Single%20Photons%20from%20Relativistic%20Heavy%20Ion%20Collisions%20:%20A%20Look%20at%20the%20Beginnings

1
Single Photons from Relativistic Heavy Ion
Collisions A Look at the Beginnings

• Dinesh K. Srivastava
• Variable Energy Cyclotron Centre
• Kolkata 700 064, India

Initial Conditions in Heavy Ion Collisions.
Goa, September 2008
2

• Thirty two years ago
• E. L. Feinberg, Nuv. Cim. A 34 (1976) 391,
  pointed out that
• Direct photons real or virtual are
  penetrating probes for the bulk matter produced
  in hadronic collisions, as
• - They do not interact strongly.
• - They have a large mean free path.

Since then relentless efforts by researchers
from across the world have established these as
reliable probes of hot and dense matter.
3
Electromagnetic Probes
Penetrating probes are emitted at all stages
then survive unscathed (ae ltltas). Historians of
the heavy ion collision encode all sub-processes
at all times
A jet passing through QGP
Different processes different characteristic
spectra
4
Direct PhotonsDifferent Sources - Different
Slopes
Rate
Photons are result of convolutions of the
emissions from the entire history of the nuclear
collision, so we need rates a model for
evolution.
Hadron Gas Thermal Tf
QGP Thermal Ti
Pre-Equilibrium?

• Hydrodynamics.
• Cascades.
• Fire-balls.
• CascadeHydro.

New
Jet Re-interaction v(Tixvs)
pQCD Prompt xvs
Eg
5
Partonic Processes for Production of Prompt
Photons in Hadrons
Fragmentation
Compton
Annihilation
Calculate using NLO pQCD with shadowing
scaling with TAA(b) for AA, partons remain
confined to individual nucleons do not forget
the iso-spin! The quarks will lose energy
before fragmenting if there is QGP suppressing
the fragmentation contribution. See e.g., Jeon,
Jalilian-Marian, Sarcevic, NPA 715 (2003) 795,
QM-2002.
6
NLO pQCD description of world prompt- photon data.
Aurenche et al. PRD 73 (2006) 094007.
See also Gordon Vogelsang (1994).
Most suitable scale is mmRmFpT/2. Do not
forget that mpT/3 for pions !!! Why should they
be different?
7
Neutrons are different from protons!
Chatterjee, Jeon, and S., to be published.
8
In the QGP we also have
g
Medium induced bremsstrahlung First calculated
by Zakharov, JETP Lett. 80 (2004) 1 Turbide et
al, PRC 72 (2005) 014905. Zhang, Kang, Wang,
hep-ph/0609159.
Annihilation with scatterring First calculated
by Aurenche et al, PRD 58 (1998) 085003.
Complete leading order results Arnold, Moore,
Yaffe, JHEP 0112 (2001) 009. How about NLO?
9
Examples of Hadronic Processes Involving p r
for Production of Photons

• Include pr?a1? pg
• Xiong et al, PRD 46 (1992) 3798
• Song, PRC 47 (1993) 2861.
• Include baryonic processes.
• Alam et al, PRC 68 (2003) 031901.
• Medium modifications (Series of
• valuable papers, T and mb)
• Alam et al, Ann. Phys. 286 (2001)
• 159.
• Include strange sector, massive
• Yang- Mills theory, form-factors,
• baryons, t-channel exchange of
• w mesons etc.
• Turbide, Rapp, Gale, PRC 69
• (2004) 014903.

First calculated by Kapusta, Lichard, Seibert,
PRD 44 (1991) 2774.
10
Complete Leading Order Rates from QGP
Exhaustive Reactions in Hadronic Matter
Rates are similar !!
We need QGP at higher T0 for golden photons to
clearly outshine others.
Arnold, Moore, Yaffe, JHEP 0112 (2001)
009. Turbide, Rapp, Gale, PRC 69 (2004) 014903.
11
The rate of production of photons having total
energy E and momentum p can be written as
T(x, y, t), vT(x, y, t), and fi using
hydrodynamics and Equation of State.
12
Upper Limit of Single Photons, WA80
Ruled out hadronic gas with limited hadrons p,
r, w, h.
PbPb_at_SPS
S. and Sinha, PRL 73 (1994) 2421 Dumitru et al.,
PRC 51 (1995) 2166. Sollfrank et al., Lee
Brown, Arbex et al., .
Cleymans, Redlich, S., PRC 55 (1997)
1431. However, nhad gtgt 2-3 /fm3 ! For No Phase
Transition.
13
QGP or Hot Hadrons? Enter WA98
QGP
prompt g
v0.ne.0
Hadrons (mhad ?0 at Ti205 MeV for all hadrons)
Huovinen et al, PLB 535 (2002) 109. QGP or
hadrons ( nhad gtgt 1/fm3 at Ti 245-275 MeV)
Alam et al, PRC 63 (2001) 021901 ( R).
14
2-loop ? Complete O(aS) for QGP pra1?
Exhaustive Hadronic Reactions for hadrons
S., PRC 71 (2005) 034905.
S. Sinha, PRC 64 (2001)034902 (R ).
Hydrodynamics, QGP rich EOS for hadrons
accounting for the prompt photons
15
So, what did we learn from the single photon
data at SPS energies?

