Ion Collisions?
Thermal radiation from hadronic collisions:
An old but still hot idea!
“thermal” radiation
Experimental Challenges
Experimental attempts to measure thermal radiation
First successful experiment: CERES
State of the Art experiment: NA60
Energy frontier: PHENIX
Future perspectives
Lessons learned and conclusions from them
Dedicated experiment: Technical specifications
Dedicated experiment: A strawman design

2. Thermal Radiation from QGP
Axel Drees

3. Shuryak 1978: Birth of the Quark Gluon Plasma
Shuryak PLB 78B (1978) 150
J/
Drell-Yan
QGP
Data from 400 GeV p-A at FNAL
e+e- for high mass PRL 37 (1976) 1374
m+m- for high mass PRL 38 (1977) 1331
p-A 400 GeV
Ultimately the wrong explanation, but this paper was landmark and kicked off the search for the QGP and its radiation!
NA38 experiment was originally
proposed to measure thermal radiation
Key lesson: Know your backgrounds!
In particular charm and bottom!
Axel Drees

4. Naming Convention: Thermal??
log t (fm/c)
Photons from A+A
Direct photons
Photons from hadron decays
“Prompt”
hard scattering
Pre-equilibrium
Quark-Gluon Plasma
Hadron gas
Thermal
Non-thermal
Need to be more clear in what we mean by
“thermal” and thermal equilibrium
Axel Drees

5. Thermal Radiation Thermal photons a ~10% contribution to p0 photons
Black body radiation
Real Photons and virtual photons (lepton pairs)
Static source back of envelope estimate:
Thermal photons a ~10% contribution to p0 photons
Range to look few 200 MeV to 2 GeV
Axel Drees

6. An Expanding Source in Local Equilibrium
Real and virtual photon momentum spectrum at mid rapidity:
Temperature information
Integrated over space time evolution
due to T4 dependence sensitive to early times
Collective expansion
Radial expansion results in blue and red shift
Longitudinal expansion results in red shift
Virtual photon mass spectrum
Not sensitive to collective expansion
Mass and momentum dependence allows to disentangle flow from temperature contributions!!
Axel Drees

7. Microscopic View of Thermal Radiation
Production process: real or virtual photons (lepton pairs)
hadron gas:
photons low mass lepton pairs
QGP:
photons medium mass lepton pairs
Key issues:
In medium modifications of mesons
pQCD base picture requires small as
But as can not be small for dNg/dy ~ 1000
(i.e. in a strongly coupled plasma) p r* g* e- e+ g p r q g q q e- e+
Additional issue:
Need to know time evolution!!
Axel Drees

8. Experimental Challenge
Thermal radiation
compete with “cocktail” of g and l+l-
from hadron decays after freeze-out
Real photons:
p0g, hg, wp0g, ...
More than 90% of photon yield
Virtual photons:
p0e+e-, h, wp0e+e- and direct decays r,w,f  e+e-, J/y  e+e- ...
Semileptonic decays of heavy flavor
Drell Yan
Dileptons have mass  remove contribution from p0
 more sensitive to thermal radiation than photons
Measure hadron decay contributions
Focus on Dileptons, they are more sensitive than photons
Axel Drees

9. Dilepton Experimental Challenges (I)
Uncorrelated background: l+ and l- from different uncorrelated source
Veto as many of the pairs actively by finding partner (rejection scheme)
Remove remaining background statistically
Like and unlike sign combinatorial pairs.
Unlike sign background can be determined from like sign background, either measured (FG) or determined by event mixing (BG).
Total number of pairs related by geometrical mean.
True for e+e- since they are produced as pairs, even for different efficiencies.
For m, produced as singles, strictly true only if produced with Poisson distribution.
For different acceptance for singles or pairs need relative acceptance correction (obtained from mixed events)
Axel Drees

10. Dilepton Experimental Challenges (II)
Unphysical correlated background
Limited double track/hit resolution
False track match between detectors
Not equal in like and unlike; Not reproducible by mixed events
MUST be removed from event sample
Correlated background: l+ and l- from same source but not signal
“Cross” pairs  “jet” pairs
Need case by case investigation with MC simulations and subtraction
If background produces same numbers of ++ and - - pair same method as for different pair acceptance works X π0 e+ e- γ
Axel Drees

