1. Thermal Radiation Mapping the Space-Time Evolution Thermal radiation from hadronic collisions: An old but still hot idea! Mapping the time evolution Experimental results on “thermal” radiation State of the Art experiment: NA60 Energy frontier: PHENIX Future perspectives PHENIX VTX and HBD upgrades Lessons learned and future opportunities Promise to help unravel time evolution Time evolution not understood; Hints for new physics? Unique possibilities for future Axel Drees, WWND 2012, Dorado del Mar, Puerto Rico

2. Thermal Radiation from QGP Axel Drees 2

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

4. Thermal Radiation from Expanding Source Radiation from longitudinally and radially expanding fire ball in “local equilibrium” Real and virtual photon momentum spectrum at mid rapidity: Temperature information Integrated over space time evolution due to T 4 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 Temperature information Not sensitive to collective expansion Axel Drees 4 Mass and momentum dependence allows to disentangle flow from temperature contributions!! Planck spectrum: yield  T 4, mean  T boosted by collective motion

5. Production process: real or virtual photons (lepton pairs) hadron gas: photons low mass lepton pairs QGP: photonsmedium mass lepton pairs Microscopic View of Thermal Radiation q qq e-e- e+e+       ** ** e-e- e+e+ q qg  Experimentally observed yield integrated over full time evolution! Key issues: In medium modifications of mesons pQCD base picture requires small  s But  s can not be small for dNg/dy ~ 1000 (i.e. in a strongly coupled plasma) Axel Drees 5 Equilibrium of strong interaction! Equilibrium not a necessary condition!

6. Experimental Issue: Isolate Thermal Radiation Axel Drees 6 1 10 10 7 log t (fm/c) ,  * from A+A Direct Hadron Decays “Prompt” hard scattering Pre-equilibrium Quark-Gluon Plasma Hadron Gas Thermal Non-thermal Sensitive to space-time evolution Need to subtract decay and prompt contributions

7. Axel Drees Map Out Time Evolution with Thermal Radiation Experimental method Measure inclusive  and l  l  Subtract experimental background e.g. combinatorial pair background Subtract “cocktail” of known sources, i.e. hadron decays after freeze out Isolate thermal radiation Real photons cocktail:  0 ,  0  More than 90% of photon yield Lepton pair cocktail:  0  e  e ,  0 e  e  and direct decays  e  e   J    e  e   Semileptonic decays of heavy flavor Drell Yan Dileptons have mass  remove contribution from  0  more sensitive to thermal radiation than photons Dileptons are more sensitive than photons 7

8. NA60 features Classic muon spectrometer Precision silicon pixel vertex tracker tagging of heavy flavor decay muons Reduction of combinatorial background by vetoing , K decay muons Double dipole for large acceptance (low mass) High rate capability Axel Drees 8 State of the Art Measurements with NA60 2.5 T dipole magnet beam tracker vertex tracker Muon Other hadron absorber muon trigger and tracking target magnetic field Next slides mostly derived from talks given by Sanja Damjanovic NA60 can isolate “thermal” contribution

9. Continuum Excess Measured by NA60 Axel Drees 9 Planck-like mass spectrum, falling exponentially (T > 200 MeV) For m>m  good agreement with three models in shape and yield Main Sources m < 1 GeV            Sensitive to medium spectral function Main sources m > 1 GeV qq       a 1      (Hess/Rapp approach) Eur. Phys. J. C 59 (2009) 607; CERN Courier 11/2009 Evidence for thermal dilepton radiation ~ 1/m exp(-m/T) 200 MeV 300 MeV Fully acceptance corrected

10. Axel Drees Sensitivity to Spectral Function Models for contributions from hot medium (mostly  from hadronic phase) Vacuum spectral functions Dropping mass scenarios Broadening of spectral function Broadening of spectral functions clearly favored!  annihilation with medium modified  works very well at SPS energies! 10 Not acceptance corrected Phys.Rev.Lett. 96 (2006) 162302 Hadronic contributions explored and exhausted

11. Dominance of partons for m>1GeV Schematic time evolution of collision at CERN energies Partonic phase early emission: high T, low v T Hadronic phase late emission: low T, high v T Experimental Data: thermal radiation Mass < 1 GeV from hadronic phase  th   MeV < T c Mass > 1 GeV from partonic phase  th   MeV >T c Axel Drees 11 hadronic     →  →     partonic qq→     Dileptons for M >1 GeV dominantly of partonic origin Eur. Phys. J. C 59 (2009) 607 T eff ~ + M 2 Phys. Rev. Lett. 100 (2008) 022302

