Direct photons at GSI

1
Direct photons at GSI

• Sergey Kiselev, ITEP Moscow, for the ECAL group
• Introduction
• Prompt g
• g from transport codes
• Experimental methods
• Conclusions

2
Introduction - definitions

• In AB collision direct photons - photons that
  originate during time of the collision, ctAB
  50-100 fm po, h, h? rg/?g/2g, have
  ct ctAB - main origin of non-direct photons.
• Quark gluon level qq ? gg, qg ? qg, qq(g) ?
  qq(g)g Initial hard NN collisions,
  pQCD ? prompt g.
  Thermalised QGP
  stage ? thermal g from QGP.
• Hadron level
• decays w ? pg, a1 ? pg, D ? Ng, K ? Kg, f ? hg,
  r ? ppg, . . .
• meson scatterings pp ? rg, pr ? pg, pK ? Kg, Kr
  ? Kg, KK ? pg, pK ? Kg, . . .
  
  Thermalised hadron stage ?
  thermal g from hadron gas.

3
Introduction aims

• To estimate for the CBM energy
• Prompt ? contribution
• ? contribution from hadron sources (decays and
  rescatterings)
• Possibility to explore the state-of-the-art
  experimental methods

4
Prompt g yields at CBM

• Data fit (xTgt0.1) Ed3spp/d3p 575(vs)3.3
  /(pt)9.14 pb/GeV2 EPJ C22 (2001)
  129
• If extrapolation to the CBM energy is correct one
  can estimate yields
• 10-4 prompt g with pt gt 2 GeV/c per AuAu
  central event at 25 AGeV
• At beam intensity 109/s 1 interaction 10
  centrality ? prompt g rate 100/s

5
Prompt g PYTHIA
PYTHIA subprocesses q g ? q ?, q qbar ? g ?, q
qbar ? ? ?
Lowest c.m. energy PARP(2)7 instead of default
10.
s 2 10-4 mb 85 (q g ? q ?) 15
(q qbar ? g ?)
6
g from transport codes

• Photons from existing transport codes at the CBM
  energy (central AuAu at 25 AGeV)

g origin UrQMD HSD RQMD HIJING
prompt - - - BUT -
decays BUT - BUT ? h, w 5 ? per event h, w, D, f, K 10 ? per event
meson scatterings p? ? p?, pp ? ??, BUT - - - -
No a transport code with direct photons! Need the
meson scattering source of ?
At pt gt 1.5 GeV/c main contributions ? ? po ?, p
? ? p ?
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g from RQMD

• RQMD (Relativistic Quantum Molecular
  Dynamics)Phys. Rev. C52 (1995) 3291
• 103 AuAu central, blt3 fm, events at T/A25 GeV
  have been generated
• 5.4 g per event
• 2/3 from h
• 1/3 from w

8
g from HSD meson scatterings
Agreement with the HSD team to implement p? ? p?,
pp ? ?? Kapusta et.al. (Phys.Rev.D44 (1991) 2774)

ITEP team FORTRAN subroutines with cross
sections were prepared for the HSD code HSD team
implementation of the cross sections into the HSD
code
9
Experimental methods for direct photons

• Last years large progress in experimental
  techniques to extract direct photons
• Direct photons have been revealed by methods
• Subtraction method, WA98/2000, PHENIX/2005
• Momentum correlation method, WA98/2004
• Internal conversion method, PHENIX/2005
• ? Real photon conversion method, PHENIX/2006

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Internal conversion - details
Main assumption the Kroll-Wada (KW) formula,
Phys.Rev.98(1955)1355 A given reaction produces,
instead of real ?, ? ? ee-. For decays
N? number of real ? po ? ?? po ? ?? ?
?ee- I?KW 0.0059 ? ? ?? ? ? ?? ?
?ee- I?KW 0.0081 ?direct ?direct
? ee- I?KW 0.0162
phase space factor 1, if peeT mee
But for a fixed target experiment there is an
additional source of ee- in the A region from
real ? conversion in a target
The method must be modified for the fixed target
experiment
?
11
Real photon conversion

• Hot Quark 2006 real photon conversion in the
  beam pipe, PHENIX, nucl-ex/0608009
• Clear ? ? ee- identification
• Very good, dp/p 1, of tracking system
• While a result can not be presented yet
  (systematic 25), the method seems promising.

12
Conclusions

• Prompt ? reasonable agreement at pT2 GeV/c
  between PYTHIA predictions and data extrapolation
• ? from meson scatterings cross sections for p? ?
  p? pp ? ?? have been prepared, waiting for its
  implementation into the HSD code ? direct photon
  dynamics at the CBM energy
• Experimental methods
• Internal conversion method modification for the
  fix target experiment
• Real photon conversion method no final result,
  QM06 ?