11. Pioneering Dilepton Results form CERES/NA45
CERES PRL 92 (95) 1272 with 466 citations
Discovery of low mass dilepton enhancement in 1995
p-Be and p-Au well described by decay cocktail
Significant excess in S-Au (factor ~5 for m>200 MeV)
Onset at ~ 2 mp suggested p-p annihilation
Maximum below r meson near 400 MeV p r* g* e- e+
Launched massive theoretical investigation
of meson properties in medium
Axel Drees

12. Precision Measurements with NA60
Next slides mostly derived from talks given by Sanja Damjanovic
2.5 T dipole magnet
beam tracker
vertex tracker
Muon
Other
hadron absorber
muon trigger and tracking
target
magnetic field
NA60 features
Precision silicon pixel vertex tracker
Classic muon spectrometer
Double dipole for large acceptance (low mass)
High rate capability
Axel Drees

13. Low Mass Data Sample for 158 AGeV In-In
NA60 can isolate continuum excess (including r meson) from decay background
Phys. Rev. Lett. 96 (2006)
Experimental advance
High statistics
Excellent background rejection
Precision control of decay cocktail
Example: NA60 can measure electromagnetic transition form factors for of η→μ+μ-γ and ω→μ+μ-π0
(Phys. Lett. B 677 (2009) 260)
Axel Drees

14. Intermediate Mass Data for 158 AGeV In-In
Intermediate Mass Range prompt continuum excess
2.4 x Drell-Yan
Eur.Phys.J. C 59 (2009) 607
Experimental Breakthrough
Separate prompt from heavy flavor muons
Isolate prompt continuum excess
Axel Drees

15. Continuum Excess Measured by NA60
Fully acceptance corrected
Planck-like mass spectrum, falling exponentially (T > 200 MeV)
For m>mr good agreement with three models in shape and yield
Main sources m > 1 GeV
qq  m+m-
p a1  m+m (Hess/Rapp approach)
Main Sources m < 1 GeV
p+p-  r  m+m-
Sensitive to medium spectral function
~ 1/m exp(-m/T)
300 MeV
200 MeV
Eur. Phys. J. C 59 (2009) 607; CERN Courier 11/2009
Evidence for thermal dilepton radiation
Axel Drees

16. Sensitivity to Spectral Function
Not acceptance corrected
Models for contributions from hot medium (mostly pp from hadronic phase)
Vacuum spectral functions
Dropping mass scenarios
Broadening of spectral function
Broadening of spectral functions clearly favored!
pp annihilation with
medium modified r
works very
well at SPS energies!
Axel Drees

17. Transverse Mass Distributions of Excess Dimuon
transverse mass: mT = (pT2 + m2)1/2
Phys. Rev. Lett. 100 (2008)
Eur. Phys. J. C 59 (2009) 607
All mT spectra exponential for mT-m > 0.1 GeV
Fit with exponential in 1/mT dN/mT ~ exp(-mT/Teff)
Soft component for <0.1 GeV ??
Only in dileptons not in hadrons (speculate red shift???)
Axel Drees

18. Rise and Fall of Teff of Thermal Dimuons
Mass < 1 GeV
Linear increase of Teff with m
Similar trend observed in hadrons
Interpretation at SPS: Radial flow in hadronic phase!
Dileptons sense flow of in-medium r
p+p-→r→m+m-
Mass > 1 GeV
Sudden drop to Teff ~ 200 MeV
Remains independent of mass
Phys. Rev. Lett. 100 (2008)
Teff ~ Tf + M <vT>2
Indication that source of thermal dileptons is different for low and large masses!!
Axel Drees

19. Dominance of partons for m>1GeV
Schematic time evolution of collision at CERN energies
Partonic phase early emission: high T, low vT
Hadronic phase late emission: low T, high vT
Experimental Data: thermal radiation
Mass < 1 GeV from hadronic phase
<Tth> = MeV < Tc
Mass > 1 GeV from partonic phase
<Tth> = 200 MeV >Tc
hadronic
p+p-→r→m+m-
partonic
qq→m+m-
Dileptons for M >1 GeV dominantly of partonic origin
Axel Drees