12. Status: Thermal Radiation at SPS energies State of the art dilepton experiment: NA60 Isolate thermal radiation Planck like exponential mass spectra exponential m T spectra zero polarization general agreement with models of thermal radiation Emission sources of thermal dileptons (from m-p T ): hadronic (      annihilation) dominant for M<1 GeV partonic (qq annihilation) visible M>1 GeV In-medium  spectral function identified: no significant mass shift of the intermediate  only broadening. Axel Drees 12

13. Axel Drees Thermal radiation at RHIC Energies: PHENIX Photons, neutral pion  0     e-e- ee Calorimeter PC1 PC2 PC3 DC magnetic field & tracking detectors e + e - pairs E/p and RICH Disclaimer: ongoing analysis from STAR analysis 13 No background rejection! dilepton S/B < 1:150 HBD upgrade!

14. Axel Drees 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 14 Consistent with PHENIX single electron measurement  c = 567±57±193  b

15. Axel Drees Dilepton Continuum in d+Au Collisions 15 Consistent with known sources Data will constrain known sources with high precision In particular bottom cross section PHENIX preliminary

16. Axel Drees Au+Au Dilepton Continuum Excess 150 <m ee <750 MeV: 3.4 ± 0.2(stat.) ± 1.3(syst.) ± 0.7(model) Charm from PYTHIA filtered by acceptance  c = N coll x 567±57±193  b Charm “thermalized” filtered by acceptance  c = N coll x 567±57±193  b Intermediate-mass continuum: consistent with PYTHIA since charm is modified room for thermal radiation hadron decay cocktail tuned to AuAu 16 PHENIX VTX upgrade PHENIX Phys. Rev. C 81 (2010) 034911

17. Axel Drees Models calculations with broadening of spectral function: Rapp & vanHees Central collisions scaled to mb + PHENIX cocktail Dusling & Zahed Central collisions scaled to mb + PHENIX cocktail Bratkovskaya & Cassing broadening  annihilation with medium modified  insufficient to describe RHIC data! Large low mass enhancement 17 Excess not from hadronic phase!! Low Mass Dilepton Puzzle PHENIX Phys. Rev. C 81 (2010) 034911

18. Soft Low Mass Dilepton Puzzle m T spectrum of excess dileptons Subtract cocktail Correct for pair acceptance Fit two exponentials in m T –m 0 Eludes any theoretical interpretation Hint also in NA60 data Insufficient date for more detailed information Axel Drees 18 92  11  9 MeV 258  37  10 MeV 300 < m < 750 MeV Soft component below m T ~ 500 MeV: T eff ~ 100MeV independent of mass more than 50% of yield Excess has 2 components : (1) Thermal radiation (next slides); (2) Soft “exotic” source, red shift, glasma, color B-field acceptance corrected PHENIX Phys. Rev. C 81 (2010) 034911

19. pQCD  * (m  0)  T ~ 220 MeV First Measurement of Thermal Radiation at RHIC Direct photons from real photons: Measure inclusive photons Subtract    and  decay photons at S/B < 1:10 for p T <3 GeV Direct photons from virtual photons: Measure e + e - pairs at m  < m << p T Subtract  decays at S/B ~ 1:1 Extrapolate to mass 0 Axel Drees 19 First thermal photon measurement: T ini > 220 MeV > T C  * (m→0) =  ; m << p T Need to consider radial flow! PHENIX Phys. Rev. C 81 (2010) 034911

20. Calculation of Thermal Photon Yield 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, 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.) Observations comparing models: Correlation between T and   Yield typically to low Yield not correlated with T ini T ini = 300 to 600 MeV   = 0.15 to 0.5 fm/c 20 Axel Drees Low yield  earlier emission (yield  T 4 ) increase factor 2 with <20% change in T PHENIX Phys. Rev. C 81 (2010) 034911

21. 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 Axel Drees 21 Large v 2 of low p T thermal photon PHENIX arXiv:1105.4126

22. Thermal Photon Puzzle Large flow requires late emission! Apparent contradiction with yield, which points towards early emission! Axel Drees 22 Hees/Gale/Rapp Phys.Rev.C84:054906,2011. R. Chatterjee and D. K. Srivastava PRC 79, 021901(R) (2009) PRL96, 202302 (2006) Models fail to describe simultaneously photon yield, T and v2! T ini ~ 325MeV

