Transcript Slide 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
• 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.
Electromagnetic Probes
Penetrating probes are emitted at all stages then survive unscathed (ae <<as).
“Historians” of the heavy ion collision: encode all sub-processes at all times
A jet passing through QGP
Different processes: different characteristic spectra
Direct Photons
Different 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”?
“New”
Jet Re-interaction √(Tix√s)
pQCD Prompt x√s
Eg
• Hydrodynamics.
• Cascades.
• Fire-balls.
• Cascade+Hydro.
Partonic Processes for Production
of Prompt Photons in Hadrons
Compton
Annihilation
Fragmentation
Calculate using NLO pQCD [with shadowing & scaling with TAA(b) for AA,
partons remain confined to individual nucleons; do not forget the isospin! ]
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”.
NLO pQCD
description of
world promptphoton data.
Aurenche et al.
PRD 73 (2006)
094007.
See also
Gordon &
Vogelsang (1994).
Most suitable scale
is m=mR=mF=pT/2.
Do not forget that
m=pT/3 for pions !!!
Why should they be
different?
Neutrons are different from
protons!
Chatterjee, Jeon, and S., to be published.
In the QGP we also have:
g
Annihilation with scatterring; First
calculated by Aurenche et al,
PRD 58 (1998) 085003.
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.
Complete leading order results: Arnold, Moore, Yaffe,
JHEP 0112 (2001) 009. How about NLO?
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.
First calculated by Kapusta, Lichard,
& Seibert, PRD 44 (1991) 2774.
• Include strange sector, massive
Yang- Mills theory, form-factors,
baryons, t-channel exchange of
w mesons etc.
Turbide, Rapp, Gale, PRC 69
(2004) 014903.
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.
The rate of production of photons having total
energy E and momentum p can be written as:
dN
2 E /T
E 4 3 ~T e
d xd p
 dR 

f QGP ( x, y, ) 
 E 3 
dN
 d p QGP

   d d dxdy
2

d pT dy
  E dR  f HM ( x, y, )
  d 3 p  HM

T(x, y, ), vT(x, y, ), and fi using hydrodynamics
and Equation of State.
Upper Limit of Single Photons, WA80
Ruled out hadronic gas with
limited hadrons: p, r, w, & .
Pb+Pb@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 >> 2-3 /fm3 ! For
No Phase Transition.
QGP or Hot Hadrons? Enter WA98
QGP
+ prompt g
v0.ne.0
Hadrons
(mhad 0 at
Huovinen et al, PLB 535 (2002) 109.
QGP or hadrons ( nhad >> 1/fm3
at Ti = 245-275 MeV)
Ti=205 MeV
for all
hadrons)
Alam et al, PRC 63 (2001) 021901 ( R).
2-loop  Complete O(aS) for QGP
&
pra1 Exhaustive Hadronic Reactions for hadrons
S. & Sinha, PRC 64 (2001)034902 (R ).
S., PRC 71 (2005) 034905.
Hydrodynamics, QGP + rich EOS for hadrons
& accounting for the prompt photons
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 >> 1/fm3 at 0 is not ruled out.
 QGP initial state describes the data well.
 Initial temperature could be 210 – 350 MeV, depending on 0 , profile,
& v0. What is a good 0? 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.
Interaction of hard-scattered
parton with dense matter
“External Probe!!”
Hard scattered parton
Fries, Mueller, & S. , PRL 90 (2003) 132301.
Jet-Initiated EM Radiations from QGP
• Annihilation and Compton processes peak in forward and
backward directions:
d u t
~ 
dt
t u
pg  p q
pg  p q
d t s
~ 
dt
s t
• one parton from hard scattering, one parton from the thermal
medium; cutoff pg,min > 1 GeV/c.
 photon carries momentum of the hard parton
 Jet-Photon Conversion
 This puts photon production and jet-quenching on the same
page!!
Jet-Photon Conversion: Rates
• Annihilation and Compton rates:
quark (-jet) distribution
16 Eg N f
dN ( A)
Eg 4 3 
f ( pg )
6  q
d xd pg 2(2p ) q 1
  d p f q ( p)[1  f g ( p)]
3
16 Eg
dN (C )
Eg 4 3 
d xd pg 2(2p )6
( A)
Nf
f
q 1
q
( pg )
  d p f g ( p)[1  f q ( p)]
3
s ( s  4m 2 )
( s)
 (q  q)
2 Eg E
(C )
s  m2
( s)
 (q  q)
2 Eg E
• thermal medium:
dNg aa s
4 Eg T