20. Status: Thermal Radiation at SPS energies
History
Search started in 1986
First pioneering results on dileptons and photons (mostly limits) after 1995
Breakthrough with precise measurements (NA60) after 2006
Current status from dilepton experiment NA60
Planck like exponential mass spectra, exponential mT spectra, zero polarization and general agreement with thermal models consistent with interpretation of excess dimuons as thermal radiation
Emission sources of thermal dileptons mostly hadronic (p+p- annihilation) for M<1 GeV, and mostly partonic (qq annihilation) for M>1 GeV
In-medium r spectral function identified; no significant mass shift of the intermediate , only broadening.
Axel Drees

21. Thermal radiation at RHIC Energies: PHENIX
Disclaimer: ongoing analysis from STAR and potentially at LHC, but not finalized yet
Photons,
neutral pion
p0  g g
e+e- pairs
E/p and RICH
Calorimeter e- g g e+
PC1
PC2
PC3 DC
magnetic field &
tracking detectors
Axel Drees

22. Dilepton Continuum in p+p Collisions
Phys. Lett. B 670, 313 (2009)
Data and Cocktail of known sources represent pairs with e+ and e- PHENIX acceptance
Data are efficiency corrected
Excellent agreement of data and hadron decay contributions
with 30% systematic uncertainties
Consistent with PHENIX single electron measurement
sc= 567±57±193mb
Axel Drees 22

23. Au+Au Dilepton Continuum
Excess 150 <mee<750 MeV:
3.4 ± 0.2(stat.) ± 1.3(syst.) ± 0.7(model)
hadron decay cocktail tuned to AuAu
Charm from PYTHIA filtered by acceptance
sc= Ncoll x 567±57±193mb
Charm “thermalized”
filtered by acceptance
sc= Ncoll x 567±57±193mb
Intermediate-mass continuum: consistent with PYTHIA
since charm is modified room for thermal radiation
Axel Drees 23

24. In Medium Mesons at RHIC???
Models calculations with broadening of spectral function:
Rapp & vanHees
Central collisions scaled to mb
+ PHENIX cocktail
Dusling & Zahed
Bratkovskaya & Cassing
broadening
broadening and dropping
Au-Au mb
with modified charm
pp annihilation with
medium modified r
insufficient to describe
RHIC data!
Axel Drees

25. Contribution from Direct (pQCD) Radiation
Measuring direct photons via virtual photons:
any process that radiates g will also radiate g*
for m<<pT g* is “almost real”
extrapolate g*  e+e- yield to m = 0  direct g yield
m > mp removes 90% of hadron decay background
S/B improves by factor 10: 10% direct g  100% direct g*
1 < pT < 2 GeV
2 < pT < 3 GeV
3 < pT < 4 GeV
4 < pT < 5 GeV
pQCD q g
hadron decay cocktail
Small excess at for m<< pT consistent with pQCD direct photons
Axel Drees

26. Direct Real Photons from Virtual Photons
Significant direct photon excess beyond pQCD in Au+Au
Axel Drees

27. First Measurement of Thermal Radiation at RHIC
Direct photons from real photons:
Measure inclusive photons
Subtract p0 and h decay photons at S/B < 1:10 for pT<3 GeV
Direct photons from virtual photons:
Measure e+e- pairs at mp < m << pT
Subtract h decays at S/B ~ 1:1
Extrapolate to mass 0
T ~ 220 MeV
g* (e+e-)
 m=0 g
pQCD
First thermal photon measurement:
Tini > 220 MeV > TC
Axel Drees

28. Calculation of Thermal Photons
Reasonable agreement with data
factors of two to be worked on ..
Initial temperatures and times from theoretical model fits to data:
0.15 fm/c, 590 MeV (d’Enterria et al.)
0.2 fm/c, MeV (Srivastava et al.)
0.5 fm/c, MeV (Alam et al.)
0.17 fm/c, MeV (Rasanen et al.)
0.33 fm/c, MeV (Turbide et al.)
Correlation between T and t0
Tini = 300 to 600 MeV
t0 = 0.15 to 0.5 fm/c
D.d’Enterria, D.Peressounko, Eur.Phys.J.C 46 (2006)
Axel Drees