• Hadronic gas with limited degrees of freedom is
  definitely ruled out.
• Massively medium modified hadonic gas with mhad?0
  at T0 200 MeV or an over-dense hadronic gas
  with nhad gtgt 1/fm3 at t0 is not ruled out.
• QGP initial state describes the data well.
• Initial temperature could be 210 350 MeV,
  depending on t0 , profile, v0. What is a good
  t0? Should v0 be non-zero?

RHIC should provide higher initial
temperatures. More importantly RHIC ushers in a
paradigm shift by producing jets and quenching
them! Reference pp data available.
Hope
16
Interaction of hard-scattered parton with dense
matter
Jet Photon Conversion
External Probe!!
Hard scattered parton
Fries, Mueller, S. , PRL 90 (2003) 132301.
17
Jet-Initiated EM Radiations from QGP

• Annihilation and Compton processes peak in
  forward and backward directions
• one parton from hard scattering, one parton from
  the thermal medium cutoff p?,min gt 1 GeV/c.
• photon carries momentum of the hard parton
• Jet-Photon Conversion
• This puts photon production and jet-quenching on
  the same page!!

18
Jet-Photon Conversion Rates

• Annihilation and Compton rates
• thermal medium

quark (-jet) distribution
19
Photons from Passage of Jets through QGP
Fries, Mueller, S., PRL 90 (2003) 132301.
This bremsstrahlung contribution will be
suppressed due to E-loss and there will be an
additional jet-induced bremsstrahlung, which is
also similarly suppressed, leaving the
jet-conversion photons as the largest source for
pT 4-10 GeV.
20
FMS Results Comparison to Data
calibrate pQCD calculation of direct and
Bremsstrahlung photons via pp data

• for ptlt6 GeV, FMS photons give significant
  contribution to photon spectrum 50 _at_ 4GeV.

Proper Isospins Shadowing !!!
Fries, Mueller, S., PRC 72 (2005) 041902( R).
21
Evolution of jet distribution due to E-loss
AMY
22
AMY and One-Stop Treatment of Jet-Quenching and
Jet-Initiated Photons
Turbide, Gale, Frodermann, Heinz, PRC77 (2008)
024909.
23
QpT/sqrt(2) for prompt calculations, Turbide et
al. (see also Arleo, JHEP 0609, 015 (2006), Liu
Werner, hep-ph/0712.3619 and Liu Fries,
nucl-th/0801.0453 . ).
24
Parton Cascade Model
Embedded in the partonic cascades
Renk, Bass, S., PLB 632 (2006) 632.
Bass, Mueller, S., PRC 66 (2002) 061902 (R).
LPM plays a significant role.
25
Thermal photons from AuAu_at_RHIC
dEnterria Peressounko, EPJC 46 (2006) 451.
26
So, what are we seeing at RHIC ?

• QGP or partonic initial state describes the data
  well.
• Models of evolution hydrodynamics, cascades,
  fireballs.
• Initial temperature could be 300 580 MeV,
  depending on t0 0.6 - 0.15 fm/c. Chemical
  equilibrium assumed.
• Emerging evidence for photons from the passage
  of jets through the plasma. Dileptons should
  follow.

S., Mustafa, Mueller, PRC 56 (1997) 1064.
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Not Just T0!

• Measure evolution of elliptic flow with thermal
  photons (v2 gt0)!
• Help understand dE/dx with photon or dilepton
  tagged jets.
• Study evolution of size with intensity
  interferometry of photons!