4 2
2
Eg 3  2  d x  f q ( pg )  f q ( pg )  T  ln 2  C 
d pg 8p
3
m


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.
FMS Results: Comparison to Data
calibrate pQCD calculation of
direct and Bremsstrahlung
photons via p+p data:
 for pt<6 GeV, FMS photons give
significant contribution to
photon spectrum: 50% @ 4GeV.
Proper Isospins & Shadowing !!!
Fries, Mueller, & S., PRC 72 (2005) 041902( R).
Evolution of jet distribution due to E-loss
AMY and One-Stop Treatment of
Jet-Quenching and Jet-Initiated Photons
Turbide, Gale, Frodermann, & Heinz, PRC77 (2008) 024909.
Q=pT/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 . ).
Parton Cascade Model
Embedded in the partonic cascades
Renk, Bass, & S., PLB 632 (2006) 632.
LPM plays a significant role.
Bass, Mueller, & S.,
PRC 66 (2002) 061902 (R).
Thermal photons from Au+Au@RHIC
d’Enterria & Peressounko,
EPJC 46 (2006) 451.
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
0 : 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.
Not Just T0!
 Measure evolution of elliptic flow with thermal
photons (v2 >0)!
 Help understand dE/dx with photon or dilepton
tagged jets.
 Study evolution of size with intensity interferometry of
photons!
Elliptic Flow of Thermal Photons:
Measure Evolution of Flow !
Chatterjee, Frodermann, Heinz, and S., PRL 96 (2006) 202302.
Impact Parameter Dependence of v2
Elliptic Flow of Thermal Dileptons:
Measure Evolution of Flow !
Chatterjee, Heinz, Gale, & S., PRC 75, 054909 (2007). See poster by Rupa
Evolution of flow for thermal photons
Rupa Chatterjee et al. to be published
Azimuthal Anisotropy of Photons from Passage of
Jets Through QGP
• Jet-photon conversion
(v2 < 0)
• Jet-bremsstrahlung
(v2 < 0)
• Jet-fragmentation
(v2 > 0)
• pQCD (v2 = 0)
Only the v2 for thermal photons survives.
Elliptic Flow of Hadrons and Formation
Time of Quark Gluon Plasma
Elliptic Flow of Photons and
Formation Time of QGP
R. Chatterjee and S., arXiv:0809.0548
Elliptic Flow of Photons at
SPS: Would it were so.
Rupa Chatterjee et al., to be pulished
Elliptic Flow of Photons & Pions
& Formation Time of QGP at SPS
Rupa Chatterjee et al. to be published.
Why does it happen?
Intensity Interferometry of Thermal Photons
WA98; 8.3+/-2 fm.
D. K. Srivastava, PRC 71 (2005) 034905.
Photon tagged jets
g-jet correlation
– Eg = Ejet
– Opposite direction
• Direct photons are not affected
by the medium
• Parton in-medium-modification
g
through the fragmentation function
D(z), z = phadron/Eg
• Golden Channels :
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.
g + q → γ + q (Compton)
q+q→γ+g
(Annihilation)
• pT > 10 GeV/c
Isolation cut can remove the bremmstrahlung photons
q
γ
Compton
g
q
q
γ
q
g
isolated
photons
q
g
γ
Bremsstrahlung
g
q
γ
q
Annihilation
Jet
g
g
q
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.
Azimuthal tagging of jets with photons/dileptons
Jets of a given enegy traversing
different path-lengths!
Jet
Eg=Ejet
g
Eg=Ejet
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 < L > 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 m environments- not well explored as yet.
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 Eloss 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.
Thank You
For Your
Attention
Back up slides
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/mm
• 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).
The Information Content of EM Probes
m+
Emission rates:
Photons:
m-
d 3R
g m
1
R
w 3 
Im  m (k ) w
3
d k
(2p )
e 1
6
2
d
R
2
e
1
1
Dileptons:
mv