23. Status: Thermal Radiation at RHIC energies PHENIX e + e  and  from √s NN = 200 GeV Soft low mass dilepton puzzle larger excess beyond contribution from hadronic phase with medium modified  -meson properties … not from hadronic phase soft momentum distribution … not from hot partonic phase Thermal photon puzzle Large thermal yield with T > 220 MeV (20% of decay photons) … suggests early emission Large elliptic flow (v2) … suggests late emission Axel Drees 23 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  0

24. 10% central, 62 GeV: efficiency 60% Rejection 90% Near Future: PHENIX HBD upgrade HBD fully operational: Single electron ~ 20 P.E. Conversion rejection ~ 90% Dalitz rejection ~ 80% Improvement of S/B factor 5 to published results Axel Drees 24 Window less CF4 Cherenkov detector GEM/CSI photo cathode readout Operated in B-field free region p+p data in 2008/9 Au+Au data in 2009/10 Au+Au background subtraction needs to be finalized, results at QM p+p with HBD uncorrected Improve S/B by rejecting combinatorial background

25. Near Future: PHENIX VTX upgrade Axel Drees 25 VTX in 2010/11 FVTX in 2011/12 Tracking with 4 layers of silicon vertex detector Online display of Au+Au collision 49.6  m  24.8  m (cm) Vertex resolution in Y 29.2  m (sim) 300  m DCA resolution  DCA ~ 80  m Promise to tag e+e  pairs from ccbar Opens opportunity to measure thermal radiation above M = 1 GeV Drawback added material, increased background Not compatible with HBD, no rejection Impact on dilepton measurement unclear

26. Summary of Findings We have discovered “thermal” radiation from heavy ion collisions Dileptons allow to disentangle space-time evolution of collision NA60 established method with     from In-In at 158 AGeV PHENIX e + e  and  from √s NN = 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 RHIC) PHENIX HBD & VTX data STAR with full detector upgrades Significant progress will requires a new experiment at RHIC dedicated to thermal radiation measurements! Axel Drees 26

27. Backup Slides Axel Drees 27

28. 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 < 3 10 -12 fm Axel Drees 28 Homogeneity of background radiation Requires inflation phase!

29. STAR p+p Dilepton Data Axel Drees 29 STAR arXiv:1204.1890 STAR charm cross section  = 920  b PHENIX cocktail in STAR acceptance MC@NLO for heavy flavor resolution not tuned for STAR

30. Lesson learned to Pursue Thermal Radiation Build dedicated thermal radiation experiment Map thermal radiation in phase space Deconvolve temperature and flow Map time evolution of system Focus on Dileptons e + e  preferred for collider and y=0  in coincidence is a must to tag background     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 Axel Drees 30 Strong Physics Program Large Discovery Potential

31. 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 p T virtual photons? Detectors not setup for dilepton measurements Axel Drees 31 Strong physics program at RHIC with little competition from LHC

32. Thermal Radiation Experiment Axel Drees 32 Design requirement (educated guess) Large acceptance (2  ;  y=2) For high statistics and better systematics Charged tracking Good electron id (1:1000  rejection) Excellent momentum resolution (  p/p < 0.2% p) Combinatorial background rejection Passive: minimize material budget (in particular before first layer) Active: solid Dalitz rejection scheme Heavy flavor detection Low mass precision vertex tracker (<10-20  m 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!

33. Transverse Mass Distributions of Excess Dimuon All m T spectra exponential for m T -m > 0.1 GeV Fit with exponential in 1/m T dN/m T ~ exp(-m T /T eff ) Soft component for <0.1 GeV ?? Only in dileptons not in hadrons (speculate red shift???) Axel Drees 33 transverse mass: m T = (p T 2 + m 2 ) 1/2 Phys. Rev. Lett. 100 (2008) 022302 Eur. Phys. J. C 59 (2009) 607

34. Intermediate Mass Data for 158 AGeV In-In Experimental Breakthrough Separate prompt from heavy flavor muons Isolate prompt continuum excess Axel Drees 34 Intermediate Mass Range prompt continuum excess 2.4 x Drell-Yan Eur.Phys.J. C 59 (2009) 607

35. Axel Drees Interpretation as Direct Photon Relation between real and virtual photons: Extrapolate real  yield from dileptons: 35 Virtual Photon excess At small mass and high p T Can be interpreted as real photon excess no change in shape can be extrapolated to m=0

36. Axel Drees Search for Thermal Photons via Real Photons 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 The internal conversion method should also work at LHC! internal conversions 36