29. Thermal Photons also Flow
How to determine elliptic flow of thermal photons?
Establish fraction of thermal photons in inclusive photon yield
Predict hadron decay photon v2 from pion v2
Subtract hadron decay contribution from inclusive photon v2
Large v2 of low pT thermal photon
Axel Drees

30. Thermal Photon v2 Model Comparison
Current models fail to describe direct photon v2
Hees/Gale/Rapp 
Phys.Rev.C84:054906,2011.
R. Chatterjee and D. K. Srivastava
PRC 79, (R) (2009)
PRL96, (2006)
Direct emission from hadronic phase insufficient!
Axel Drees

31. Quark Scaling Behavior of v2
All hadrons flow collectively
in common velocity field
works for f and D mesons too
favors a pre-hadronic origin
Hadrons form from constituent quarks
Flow builds up in partonic phase?!
Common wisdom about space-time evolution may not be correct!
Axel Drees

32. Summary of Findings
We have discovered “thermal” radiation from heavy ion collisions
NA60 m+m- from In-In at 158 AGeV
Thermal source isolated experimentally
Planck like mass-spectrum of thermal radiation
Hadronic phase largest contributor (m < 1 GeV)
Observe melting of r meson in medium
Contribution from partonic phase (m > 1GeV) with <T> ~ 200MeV
PHENIX e+e- and g from √sNN = 200 GeV
Low mass excess larger than expected thermal contribution from hadron phase
Thermal photons with <T> > 200 MeV (from g* extrapolated to m=0)
Large elliptic flow (v2) of thermal photons which exceeds expected contribution from hadron phase
More data to come in next years
PHENIX HBD, STAR
Expect significant progress requires new dedicated effort!
Axel Drees

33. Short Detour on Cosmic Background Radiation
Discovered by chance in 1962
Perfect Black Body spectrum with T=2.37 K in 1992 (COBE)
WMAP power spectrum 2006
First data from Planck Satellite search for finger print of Inflation probing early evolution at t < fm
Much to learn from thermal
radiation beyond temperature!
Axel Drees

34. Lesson learned: Build a Dedicated Experiment
Build dedicated thermal radiation experiment
Map thermal radiation in phase space
Deconvole temperature and flow
Map time evolution of system
Focus on Dileptons
e+e- preferred for collider and y=0
g in coincidence is a must to tag background
m+m- good at forward rapidity might be nice addition at y=0
Measure heavy flavor simultaneously
Open and closed heavy flavor and much more as by product
Strong Physics Program
Large Discovery Potential
Axel Drees

35. Comment on RHIC vs SPS vs LHC
RHIC is at a sweet spot
System is well in partonic phase
Proof of principle to measure thermal radiation exists
Many unsolved puzzle – which are not small!
large unknown source, large partonic contribution, rapid thermalization, time evolution?
SPS at to low energy
Dominated by hadronic phase
Little to learn about early phase
Program at its end (or already beyond)
LHC at to high energy
System created at very similar condition compared to RHIC temperature
Dilepton continuum inaccessible due to background
Charm cross section so high that irreducible background (both physics and random) becomes prohibitive for precision measures
Thermal photons may be possibly via low mass high pT virtual photons?
Detectors not setup for dilepton measurements
Strong physics program at RHIC
with little competition from LHC
Axel Drees

36. Thermal Radiation Experiment
Design requirement (educated guess)
Large acceptance (2p ; Dy=2)
For high statistics and better systematics
Charged tracking
Good electron id (1:1000 p rejection)
Excellent momentum resolution (dp/p < 0.2% p)
Combinatorial background rejection
Passive: minimize material budget (in particular before first layer)
Active: Dalitz rejection scheme
Heavy flavor detection
Low mass precision vertex tracker (<10-20mm DCA)
Photon measurement
Sufficient energy resolution (<10%/√E; small constant term)
High DAQ rate (all min bias you can get ~ 40 kHz)
Do not compromise
on requirements!
Axel Drees

