1. Thermal Radiation Mapping the Space-Time Evolution
Axel Drees, Hard Probes 2012, June 1st, Cagliari
Thermal radiation from heavy ion collisions:
Mapping the space-time evolution
The state of the art experiment: NA60
Energy frontier: PHENIX
Thermal photons and flow
Dilepton puzzle
Future perspectives
PHENIX VTX and HBD upgrades
Summary and outlook

2. Schematic View of Thermal Radiation
Longitudinally and radially expanding fire ball in “local equilibrium”
Real and virtual photons
Integrated over space-time
Planck spectrum
yield  T4 , mean  T
boosted by collective motion
Real and virtual photon momentum spectrum
Temperature information
sensitive to early times due to T4 dependence
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
Measure mass and pT
disentangle flow and T
Axel Drees

3. 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
Rates not well established for strongly
coupled plasma p r* g* e- e+ g p r q g q q e- e+
Equilibrium of strong interaction!
Equilibrium not a necessary condition!
Axel Drees

4. Experimental Issue: Isolate Thermal Radiation
log t (fm/c)
, * from A+A
Direct
Hadron Decays
“Prompt”
hard scattering
Pre-equilibrium
Quark-Gluon Plasma
Hadron Gas
Thermal
Non-thermal
Need to subtract decay and prompt contributions
Sensitive to space-time evolution
Axel Drees

5. State of the Art 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
Classic muon spectrometer
Precision silicon pixel vertex tracker
tagging of heavy flavor decay muons
Reduction of combinatorial background by
vetoing p, K decay muons
Simultaneous measurement of decay contributions
NA60 can isolate “thermal” contribution
Axel Drees

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

7. In Medium Spectral Function
Not acceptance corrected
Models for contributions from hot medium (mostly pp annihilation in 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!
Phys.Rev.Lett. 96 (2006)
Contributions from hadronic phase
explored and exhausted!
Axel Drees

8. Unfolding Time-Evolution Using m-pT Dependence
Phys. Rev. Lett. 100 (2008)
Schematic hydrodynamic evolution
Partonic phase early emission: high T, low vT
Hadronic phase late emission: low T, high vT
“thermal” dimuons
Experimental Data: thermal radiation
Mass < 1 GeV from hadronic phase
<Tth> = MeV < Tc
Mass > 1 GeV from partonic phase
<Tth> = 200 MeV >Tc
Teff ~ <Tth> + M <vT>2
hadronic
p+p-→r→m+m-
partonic
qq→m+m-
In-In data from SPS consistent with hydro-evolution of QGP
Eur. Phys. J. C 59 (2009) 607
Axel Drees

9. Status: Thermal Radiation at SPS energies
State of the art dilepton experiment: NA60
Isolate thermal radiation
Planck like exponential mass spectra
exponential mT spectra
zero polarization
general agreement with models of thermal radiation
Emission sources of thermal dileptons (from m-pT):
hadronic (p+p- annihilation) dominant for M<1 GeV
partonic (qq annihilation) visible M>1 GeV
In-medium r spectral function identified:
no significant mass shift of the intermediate 
only broadening.
Axel Drees

10. Thermal Radiation at RHIC Energies: PHENIX
Photons,
neutral pion
p0  g g
e+e- pairs
E/p and RICH
Calorimeter e- g g e+
No background
rejection!
dilepton
S/B < 1:150
HBD upgrade!
PC1 DC
magnetic field &
tracking detectors
Axel Drees

11. Prompt Photons from p+p and d+Au
PHENIX arXiv:
PHENIX photon data
High pT (4 to 25 GeV) from calorimeter
Low pT (<4 GeV) from virtual photons
p+p data consistent with pQCD
xT scaling of cross section
NLO calculation agree well with data
d+Au data consistent with Ncoll scaling
No evidence for cold nuclear matter effects
Well established reference
for prompt photons
Axel Drees

