Direct Photon Production from Heavy Ion Collisions

Transcription

1 Direct Photon Production from Heavy Ion Collisions Bjørn Bäuchle The UrQMD Group ( based on PRC 81 (2010) and ongoing work FIAS Frankfurt Institutt for fysikk og teknologi, Universitetet i Bergen 28/

2 Collaborators The UrQMD-Group Marcus Bleicher, Horst Stöcker, Michael Mitrovski, Elvira Santini, Bjørn Bäuchle, Thomas Lang, Jan Steinheimer-Froschauer, Gunnar Gräf Sponsors HGS-HIRe Helmholtz Graduate School for Hadron and Ion Research

3 Introduction Model Photon Spectra at SPS Results for FAIR Results for RHIC Summary & Outlook

4 Why Heavy Ion Collisions? History of the Universe Hot nuclear matter present until 1 µs after Big Bang Direct observation of Big Bang only until 300ka after BB Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

5 Why Heavy Ion Collisions? History of the Universe Hot nuclear matter present until 1 µs after Big Bang Direct observation of Big Bang only until 300ka after BB No Problem: Recreate parts of Big Bang in the lab! Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

6 Why Heavy Ion Collisions? T Quark Gluon Plasma Collective behaviour of QCD-Matter Explore Phase Diagram Hadron Gas?Liquid/Gas 922 MeV Color Super Conductor µ B Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

7 Why Heavy Ion Collisions? T Quark Gluon Plasma µ B = 0, T 170 MeV: Cross-over Hadron Gas µ B =?, T =?: Critical End Point? Liquid/Gas 922 MeV high µ B 1 st Order PT? Color Super Conductor µ B Collective behaviour of QCD-Matter Explore Phase Diagram: Critical Point Deconfinement Chiral Restauration Get there by smashing nuclei! Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

8 Why direct photons? Interesting scattering in fireball Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

9 Why direct photons? Interesting scattering in fireball Hadronic decay products... Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

10 Why direct photons? Interesting scattering in fireball Hadronic decay products......rescatter. Information lost. Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

11 Why direct photons? Interesting scattering in fireball Hadronic decay products......rescatter. Information lost. Photons do not rescatter keep information! Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

12 Why direct photons? Interesting scattering in fireball Hadronic decay products......rescatter. Information lost. Photons do not rescatter keep information! Plenty uninteresting photon sources Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

13 Photon Sources in A+A Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

14 Direct Photon Experiments Helios, WA 80, CERES (SPS) 1 upper limits WA 93 (SPS) and STAR (RHIC) no results (yet) WA 98 2 first measurements at SPS PHENIX 3 (RHIC) various results 1 Helios: Z. Phys. C 46, 369 (1990); WA 80: Z. Phys. C 51, 1 (1991); PRL 76, 3506 (1996); CERES: Z. Phys. C 71, 571 (1996) 2 PRL 85, 3595 (2000) 3 e.g. PRL 94, (2005) Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

15 Direct Photon Experiments Helios, WA 80, CERES (SPS) 1 upper limits WA 93 (SPS) and STAR (RHIC) no results (yet) WA 98 2 first measurements at SPS PHENIX 3 (RHIC) various results Photons from hadronic decays make 97 % of all photons Uncertainties in hadron yield significantly change direct photon yield EM-Calorimeters expensive, coverage therefore usually small 1 Helios: Z. Phys. C 46, 369 (1990); WA 80: Z. Phys. C 51, 1 (1991); PRL 76, 3506 (1996); CERES: Z. Phys. C 71, 571 (1996) 2 PRL 85, 3595 (2000) 3 e.g. PRL 94, (2005) Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