L Im   w
3
3
6
4 mv
d p d p (2p ) k
e 1
In- medium photon self energy:
Directly related to the in-medium
vector spectral
densities!
• McLerran & Toimela,
PRD 31 (1985) 545;
• Weldon, PRD 42 (1990) 2384;
• Gale & Kapusta,
NPB 357 (1991) 65.
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.
FMS: Centrality Dependence and
Jet-Quenching
• centrality dependence well described
• effect of energy-loss on jets before
conversion ~ 20%
Larger kT or Larger Ti?
Turbide, Rapp, & Gale, PRC 69 (2004) 014903;
Fire-ball, QGP + rich EOS for hadrons
Intermediate Mass; NA50
Kvasnikova, Gale, & Srivastava,
PRC 65 (2002) 064903.
Acceptance and detector resolution
accurately modeled.
See also Rapp & Shuryak, PLB 473 (2000) 13.
Photons: pre-equilibrium vs. thermal
pre-equilibrium contributions are
easier identified at large pt:
•window of opportunity above pt=2
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 τ0=0.3
fm/c allows for a smooth continuation
of emission rate
 caveat: medium not equilibrated at τ0
Photons: HBT Interferometry
•pt=2 GeV: prethermal photons
dominate, small
radii
•pt=1 GeV:
superposition of
pre- & thermal
photons:
increase in radii
Bass, Mueller, & Srivastava, PRL 93 (2004) 16230;
Srivastava, PRC 71 (2005) 034905.
Determining TC: w Lends a Hand
p
r
p
w, f
g*
p
Srivastava et al (to be published);
Lichard, PRD 49 (1994) 5812.
Charmonium: “suppression of c
as QGP indicator”!
The same idea for J/y
suppression carries
over to c.
Full width for c is 17.3
MeV. It should stand
“tall and proud” unless
it is disturbed by the
QGP!!
The other charmonium
states represent
additional tools---it’s all
good.
K. Haglin, Talk given
at Hot Quarks 2006.
Relative Contribution of Hadronic Reactions
Cross-over; pp dominant to pr dominant
Elliptic Flow of Thermal Dileptons
Photon tagged jets.
Wider window will open once
heavy quarks loose energy!
With-out E-loss by heavy quarks.
g-tagged hadrons and mechanism for E-loss
Renk, PRC 74 (2006) 034906.
3-pion reactions for dilepton production
Srivastava et al., to be published
Scaling of Single Photon Production
D. K. Srivastava, EPJC 22 (2001) 129.
Elliptic Flow of Decay Photons
d ~0.2 GeV
Layek, Chatterjee, Srivastava,
PRC 74 (2006) 044901.
The assertion of the iso-spin
S. , Jeon, and Gale, to be published.
Most Reliable Historians of Ancient India
“A Record of Budhist Kingdoms”:
Fa Hien (337-422 AD):
visited India during 399-414 AD.
“Journey to the Western World”:
Huen Tsang (Yuoan Chwang) 603-664 AD:
visited India during 630-645 AD.
They traversed India like photons and dileptons and left most valuable records!!
Terry C. Awes
Steffen A. Bass
Rupa Chatterjee
Jean Cleymans
Rainer J. Fries
Evan Frodermann
Charles Gale
Ulrich Heinz
Berndt Mueller
M. G. Mustafa
Thorsten Renk
Krzysztof Redlich
Bikash Sinha
Simon Turbide
Collaborators