12. Prompt Photons from Au+Au Collisions
PHENIX arXiv:
PHENIX photon data
High pT (4 to 25 GeV) from calorimeter
Nuclear Modification Factor
Consistent with binary scaling of p+p
No evidence for cold nuclear matter or hot medium effects out to 20 GeV/c
Hard scattering × Ncoll
describes prompt g from Au+Au
for pT above 5 GeV/c
PHENIX arXiv:
Axel Drees

13. Thermal Photons from Au+Au Collisions
PHENIX Phys. Rev. C 81 (2010)
* (m→0) =  ; m << pT
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* (m0)
First thermal photon measurement:
Tini > 220 MeV > TC g
pQCD
Need to consider radial flow! Compare to models!
Axel Drees

14. Thermal Photons Measured via Real Photons
PHENIX has developed new method to detect direct photons:
Use photon conversions to e+e-
Tag contribution from p0 decays
Independent systematic uncertainties
PHENIX preliminary
measured
raw yields
conditional
tagging efficiency
simulated based
On hadron data
Thermal photons observed in virtual and real photons
consistent within systematic uncertainties
Axel Drees

15. Model Comparison to Thermal Photon Yield
PHENIX Phys. Rev. C 81 (2010)
Reasonable agreement between data and models
Typically yield underestimated
Correlation between T and t0
Yield not correlated with Tini
Initial temperatures and times from theoretical model fits to data:
Tini = 300 to 600 MeV
t0 = 0.15 to 0.5 fm/c
Low yield  earlier emission (yield  T4)
increase factor 2 with <20% change in T
Axel Drees

16. Elliptic Flow of Thermal Photons
How to determine elliptic flow of thermal photons?
Establish Rg, i.e. fraction of thermal photons in inclusive photon yield
Measure inclusive photon v2incl
Predict hadron decay photon v2hadr from pion v2p0
Subtract hadron decay contribution
PHENIX arXiv:
Large v2 of low pT thermal photon
Axel Drees

17. Large elliptic flow of thermal photons
Thermal Photon v2
Independent analysis based on photon conversions →e+e
Background free inclusive photon sample
Rg from same data
Completely independent systematic uncertainties
pT reach extended down to 0.5 GeV/c
Two independent and consistent results!
photon conversions →e+e
PHENIX preliminary
arXiv:1105:4126
reaction plan: 1< |h|<2.8
Au+Au 200 GeV
min. bias
Large elliptic flow of thermal photons
Maximum v2~ 0.2 at 2 GeV/c
Axel Drees

18. Models fail to describe simultaneously photon yield, T and v2!
Thermal Photon Puzzle
Models fail to describe simultaneously photon yield, T and v2!
Hees/Gale/Rapp 
Phys.Rev.C84:054906,2011.
Tini ~ 325MeV
R. Chatterjee and D. K. Srivastava
PRC 79, (R) (2009)
PRL96, (2006)
Large flow requires late emission! Apparent contradiction with yield, which points towards early emission!
Axel Drees

19. Dilepton Continuum in p+p Collisions
PHENIX 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 19

20. Dilepton Continuum in d+Au Collisions
PHENIX preliminary
sbb= Ncoll × 3.9 ±2.5±3 mb
Consistent with known sources!
Data will constrain known sources with high precision,
e.g. bottom cross section.
Axel Drees 20

21. Au+Au Dilepton Continuum
Excess 150 <mee<750 MeV:
4.7 ± 0.4(stat.) ± 1.5(syst.) ± 0.9(model)
hadron decay cocktail tuned to AuAu
PHENIX Phys. Rev. C 81 (2010)
Charm from PYTHIA filtered by acceptance
scc= Ncoll × 567±57±193mb
Charm “thermalized”
filtered by acceptance
scc= Ncoll × 567±57±193mb
PHENIX VTX
upgrade
Intermediate-mass continuum: consistent with PYTHIA
since charm is modified room for thermal radiation
Axel Drees 21

22. Low Mass Dilepton Puzzle
Large low mass enhancement
Models calculations with broadening of spectral function:
Rapp & vanHees
Central collisions scaled to mb
+ PHENIX cocktail
Dusling & Zahed
Bratkovskaya & Cassing
broadening
pp annihilation with
medium modified r
insufficient to describe
RHIC data!
PHENIX Phys. Rev. C 81 (2010)
Excess not from hadronic phase!!
Axel Drees