16 Theory: Underlying Models pqcd Good for high p, good for pp. In AA, nuclear effects can be parametrized 4. Important at RHICand LHC-energies, what about SPS? Hydro Good for thermalized systems, phase transitions. Input: Parametrized Rates E dn d 3 pd4x (p,t,µ) Application: numerical challenge 5! Transport Good for not-too-dense systems. Input: Cross-sections dσ dt (s,t,ρ) Application: straight-forward 6 4 E.g. Aurenche, Fontannaz et. al, PRD 73, (2006) 5 Turbide et al., PRC 69, (2004); Turbide et al., PRC 72, (2005); Liu et al., arxiv: [hep-ph]; Vitev et al. arxiv: [hep-ph]; Haglin, PRC 50, 1688 (1994); Haglin, JPG 30, L27 (2004), BB et al., PRC 81 (2010) Dumitru et al., PRC 57, 3271 (1998); Huovinen et al., PRC 66, (2002); Li et al. arxiv:nucl-th/ ; BB et al., PRC 81 (2010) Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

17 Theory: Unknown processes Branching ratios of many processes not known, e.g. a 1 γπ. PDG says seen. What of it? Data from Zielinski 7 show odd behaviour; this work: take calculations from Xiong 8. Difference factor 2.5 may make a big difference! 7 Zielinski et al., PRL 52, 1195 (1984) 8 Xiong, Shuryak and Brown, PRD 46, 3798 (1992) Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

18 Theory: Unknown processes Branching ratios of many processes not known, e.g. a 1 γπ. PDG says seen. What of it? Data from Zielinski 7 show odd behaviour; this work: take calculations from Xiong 8. Difference factor 2.5 may make a big difference! Which channels are implemented? Bremsstrahlung? Hadronic decays? 7 Zielinski et al., PRL 52, 1195 (1984) 8 Xiong, Shuryak and Brown, PRD 46, 3798 (1992) Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

19 Theory: Unknown processes Branching ratios of many processes not known, e.g. a 1 γπ. PDG says seen. What of it? Data from Zielinski 7 show odd behaviour; this work: take calculations from Xiong 8. Difference factor 2.5 may make a big difference! Which channels are implemented? Bremsstrahlung? Hadronic decays? Hadronic decays are not in data! But: Do we believe that? 7 Zielinski et al., PRL 52, 1195 (1984) 8 Xiong, Shuryak and Brown, PRD 46, 3798 (1992) Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

20 UrQMD Classical propagation of hadrons QM scattering cross-sections UrQMD Ultra-Relativistic Quantum Molecular Dynamics Cross-sections fitted to data or calculated via detailed balance or parametrized via additive quark model All hadrons from PDG up to m = 2.2 GeV Full microscopic collision history availavble Version 3.3 out now! Go to Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

21 UrQMD+Hydro (new in u3.3) High-density part of evolution optionally substituted by ideal hydrodynamics Macroscopic description Microscopic initial state from transport (UrQMD) mapped to densities and flow velocities Hydro propagation with variable Equation of State Back to transport when ǫ < 5ǫ 0 in all cells seperately for all z (gradual)... in complete system (isochronous) Rescatterings and decays with UrQMD See also Petersen et al. Phys. Rev. C 78 (2008) Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

22 Equations of State Hadron Gas Includes all particles from UrQMD no phase transition Bag Model MIT Bag Model First Order Phase Transition at T C = 170 MeV Chiral Model Chirally restored phase First Order PT, Cross-over and CEP Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

23 Photons from the model Both models Only Cascade π + π γ + ρ, π + ρ γ + π Cross-sections from Kapusta, Lichard & Seibert (PRD 44 (1991) 2774) Only Hydro π + π γ + η, π + η γ + π, π + π γ + γ Cross-sections known, applied to every scattering Parametrizations from Turbide, Rapp & Gale (PRC 69 (2004) ) π + K γ + K, π + K γ + K, ρ + K γ + K, K + K γ + π Thermal rates known, applied to every cell Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

24 Photons from the model Cascade Emitted photons may be only a fraction of a photon Each collision and channel: 100 photons produced with different mandelstam t-values and appropiate weight N = dσγ dt t/σ tot less events calculated, better statistics Hydro Take care of proper Lorentz-Transformation (mind Cooper-Frye): Generate random p µ u µ according to thermal rate, then generate p so that it yields desired p µ u µ. For all cells, every implemented rate: one photon-information (with weight N = d 3 p dr E V t E ) is created. d 3 p Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

