1. Axel Drees, Stony Brook University, Lectures at Trento June 16-20, 2008 1 10 10 7 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 Thermal Non-thermal

2. Axel Drees Measuring Photons is not Hard!

3. Axel Drees Photons from Hadron Decays l Example RHIC Au-Au Main contribution   (~85%) 2 nd largest contribution  meson (~12%) l 3 rd largest contribution w meson (~3%) l All other contributions negligible l These are high p T values; some variation at lower p T

4. Axel Drees hadron gas: QGP: Direct Contributions l Direct photons l From initial hard scattering “prompt” l From medium: “thermal”, “pre- equilibrium”, other effects prompt thermal decays Direct contributions small (<10%) compared to hadron decay contribution  measurement limited by systematic uncertainties     q qg  q qg 

5. Axel Drees Theoretical Expectation for RHIC Window for thermal radiation: 0.5 to 2.5 GeV thermal hard Turbide, Rapp & Gale PRC (2004)

6. Axel Drees Search for Direct “Thermal” Photons at the SPS l 1 st and 2 nd generation experiments gave upper limits l With oxygen and sulfur beams Measurement limited by systematic errors on data analysis &  production Experime nt publishedyp T (GeV/c)systemUpper limit HELIOS 2Z.Phys. C46 (90)1.0-1.90.1 – 1.5p-W, O-W, S-W13% WA80Z.Phys. C51 (91)1.5-2.10.4 – 2.8O-Au15% WA98PRL (96)2.1-2.90.5 – 2.5S-Au12.5% CERESZ.Phys. C71 (96)2.1–2.650.4 – 2.0S-Au14% ~13% upper limits on direct photon production from central O and S beams

7. Axel Drees Measurement of Direct Photons Measure p T spectrum of   and  mesons with high accuracy Calculate number of decay photon per   l Usually with Monte-Carlo m T scaling for (  ),  ’, , … l Get clean inclusive photon sample l Charged background subtraction l Finally: Subtract decay background from inclusive photon spectrum Handy formula:

8. Axel Drees Why this is Difficult? Signal ! Reduce systematic uncertainties: (e.g. energy scale non-linearity) partially cancel in this ratio “Subtraction method”

9. Axel Drees WA98 Result l 20% direct photon excess at high p T in central Pb+Pb collisions at CERN SPS l No signal within errors in peripheral collisions

10. Axel Drees WA98 Result and Interpretation l WA98 data from Pb-Pb collisions l Published 2000 l 14 years after start of SPS program l Clear signal above 2 GeV/c l Access beyond prompt component l Consistent T init ~200-270 MeV l Remains ambiguous l Upper limits below 1.5 GeV/c l Systematic errors at low p T remain prohibitive Data: WA98, PRL 85 (2000) 3595 Theory: Turbide, Rapp & Gale PRC (2004) WA98 Pb-Pb First hint of direct photons from Pb-Pb

11. Axel Drees Direct Photon Search in the RHIC Era l Significant progress with PHENIX: l Better input to decay cocktail  0 and  measured more accurately

12. Axel Drees Reference Data from p+p l PHENIX preliminary result. l NLO-pQCD calculation l Private communication with W.Vogelsang l CTEQ6M PDF. l Sum of direct photon bremsstrhlung photon l 3 scales (1/2p T,1p T,2 p T ) For renormalization scale factorization scale pQCD calculation consistent with PHENIX data

13. Axel Drees Comparison with Other Experiment Systematic errors are not shown PHENIX Preliminary proton-proton collisions proton-antiproton collisions

14. Axel Drees l Excepted from QCD, if l Q 2 -Scaling of PDF,FF l No running coupling constant(  s ) n=constant x T =2p T /  s l Can be express as two terms Interaction Structure l If leading order n=4 Next-to-leading order: n=4+  Perturbative QCD: x T Scaling All data consistent with x T -Scaling n=~5 PHENIX data preliminary

15. Axel Drees Analysis method:  0 tagging method as used in p+p l NLO pQCD Calculation l p+p collisions l Calculated by W.Vogelsang l CTEQ6M l Scale(renormalization and factorization scale) 0.5,1.0,2.0 p T l Binary scaling to d+Au l 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

16. Axel Drees Blue line: N coll scaled p+p cross-section Direct Photons from Au-Au Collisions PRL 94, 232301 (2005) + preliminary data at high pt Au-Au data consistent with pQCD calculation scaled by N coll

17. Axel Drees Direct Photons are a Key Calibration for Jet Production l Jet quenching in Au-Au collisions l Direct photons follow binary collision scaling l Pions are suppressed by factor of 5

18. Axel Drees Most Recent Data out to 20 GeV l Use pp data as reference rather than pQCD l Use most recent data analysis R AA for direct photons drops? Shadowing? Isospin effects? Data wrong?

19. Axel Drees But what about thermal photons? l Go back to quantity actually measured! l 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

20. Axel Drees Alternative Approaches with Real Photons l Tagging method (explain of black board) l Photons detected by calorimeter Photons detected by conversions, i.e. e + e  pairs Needs still more work and more statistics to get conclusive result

21. Axel Drees Dileptons at low mass but high p T ? 0<p T <0.7 GeV/c 0.7<p T <1.5 GeV/c 1.5<p T <8 GeV/c 0<p T <8.0 GeV/c p+p Au+Au m<<p T Can we distil thermal photons from dileptons??

22. Axel Drees phase space factorform factor invariant mass of virtual photon invariant mass of Dalitz pair form factor invariant mass of Dalitz pair invariant mass of virtual photon Compton q  g q q  g q e+e+ e-e- The idea l Start from Dalitz decay l Calculate inv. mass distribution of Dalitz pairs‘ l Now direct photons Any source of real  produces virtual  with very low mass l Rate and mass distribution given by same equation Form factor * phase space factor converge towards unity for m ee << p T or m ee  0 phase space factor N.M.Kroll and W.Wada, Phys. Rev. 98 (1955) 1355 (m ee << p T )

23. Axel Drees A Closer Look at Mass Region 150 to 300 MeV l p+p l Well described for p T <2 GeV l Small excess at higher p T l Au+Au l Large exces at all p T arXiv:0804.4168v1, 25 April 2008

24. Axel Drees How to Extract a Direct Yield: l Example: one pT bin for Au+Au collisions arXiv:0804.4168v1, 25 April 2008

25. Axel Drees Fraction of Direct Photons arXiv:0804.4168v1, 25 April 2008

26. Axel Drees First Measurement of Thermal Radiation at RHIC arXiv:0804.4168v1, 25 April 2008 l Slope analysis of data: l pQCD + exp. l Fix B, b, and n from p+p l Inverse Slope: (min. bias Au-Au) T = 224  16 (stat)  18 (sys) l Initial temperatures and times from theoretical model fits to data: l 0.15 fm/c, 590 MeV (d’Enterria et al.) l 0.2 fm/c, 450-660 MeV (Srivastava et al.) l 0.5 fm/c, 300 MeV (Alam et al.) l 0.17 fm/c, 580 MeV (Rasanen et al.) l 0.33 fm/c, 370 MeV (Turbide et al.) From data: T ini > 220 MeV > T C From models: T ini = 300 to 600 MeV   = 0.15 to 0.5 fm/c