37. Strawman Design active beam pipe Solenoid with ~2 T Dy = 2
~30 cm
vacuum pipe
2 cm
6 cm
beam axis
MAPS active pixel
sxy < 20 mm
X/X0 < 1%
sDCA ~ 15 mm
Solenoid with ~2 T
Dy = 2
Silicon strip with
sf ~100 mm
dp/p < 0.1%p 1m
active
beam pipe
GEM tracker,
med. resolution vector
with dE/dx, or RICH/HBD
0.7 m
0.1 m
EMCal
longitudinal segmented
few 100ps resolution
Rejection scheme:
full track + tracklet mass cut
tracklet: active beam pipe + inner GEM
eID via dE/dx ~ 1/10 rejection
few % p-resolution
Electron ID:
GEM tracker dE/dx
EMCal TOF/ shower shape
E/p
Axel Drees

38. My Personal Conclusion
Heavy ion physics at RHIC beyond PHENIX and STAR
(>2015) should focus on “thermal” radiation
Axel Drees

39. Backup Slides
Axel Drees

40. Search for Thermal Photons via Real Photons
The internal conversion
method should also work at LHC!
internal conversions
PHENIX has developed different methods:
Subtraction or tagging of photons detected by calorimeter
Tagging photons detected by conversions, i.e. e+e- pairs
Results consistent with internal conversion method
Axel Drees

41. Combinatorial Background: Like Sign Pairs
Shape from mixed events
Excellent agreements for like sign pairs
also with centrality and pT
Normalization of mixed pairs
Small correlated background at low masses from double conversion or Dalitz+conversion
normalize B++ and B- - to N++ and N- - for m > 0.7 GeV
Normalize mixed + - pairs to
Subtract correlated BG
Systematic uncertainties
statistics of N++ and N--: 0.12 %
different pair cuts in like and unlike sign: 0.2 %
--- Foreground: same evt N++
--- Background: mixed evt B++
Au-Au
Normalization of mixed events:
systematic uncertainty = 0.25%
Axel Drees

42. Au-Au Raw Unlike-Sign Mass Spectrum
arXiv:
Run with added
Photon converter
2.5 x background
Excellent agreement within errors!
Unlike sign pairs data
Mixed unlike sign pairs normalized to:
Systematic errors from background subtraction:
ssignal/signal = sBG/BG * BG/signal
 up to 50% near 500 MeV
0.25%
as large as 200!!
Axel Drees

43. p-p Raw Data: Correlated Background
Cross pairs
Simulate cross pairs
with decay generator
Normalize to like sign
data for small mass
Like Sign Data
Correlated
Signal = Data-Mix
Jet pairs
Simulate with PYTHIA
Normalize to like sign data
Mixed
events
Unlike sign pairs
same simulations
normalization from
like sign pairs
Unlike Sign Data
Alternative methode
Correct like sign correlated background with mixed pairs
Signal: S/B  1
Axel Drees

44. Centrality Dependence of Background Subtraction
Compare like sign data and mixed
background
Evaluation in 0.2 to 1 GeV range
For all centrality bins
mixed event background
and like sign data agree within
quoted systematic errors!!
Similar results for background
evaluation as function pT
Axel Drees

45. Background Description of Function of pT
Good agreement
Axel Drees

46. Fit Mass Distribution to Extract the Direct Yield:
Example: one pT bin for Au+Au collisions
1/m
Direct * yield fitted in range 120 to 300 MeV
Insensitive to 0 yield
Axel Drees

47. Interpretation as Direct Photon
Relation between real and virtual photons:
Extrapolate real g yield from dileptons:
Virtual Photon excess
At small mass and high pT
Can be interpreted as
real photon excess
no change in shape
can be extrapolated
to m=0
Axel Drees

48. Interpretation as Direct Photon
Relation between real and virtual photons:
Virtual Photon excess
At small mass and high pT
Can be interpreted as
real photon excess
Extrapolate real g yield from dileptons:
Example for one pT range:
Excess*M (A.U).
no change in shape
can be extrapolated
to m=0
Axel Drees