25 Check: Rates from Transport dr de [GeV 2 fm 4 ] 1 E 1 2π T = 150 MeV Box: V = (20 fm) 3, π/ρ/a 1 -gas Box-calculations (UrQMD) Hydro-rates (Turbide et al.) ππ γρ πρ γπ E [GeV] Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

26 Stages: HG-EoS E dn d 3 p (GeV 2 ) E lab = 158A GeV b < 4.5 fm, y cm < 0.5 preliminary Only using common channels: ππ γρ πρ γπ (incl. a 1 ) All γ from hybrid Pure UrQMD p (GeV) Hard photons only from before hydro-evolution Similar yield in hybrid and cascade mode! Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

27 Stages: HG-EoS E dn d 3 p (GeV 2 ) E lab = 158A GeV b < 4.5 fm, y cm < 0.5 preliminary Only using common channels: ππ γρ πρ γπ (incl. a 1 ) Transport-γ before hydro Pure UrQMD p (GeV) Hard photons only from before hydro-evolution Similar yield in hybrid and cascade mode! Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

28 Stages: HG-EoS E dn d 3 p (GeV 2 ) E lab = 158A GeV b < 4.5 fm, y cm < 0.5 preliminary Only using common channels: ππ γρ πρ γπ (incl. a 1 ) Transport-γ before hydro Hydro-γ Pure UrQMD p (GeV) Hard photons only from before hydro-evolution Similar yield in hybrid and cascade mode! Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

29 Stages: HG-EoS E dn d 3 p (GeV 2 ) E lab = 158A GeV b < 4.5 fm, y cm < 0.5 preliminary Only using common channels: ππ γρ πρ γπ (incl. a 1 ) Transport-γ before hydro Hydro-γ Transport-γ after hydro Pure UrQMD p (GeV) Hard photons only from before hydro-evolution Similar yield in hybrid and cascade mode! Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

30 Stages: HG-EoS E dn d 3 p (GeV 2 ) E lab = 158A GeV b < 4.5 fm, y cm < 0.5 preliminary Only using common channels: ππ γρ πρ γπ (incl. a 1 ) Transport-γ before hydro Hydro-γ Transport-γ after hydro All γ from hybrid Pure UrQMD p (GeV) Hard photons only from before hydro-evolution Similar yield in hybrid and cascade mode! Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

31 Stages: Cascade E dn d 3 p [GeV 2 ] complete cascade initial stage intermediate stage final stage UrQMD Pb+Pb 158 AGeV b < 4.5 fm, y c.m. < p [GeV] Split by time Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

32 Stages: BM-EoS E dn d 3 p [GeV 2 ] complete hybrid initial stage intermediate stage final stage Hybrid, Bagmodel EoS Pb+Pb 158 AGeV b < 4.5 fm, y c.m. < p [GeV] Hydro enhanced! Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

33 Different EoS vs. data WA98 Pb+Pb 158 AGeV Pure UrQMD Hybrid, Hadron gas EoS Hybrid, Bag Model EoS E dn d 3 p [GeV 2 ] Pb+Pb 158 AGeV b < 4.5 fm yc.m. < p [GeV] All models undershoot data, but no pqcd here! Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

34 E dn d 3 p [GeV 2 ] Diff. EoS, pqcd vs. data WA98 Pb+Pb 158 AGeV UrQMD + pqcd Hybrid + pqcd, HG EoS Hybrid + pqcd, BM EoS pqcd: Turbide et al. Pb+Pb 158 AGeV b < 4.5 fm yc.m. < p [GeV] Very good agreement for all three models Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

35 Photon Sources: Cascade E dn d 3 p (GeV 2 ) WA98 Pb+Pb 158 AGeV Processes with η ππ γγ b < 4.5 fm, y cm < 0.5 UrQMD preliminary p (GeV) Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

