Direct photons at CBM

28.09.2007 10th CBM collaboration meeting, Dresden S.Kiselev ITEP 1

Direct photons at CBMSergey 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

28.09.2007 10th CBM collaboration meeting, Dresden S.Kiselev ITEP

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I. HSD reminder : ππ ργ, πρ πγ Agreement with the HSD team to implement c.s. for πρ πγ, ππ ργKapusta et.al. (Phys.Rev.D44 (1991) 2774) :

ITEP team: FORTRAN subroutines with cross sections for the HSD codeElena Bratkovskaya: implementation of the cross sections into the HSD code

σππ ~ 10σπρ

at √s =1 GeV

Divergence σcut = 1 mb

News, thanks to Elena: 1. weighted averaging of c.s. with the ρ spectral function overcomes divergence 2. some bugs were fixed


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HSD reminder: ω π0 γ

~ 20% of γω are from ρ/ρ0>1 region and can be hardly reconstructed as from ω π0 γ direct photons


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HSD: in-medium effect

channel πρ πγ π π ργ ω(ρ/ρ0>1)

dNγ/dy(yc.m.=0) 0.37/1.05 0.27/0.36 0.14/1.20 !!!

without IME with IME

In-medium effect (IME): dropping mass + broadening

m = m0 (1 + ρ/ρ0) ω = - 120 MeV, ρ = - 150 MeV


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HSD: in-medium effect – WA98 data

Not enough to describe the data take into account of a1 meson


main input: πρ πγ main input: ω(ρ/ρ0>1)


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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)


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II. (1+1) Bjorken hydrodynamics (BHD)

Proper time and rapidity y Phys.Rev., D27 (1983) 140

There is no dependence onLorenz boost variable y:

Landau hydrodynamical model, viscosity and conductivity are neglected


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Photon spectrumPhoton spectrum: convolution of the photon production rates with the space–time evolution of the collision

For a longitudinally expanding cylinder

For proper time and rapidity y`

For an ideal hadron gas

Main parameters: initial 0 , T0 and Tf (at freeze-out)

Connection with the local rest frame


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γ production rates Thermal rates from QGP:

Perturbative QCD, the lowest order in s

qq gγ, qg qγdN/d4xd3p s ln(0.23E/sT) exp(-E/T) T2/E,

ZP C53, 433bremsstrahlung

dN/d4xd3p s exp(-E/T) T2/E, PL B510, 98 Thermal rates from hot hadron gas (HHG):

effective theory for hadron interactionsπρ πγ, ππ ργ, ρ ππγ, ω πγdN/d4xd3p ~ T2.15 exp(-E/T) / exp((1.35 ET)0.77), PL B510, 98


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Hot hadron gas (HHG) scenario C.Song and G.Fai,

Phys.Rev., C58 (1998) 1689.

parameterizations for the processes ππ →ργ , πρ → πγ, and ρ →ππγ , in which the a1 meson is taken into account properly

One can fit WA98 direct photon data by HHG scenario only

Can one go further to the CBM energy?


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III. HSD and BHD for CBM energy

channel πρ πγ π π ργ ω(ρ/ρ0>1)

dNγ/dy(yc.m.=0) 0.11/0.39 0.10/0.21 0.11/0.89 !!!


at high pt ω(ρ/ρ0>1) dominates both without and with IME


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HSD and BHD for CBM energy –cont.

HSD: steeper slope, main contribution from ω (ρ/ρ0 >1)

HSD BHD


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HSD and BHD for CBM energy – cont.

at pt > 1 GeV/c γdirect /γπ0 ~ ( > ) 2% both for HSD/IME and BHDdynamics HSD BHD

IME + -

a1 meson - +


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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

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Direct photons at CBM