13
Backup
14
Introduction motivation

• Data for excitation functions
• K/? ratios show a peak at 30 A GeV (horn).
• The kink in the K slope.
• Not reproduced by transport codes.

The aim to scan the energy region by other
observable, direct photons. It is very difficult
to extract direct photons. But high intensity
beams and new experimental methods (SPS, RHIC)
let us hope.
15
Quark gluon level
16
Prompt g pp data

• A Compilation of Data, J. Phys. G, 23 (1997) A1

CBM can cover the range vs lt 14 GeV
17
Prompt g data fit

• Data fit (xTgt0.1) Ed3spp/d3p 575(vs)3.3
  /(pt)9.14 pb/GeV2 EPJ C22 (2001)
  129
• For AB Ed3N/d3p(b) Ed3spp/d3p AB TAB(b)
  Ed3spp/d3p Ncoll/ sppin

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g from hadron gas. An example

• nucl-th/9712048, transport code based on the
  Walecka-type model

At pt gt 1.5 GeV/c main contributions ? ? po ?, p
? ? p ?
19
g from UrQMD

• UrQMD (Ultra Relativistic Quantum Molecular
  Dynamics) Prog. Part. Nucl. Phys. 41 (1998)
  225370.
• Phys. Rev. C57 (1998) 3271 Direct photons in
  PbPb at CERN-SPS
• The processes pp ? rg, pr ? pg were considered
  explicitly using cross sections given in Phys.
  Rev. D44, (1991) 2774.
• the pr ? pg and w ? pg processes are dominant in
  the range 1 GeV kT 3 GeV.

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g from UrQMD at CBM

• 103 UrQMD AuAu central events at 25 AGeV
• 14 photons per event. BUT all g are from
  decays, mainly a1 ? pg. Where are the processes
  pp ? rg, pr ? pg ?
• 22 Feb 2006, M. Bleicher
• . . . photons should not be calculated
  within the urqmd, but explicitely outside with a
  different code. everybody should ignore all
  processes with photons involved. we will move
  them out of the model in the next version.

21
g from HSD

• HSD (Hadron String Dinamics), Phys. Rep. 308
  (1999) 65, 6.6. Direct photons p. 174
• BUT in the file generated for CBM, 103 central
  AuAu events at 25 AGeV, there are NO photons at
  all !?

22
g from HIJING

• HIJING (Heavy Ion Jet Interaction Generator)
  Phys. Rev. D 44, (1991) 3501.
• Soft processes FRITIOF
• Hard processes PYTHIA
• NO rescatterings
• 25 AGeV Error too
  low CM energy, 6.982 GeV for event generation.
  Execution stopped!
• BLOCK DATA PYDATA PARP(2)6.8 instead 10.
• 103 AuAu central, blt3 fm, events at T/A25 GeV
  have been generated
• 10.5 g per event
• 60 from h
• 36 from w
• 4 from D
• Though direct g production (IHPR2(3)2) is
  included using PYTHIA, BUT at CBM energy the code
  does not generate direct g.

23
Subtraction method

• Direct photon measurement by the subtraction
  method WA98 PRL 85 (2000) 3595, PHENIX PRL 94
  (2005) 232301
• WA98 subtracted from measured photons those from
  known hadronic source from decays of
  reconstructed po, h and other hadrons (with some
  assumption of its yield and spectrum)
• At pt gt 0,5 GeV/c
• from po - 80
• from h - 13
• from w - 2
• from h - 1.3

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Subtraction method cont.
25
Momentum correlations method

• gg momentum correlations, WA98 PRL 93 (2004)
  022301, STAR nucl-ex/0511055
• gg correlations ? direct photons
• direct photons correlation provide the system
  sizes at all stages of heavy-ion collisions
• Needs larger statistics
• Rinv (gg) 6 fm Rinv (pp)
• emitted in the late, hadron gas, stage of the
  collision
• Thermal calculations substantialy underestimate
  the data.

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Low mass ee- pairs

• A novel technique (QM05) internal conversion of
  direct photons into ee-, PHENIX, nucl-ex/0511041
• any source of real g emits also virtual g with
  very low-mass
• gdirect ( gdirect /gincl.) gincl.
• Electrons in the central arms were
  identified by matching charged particle tracks to
  clusters in the ECAL and to rings in the RICH

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Low mass ee- pairs cont.

• Measurement for 1 lt pT lt 5 GeV/c consistent with
  calculations when thermal photon emission is
  taken into account (at high pT gt 5 it is
  consistent with a NLO pQCD calculation).

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