36 Photon Sources: Cascade E dn d 3 p (GeV 2 ) WA98 Pb+Pb 158 AGeV ππ γρ Processes with η ππ γγ b < 4.5 fm, y cm < 0.5 UrQMD preliminary p (GeV) Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

37 Photon Sources: Cascade E dn d 3 p (GeV 2 ) WA98 Pb+Pb 158 AGeV all πρ a 1 γπ πρ γπ ππ γρ Processes with η ππ γγ b < 4.5 fm, y cm < 0.5 UrQMD preliminary p (GeV) Dominant contribution from π + ρ γ + π (incl. a 1 ) Hard π + π-scatterings make power-law tail at high p Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

38 Photon Sources: Hybrid E dn d 3 p (GeV 2 ) WA98 Pb+Pb 158 AGeV Processes with η Processes with K or K ππ γγ b < 4.5 fm, y cm < 0.5 UrQMD+Hydro preliminary p (GeV) Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

39 Photon Sources: Hybrid E dn d 3 p (GeV 2 ) WA98 Pb+Pb 158 AGeV ππ γρ Processes with η Processes with K or K ππ γγ b < 4.5 fm, y cm < 0.5 UrQMD+Hydro preliminary p (GeV) Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

40 Photon Sources: Hybrid E dn d 3 p (GeV 2 ) WA98 Pb+Pb 158 AGeV all πρ γπ ππ γρ Processes with η Processes with K or K ππ γγ b < 4.5 fm, y cm < 0.5 UrQMD+Hydro preliminary p (GeV) Also, dominant contribution from π + ρ γ + π Very similar yield as in cascade mode! Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

41 Treatment of Strings High energies: UrQMD excites strings String ends (Quarks or Diquarks) have lower -sect What if they are excluded from γ-production? UrQMD w/o string ends UrQMD with string ends pqcd-photons (Gale) pqcd-photons (Turbide et al.) E dn d 3 p [GeV 2 ] Pb+Pb 158 AGeV b < 4.5 fm, y c.m. < p [GeV] Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

42 ρ on mass peak Scattering with incoming ρ: m 2 ρ = pµ p µ from UrQMD Scattering γρ: m ρ = 770 MeV? Maybe m ρ distributed acc. to Breit-Wigner E dn d 3 p [GeV 2 ] (on-off)/off m ρ fixed m ρ Breit-Wigner 100 UrQMD Pb+Pb 158 AGeV 10 b < 4.5 fm y c.m. < p [GeV] π ± π 0 γρ ± π ± π γρ 0 10 π ± π 0 γρ ± π ± π γρ p [GeV] Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

43 Flattening at high p E dn d 3 p [GeV 2 ] temission [fm] p [GeV] sum πρ γπ ππ γρ Processes inv. η 158 AGeV b < 4.5 fm y cm < 0.5 UrQMD p [GeV] Flattening at high p corresponds to earlier emission times Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

44 Flattening at high p E dn d 3 p [GeV 2 ] temission [fm] p [GeV] sum πρ γπ ππ γρ Processes inv. η 158 AGeV b < 4.5 fm y cm < 0.5 UrQMD E dn d 3 p [GeV 2 ] p [GeV] Flattening at high p corresponds to earlier emission times p γ [GeV] Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

45 A closer look at high p -photons dn d s (GeV 1 ) E dn d 3 p (GeV 2 ) s (GeV) All photons pure UrQMD scoll All photons p γ p (GeV) Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

46 A closer look at high p -photons dn d s (GeV 1 ) E dn d 3 p (GeV 2 ) s (GeV) All photons γ with p > 3 GeV scoll All photons γ from collisions with s > 4 GeV pure UrQMD p γ p (GeV) Most photons at high p come from high- s-collisions Hadronic treatment questionable Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

47 Emission times temission [fm] all πρ γπ ππ γρ Processes with η and ππ γγ 158 AGeV b < 4.5 fm y c.m. < 0.5 UrQMD p [GeV] dn dt [10 3 fm 1 ] Initial flash bulk emission later very long tail at intermediate p t 3 t 1, t 2 p = GeV p = GeV p = GeV Only πρ γπ t [fm] Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

