Direct photons at CBM

1. Direct photons at CBM Sergey Kiselev, ITEP γdirect in Hadron String Dynamics (HSD) code γdirect in Bjorken HydroDynamics (BHD) HSD and BHD predictions for CBM energy Conclusions and next steps 10th CBM collaboration meeting, Dresden S.Kiselev ITEP

2. I. HSD reminder : ππ ργ, πρ πγ Agreement with the HSD teamto implement c.s. for πρ πγ, ππ ργ Kapusta et.al. (Phys.Rev.D44 (1991) 2774) : σππ ~ 10σπρ at √s =1 GeV Divergence σcut = 1 mb ITEP team: FORTRAN subroutines with cross sections for the HSD code Elena Bratkovskaya: implementation of the cross sections into the HSD code News, thanks to Elena: 1. weighted averaging of c.s. with the ρ spectral function overcomes divergence 2. some bugs were fixed 10th CBM collaboration meeting, Dresden S.Kiselev ITEP

3. HSD reminder: ω π0γ ~ 20% of γωare from ρ/ρ0>1 region and can be hardly reconstructed as from ω π0γ direct photons 10th CBM collaboration meeting, Dresden S.Kiselev ITEP

4. HSD: in-medium effect In-medium effect (IME): dropping mass + broadening m = m0 (1 +  ρ/ρ0)ω = - 120 MeV, ρ = - 150 MeV without IME with IME 10th CBM collaboration meeting, Dresden S.Kiselev ITEP

5. HSD: in-medium effect – WA98 data without IME with IME main input: πρ πγ main input: ω(ρ/ρ0>1) Not enough to describe the data  take into account of a1 meson 10th CBM collaboration meeting, Dresden S.Kiselev ITEP

6. reminder: emission rates with a1 meson C.Song, PR C47, 2861 (1993). Emission rates are increased with the inclusion of a1 T=150: ~ 1 GeV, factor 3(γπρ) - 5 (γππ) 10th CBM collaboration meeting, Dresden S.Kiselev ITEP

7. II. (1+1) Bjorken hydrodynamics (BHD) Phys.Rev., D27 (1983) 140 Proper time  and rapidity y There is no dependence on Lorenz boost variable y: Landau hydrodynamical model, viscosity and conductivity are neglected 10th CBM collaboration meeting, Dresden S.Kiselev ITEP

8. Photon spectrum Photon spectrum: convolution of the photon productionrates with the space–time evolutionof thecollision For a longitudinallyexpanding cylinder For proper time  and rapidity y` Connection with the local rest frame For anideal hadrongas  Main parameters: initial0 , T0 and Tf (at freeze-out) 10th CBM collaboration meeting, Dresden S.Kiselev ITEP

9. γ production rates • Thermal rates from QGP: Perturbative QCD, the lowest order in s qq  gγ, qg  qγ dN/d4xd3p  s ln(0.23E/sT) exp(-E/T) T2/E, ZP C53, 433 bremsstrahlung dN/d4xd3p  s exp(-E/T) T2/E, PL B510, 98 • Thermal rates from hot hadron gas (HHG): effective theory for hadron interactions πρ πγ, ππ ργ, ρ ππγ,ω πγ dN/d4xd3p ~ T2.15 exp(-E/T) / exp((1.35 ET)0.77), PL B510, 98 10th CBM collaboration meeting, Dresden S.Kiselev ITEP

10. Hot hadron gas (HHG) scenario • C.Song and G.Fai, Phys.Rev., C58 (1998) 1689. parameterizations for theprocesses ππ →ργ , πρ → πγ, and ρ →ππγ , inwhich the a1 meson is takeninto account properly One can fit WA98 direct photon data by HHG scenario only Can one go further to the CBM energy? 10th CBM collaboration meeting, Dresden S.Kiselev ITEP

11. III. HSD and BHD for CBM energy with IME without IME at high ptω(ρ/ρ0>1) dominates both without and with IME 10th CBM collaboration meeting, Dresden S.Kiselev ITEP

12. HSD and BHD for CBM energy –cont. HSD BHD HSD: steeper slope, main contribution from ω (ρ/ρ0 >1) 10th CBM collaboration meeting, Dresden S.Kiselev ITEP

13. HSD and BHD for CBM energy – cont. at pt > 1 GeV/c γdirect /γπ0 ~ ( > ) 2% both for HSD/IME and BHD 10th CBM collaboration meeting, Dresden S.Kiselev ITEP

14. Conclusions and next steps • HSD: γdirect depends strongly on in-medium mass effect • For hadron scenario at CBM one can hope γdirect /γπ0 ~ > 2% at pt ~ > 1 GeV/c • next steps: • Check the in-medium effect in HSD • to implement c.s. for πρ πγ, ππ ργ with the inclusion of a1 meson 10th CBM collaboration meeting, Dresden S.Kiselev ITEP