28
Elliptic Flow of Thermal PhotonsMeasure
Evolution of Flow !
Adult life
Late times
Early times
Chatterjee, Frodermann, Heinz, and S., PRL 96
(2006) 202302.
29
Impact Parameter Dependence of v2
30
Elliptic Flow of Thermal Dileptons Measure
Evolution of Flow !
Chatterjee, Heinz, Gale, S., PRC 75, 054909
(2007). See poster by Rupa
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Evolution of flow for thermal photons
Rupa Chatterjee et al. to be published
32
Azimuthal Anisotropy of Photons from Passage of
Jets Through QGP

• Jet-photon conversion
• (v2 lt 0)
• Jet-bremsstrahlung
• (v2 lt 0)
• Jet-fragmentation
• (v2 gt 0)
• pQCD (v2 0)

Only the v2 for thermal photons survives.
33
Elliptic Flow of Hadrons and Formation Time of
Quark Gluon Plasma
34
Elliptic Flow of Photons andFormation Time of QGP
R. Chatterjee and S., arXiv0809.0548
35
Elliptic Flow of Photons at SPS Would it were so.
Rupa Chatterjee et al., to be pulished
36
Elliptic Flow of Photons Pions Formation Time
of QGP at SPS
Rupa Chatterjee et al. to be published.
37
Why does it happen?
38
Intensity Interferometry of Thermal Photons
WA98 8.3/-2 fm.
D. K. Srivastava, PRC 71 (2005) 034905.
39
Photon tagged jets

• g-jet correlation
• Eg Ejet
• Opposite direction
• Direct photons are not affected
• by the medium
• Parton in-medium-modification
• through the fragmentation function
• D(z), z phadron/Eg

Wang, Huang, Sarcevic, PRL 77 (1996) 231. See
also, Renk, PRC 74 (2006) 034906, for
differentiation of mechanisms of E-loss, and
several results at this meeting.
40
Isolation cut can remove the bremmstrahlung
photons
Bremsstrahlung
41
Dilepton vs. photon tagged jets
g

• Photon tagged jets
• Difficult measurement
• At low pT, p0 ? gg large background.
• At higher pT, background problem better
• but opening angle becomes smaller.

Compton

• Dilepton tagged jets
• Lower yield but lower back-ground.
• Charm and beauty decay could be identified.
• M and pT two handles!
• Gold plated standard via Z0 tagging at LHC.

S., Gale, Awes, PRC 67 (2003) 054904 Lokhtin
et al, PLB 599 (2004) 260.
42
Azimuthal tagging of jets with photons/dileptons
Jet
Jets of a given enegy traversing different
path-lengths!
Very Valuable.
EgEjet
EgEjet
g
43
So, where do we stand?

• SPS Results can be understood in terms of a very
  dense hot hadronic or a partonic initial state.
• RHIC Thermal radiations as well as photons from
  passage of jets through QGP seen.
• Photons from passage of jets through QGP, biggest
  source at pT 4 - 8 GeV.
• Elliptic flow for thermal photons dileptons
  will confirm start-up of flow at a very early
  stage.
• The photon or dilepton tagged jets will tell you
  about dE/dx. Tag a photon at some angle to
  reaction plain and control lt L gt covered by the
  jet !!
• The promises of photons and dileptons well beyond
  their original promise have started
  materializing!
• LHC An spectacular display of all conceivable
  aspects of direct photons and dileptons is
  guaranteed!!
• FAIR would explore large mB environments- not
  well explored as yet.

44
Let us get critical for a change

• The extent of thermal and qgp induced photons
  is total minus prompt contribution.
• The prompt contribution calculated at NLO pQCD
  depends on scale. The fragmentation part depends
  on E-loss as well. Accurate pp and pA data would
  help, though we shall never have pn and nn data.
• Does one need to include intrinsic kT? Cronin
  effect?
• The thermal contribution is known only at leading
  order.
• There is no independent check for hadronic
  reaction contribution. Medium modifications?
• Needed simultaneous description of photons and
  p0.
• Viscosity.