48 Future: FAIR E dn d 3 p [GeV 2 ] Pure UrQMD Hybrid, Hadron gas EoS Hybrid, Bag Model EoS Hybrid, Chiral EoS 45 AGeV min. bias y cm < 0.5 preliminary p [GeV] Same picture at FAIR: Hadronic models similar, Bag Model enhanced. Chiral EoS shows higher Temperatur Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

49 FAIR E dn d 3 p [GeV 2 ] UrQMD preliminary sum πρ γπ ππ γx πη γπ 45 AGeV min. bias y cm < p [GeV] No surprises from channel-differential investigation Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

50 E dn d 3 p [GeV 2 ] Comparison RHIC vs. UrQMD PHENIX-Data UrQMD-calculations Min. Bias % % % % p [GeV] 40-50% % % Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

51 E dn d 3 p [GeV 2 ] PHENIX w/ HG-EoS PHENIX-data pure UrQMD-calculations hybrid-calculations N coll -scaled NLO pqcd Au+Au, s NN = 200 GeV p [GeV] Min. Bias % Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

52 Summary UrQMD and UrQMD+Hydro ideal for investigating photons Fit data at SPS, pqcd needed pqcd negligible at FAIR Hadronic Sources not sufficient at RHIC Hydro stage with QGP: largely enhanced emission πρ dominant (hadronic) contribution at intermediate p Exponential slope from non-thermalized system possible! High-p from high- s Outlook Bag Model, Chiral EoS for RHIC Investigate Parameter t hydrostart, ε crit Investigate transition scenario (gradual vs. isochronous) Different systems: Au+Au, snn = 130, 62.4,(200) GeV, E lab = 11, 30 GeV

53 Backup-Slides

54 Photon emission -sections 2m π m η m ρ m a1 +mπ +mπ 100 π ± π γρ 0 π ± π 0 γρ ± 10 π ± π γη π ± η γπ ± π ± π γγ m ρ + m π m a1 π ± ρ 0 γπ ± π ± ρ γπ 0 π 0 ρ ± γπ ± πρ a 1 γπ σ [mb] m ρ Breit-Wigner m ρ fixed σ [mb] 10 3 s [GeV] 10 3 s [GeV] Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

55 Photon emission rates 1 T = 170 MeV ππ γρ ( 1000) πρ γπ ( 1000) 10 5 E dr d 3 p [GeV 2 fm 4 ] πk γk πk γk ρk γk ( 10 6 ) K K γπ ( 10 6 ) E [GeV] Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

56 Previous Work with UrQMD d 3 p [GeV 2 ] E dn Dumitru et al. (UrQMD v1.0) UrQMD v1.3, this work UrQMD v2.3, this work p [GeV] Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

57 Initial Temperature Profile 12 BM-EoS HG-EoS y [fm] x [fm] Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

58 Total cross-section in UrQMD σ [mbarn] σ(π + π resonance ) σ(π + π strings ) σ(π + π hard scattering) 1 s [GeV] 10 Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

59 Inclusion of PYTHIA E dn d 3 p [GeV 2 ] with PYTHIA without PYTHIA UrQMD p + p 158 GeV all charged particles p [GeV] Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

60 Why not PYTHIA for pqcd WA98 Pb+Pb 158 AGeV UrQMD w PYTHIA PYTHIA UrQMD w/o PYTHIA E dn d 3 p [GeV 2 ] p [GeV] Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

61 Total cross-section σ [mbarn] σ(π + π resonance ) σ(π + π strings ) σ(π + π hard scattering) 1 s [GeV] 10 Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43

62 Total cross-section σ [mbarn] σ(π + π resonance ) σ(π + π strings ) σ(π + π hard scattering) 1 s [GeV] 10 Bjørn Bäuchle, FIAS Direct Photon Production from Heavy Ion Collisions Bergen, 28/ / 43