23. Soft Low Mass Dilepton Puzzle
acceptance corrected
mT spectrum of excess dileptons
Subtract cocktail
Correct for pair acceptance
Fit two exponentials in mT –m0
1st component T ~ 260 MeV
Consistent with thermal photon yield
2nd “soft” component T ~ 100 MeV
Independent of mass
More than 50% of yield
Eludes any theoretical interpretation
hint also in NA60 data
300 < m < 750 MeV
258  37  10 MeV
92  11  9 MeV
PHENIX Phys. Rev. C 81 (2010)
Suggestions for soft “exotic” source:
red shift, glasma, color B-field
Axel Drees

24. Status: Thermal Radiation at RHIC Energies
PHENIX measured thermal e+e- and g from √sNN = 200 GeV
Soft low mass dilepton puzzle
larger excess beyond contribution from hadronic phase with medium modified r-meson properties … not from hadronic phase
soft momentum distribution … not from hot partonic phase
Thermal photon puzzle
Large thermal yield with T > 220 MeV (10-20% of decay photons)
… suggests early emission
Large elliptic flow (v2) … suggests late emission
PHENIX data on E&M probes seems INCONSISTENT with standard hydro space-time evolution!
And exhibits UNKOWN additional sources!
Speculation: look between impact (t=0) and t0
Axel Drees

25. Data from PHENIX HBD upgrade
Window less CF4 Cherenkov detector
GEM/CSI photo cathode readout
Operated in B-field free region
Improve S/B by rejecting
combinatorial background
HBD fully operational:
Single electron ~ 20 P.E.
Conversion rejection ~ 90%
Dalitz rejection ~ 80%
Improvement of S/B factor 5 to published results
10% central, 62 GeV:
efficiency 60%
Rejection 90%
p+p with HBD
uncorrected
p+p data in 2008/9
Au+Au data in 2009/10
Au+Au background subtraction needs to be finalized, results at QM
Axel Drees

26. Data from PHENIX VTX upgrade
Tracking with 4 layers
of silicon vertex detector
49.6mm
 24.8mm
(cm)
Vertex resolution in Y
29.2mm
(sim)
300mm
DCA resolution
sDCA
~ 80 mm
FVTX in 2011/12
VTX in 2010/11
Promise to tag e+e pairs from cc
Opens opportunity to measure thermal radiation above M = 1 GeV
Drawback
added material, increased background
Not compatible with HBD, no rejection
Online display of Au+Au collision
Impact on dilepton measurement unclear
Axel Drees

27. Summary and Outlook
We have discovered “thermal” radiation from heavy ion collisions
Dileptons allow to disentangle space-time evolution of collision
NA60 established method with m+m- from In-In at 158 AGeV
PHENIX measured e+e- and g from √sNN = 200 GeV
Soft low mass dilepton puzzle
Thermal photon puzzle
Data inconsistent with “standard hydrodynamic space-time evolution
Next steps towards state of the art experiments at high energy
Finalize ongoing analyses
PHENIX HBD & VTX data
STAR data plus detector upgrades
Ceterum censeo: Significant progress will require a new experimental effort dedicated to thermal radiation measurements!
Axel Drees

28. Backup slides
Axel Drees

29. STAR p+p Dilepton Data
STAR arXiv:
charm cross section: STAR s = 920 mb
PHENIX s = 570 mb
acceptance: STAR Df=2p |Dh|=2
PHENIX Df(pt)~2×p /2 |Dh|=0.7
PHENIX cocktail in STAR acceptance reasonable agreement with STAR data
possible shape issue in STAR data
Axel Drees

30. Thermal Photon v2 Using 2 different evaluations of Rg
Comparison of inclusive g v2
Calorimeter vs conversions
Comparison using 2 different Rg
Conversions vs virtual photons
Axel Drees

31. 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
Homogeneity of background radiation
Requires inflation phase!
Axel Drees

32. 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

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

34. 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

35. 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