45
Thank You For Your Attention
46
Back up slides
47
If I had more time

• Jamal Jalilian Marian, nucl-th/0703069 g/p0 at
  large y ? dynamics of colour gluon condensate
  saturation.
• Intensity Interferometry of direct
  photons
• D. Peressounko, PRC 67 (2003) 014905
• J. Alam et al., PRC 67 (2003) 054902
• S. A. Bass, B. Muller, D. K. Srivastava, PRL 16
  (2004) 162301
• D. K. Srivastava, PRC 71 (2005) 034905.
• New Mechanism
• Qin, Majumder, Gale, PRC 75 (2007) 064909 charge
  asymetry ? g
• Higher Twist Alternative to AMY
• Majumder, Fries, Muller, nucl-th/0711.2475
• g/mm-
• B. Sinha, PLB 128B (1983) 112.
• D. K. Srivastva and B. Sinha PLB 261 (1991)
  1064.
• J. K. Nayak et al, nucl-th/0705.1591.(see talk by
  Nayak).

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The Information Content of EM Probes
m-
Emission rates
m
Photons
Dileptons

• McLerran Toimela,
• PRD 31 (1985) 545
• Weldon, PRD 42 (1990) 2384
• Gale Kapusta,
• NPB 357 (1991) 65.

In- medium photon self energy Directly related
to the in-medium vector spectral densities!
49
Low, Intermediate, High Mass Dileptons

• Low-mass
• Medium modified spectral density
• Intermediate mass
• Radiation from QGP
• High mass
• J/y etc., suppression

The same model should explain both Single
Photons and Dileptons.
50
FMS Centrality Dependence and Jet-Quenching

• centrality dependence well described
• effect of energy-loss on jets before conversion
  20

51
Larger kT or Larger Ti?
Turbide, Rapp, Gale, PRC 69 (2004)
014903 Fire-ball, QGP rich EOS for hadrons
52
Intermediate Mass NA50
Kvasnikova, Gale, Srivastava, PRC 65 (2002)
064903.
Acceptance and detector resolution accurately
modeled.
See also Rapp Shuryak, PLB 473 (2000) 13.
53
Photons pre-equilibrium vs. thermal

• pre-equilibrium contributions are easier
  identified at large pt
• window of opportunity above pt2 GeV
• at 1 GeV, need to take thermal contributions into
  account

• short emission time in the PCM, 90 of photons
  before 0.3 fm/c
• hydrodynamic calculation with t00.3 fm/c allows
  for a smooth continuation of emission rate
• caveat medium not equilibrated at t0

54
Photons HBT Interferometry

• pt2 GeV pre-thermal photons dominate, small
  radii
• pt1 GeV superposition of pre- thermal
  photons increase in radii

Bass, Mueller, Srivastava, PRL 93 (2004)
16230 Srivastava, PRC 71 (2005) 034905.
55
Determining TC w Lends a Hand
A little digression
p
Srivastava et al (to be published) Lichard, PRD
49 (1994) 5812.
56
Charmonium suppression of hc as QGP
indicator!
The same idea for J/y suppression carries over to
hc. Full width for hc is 17.3 MeV. It should
stand tall and proud unless it is disturbed by
the QGP!! The other charmonium states represent
additional tools---its all good.
K. Haglin, Talk given at Hot Quarks 2006.
57
Relative Contribution of Hadronic Reactions
Cross-over pp dominant to pr dominant
58
Elliptic Flow of Thermal Dileptons
59
Photon tagged jets.
60
Wider window will open once heavy quarks loose
energy!
With-out E-loss by heavy quarks.
61
g-tagged hadrons and mechanism for E-loss
Renk, PRC 74 (2006) 034906.
62
3-pion reactions for dilepton production
Srivastava et al., to be published
63
Scaling of Single Photon Production
D. K. Srivastava, EPJC 22 (2001) 129.
64
Elliptic Flow of Decay Photons
d 0.2 GeV
Layek, Chatterjee, Srivastava, PRC 74 (2006)
044901.
65
The assertion of the iso-spin
S. , Jeon, and Gale, to be published.
66
Most Reliable Historians of Ancient India
A slight digression.
Journey to the Western World Huen Tsang (Yuoan
Chwang) 603-664 AD visited India during 630-645
AD.
A Record of Budhist Kingdoms Fa Hien (337-422
AD) visited India during 399-414 AD.
They traversed India like photons and dileptons
and left most valuable records!!
67
Collaborators

• Terry C. Awes
• Steffen A. Bass
• Rupa Chatterjee
• Jean Cleymans
• Rainer J. Fries
• Evan Frodermann
• Charles Gale
• Ulrich Heinz
• Berndt Mueller