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Thermal Photons and Dileptons*. Why? How? Theory Low mass dileptons Intermediate mass dileptons Photons: low and high(er) p T photons EM signature of jets. ( * Not an exhaustive review…). Why? The information carried by EM probes. Emission rates:. [photons].

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thermal photons and dileptons

Thermal Photons and Dileptons*

  • Why? How? Theory
  • Low mass dileptons
  • Intermediate mass dileptons
  • Photons: low and high(er) pT photons
    • EM signature of jets

(* Not an exhaustive review…)

why the information carried by em probes
Why? The information carried by EM probes

Emission rates:


McLerran, Toimela (85), Weldon (90), Gale, Kapusta (91)


  • The electromagnetic spectra will be direct probes of the in-medium
  • photon self-energy
  • They are hard probes:
      • EM signals as probes for hadronic tomography
the current current correlator
The current-current correlator

A model for the hadronic electromagnetic current: VMD

The current-field identity

(J. J. Sakurai)

Spectral density

The photon/dilepton signal can tell us about the in-medium

spectral densities of vector mesons. Rates need to be integrated

over the space-time history, with some dynamical model

what is expected dileptons
What is expected (dileptons)
  • Low masses receive significant contribution from radiative decays
  • High masses dominated by DY
  • Intermediate mass region interesting from QGP perspective,

(Shuryak (78), Shor (89))

  • Photons: similar story, but featureless spectra
  • Experiments: DLS, Helios, TAPS, NA38, -50, WA98, CERES, PHENIX, HADES, NA60
expectations part ii
Expectations, part II
  • Thermal QGP plasma radiation
  • Many-body, in-medium, effects on spectral densities

Weinberg (67)

Kapusta, Shuryak (94)

+ other possibilities…

in medium what medium


In-medium: what medium?

Phase-space trajectory goes through qualitatively different media

low masses vector meson spectral densities hot meson gas
Low Masses:Vector Meson Spectral Densities:Hot Meson Gas

The spectral density is flattened

and broadened

Rapp, Gale (99)

vector spectral densities from data
Vector spectral densities from data

E. V.Shuryak, Nucl. Phys. A 533, 761 (1991); V. L. Eletsky and V. L.

Ioffe, Phys. Lett. B 401, 327 (1997); Eletsky, Belkacem, Ellis, Kapusta,

Phys. Rev. C 64, 035202 (2001)

  • Should hold near the mass-shell
  • Adler decoupling enforced
two approaches
Two approaches:

Rapp & Wambach

Eletsky, Ioffe, Kapusta

  • Rates are also constrained by nuclear photoabsorption data
  • Lagrangians are constrained by hadronic phenomenology
  • Mass shifts & broadening are related by dispersion relations

(Giessen, Frankfurt, Munich)

fold in with a dynamical evolution model
Fold in With a Dynamical Evolution Model

Huovinen, Belkacem, Ellis, and Kapusta (02)

Rapp, Brown-Rho

What’s new?

e e mass spectrum comparison to the models
e+e- mass spectrum: comparison to the models

Sergey Yurevich (CERES)

calculation by R.Rapp using

Rapp/Wambach medium

modification of rho spectral


calculation by R.Rapp using

Brown-Rho scaling

B. Kämpfer, thermal emission

...added to the cocktail.

in the 0.8 < m < 0.98 GeV region:

Brown-Rho curve: 2/n= 2.4

the other two curves: 2/n ~0.3


NA60 Comparison of data to RW, BR and Vacuum 

  • Linear scale!!!
  • Quality of data enables a precise
  • determination of the spectral
  • properties.
  • The beginning of a new era…
the intermediate mass sector some background

NA50Pb-Pb 158 GeV



central collisions




The intermediate mass sector: some background
  • Direct connection to Hard Probes
  • Off-shell effects are potentially important for effective hadronic interactions Gao & Gale, PRC 57, 254 (1998)
  • A lot of data already exists!

A. Shor, PLB 233, 231 (1989)

e e data a wealth of information
e+ e- Data: A Wealth of Information
  • OLYA
  • CMD
  • DM-1(2)
  • M3N
  • gg2
a larger comparison
A larger comparison
  • Agreement across theoretical


  • Those channels are absent

from the spectral densities used in comparisons with CERES and the new NA60 data.

intermediate mass data
Intermediate mass data

A. L. S. Angelis et al. (Helios 3), Eur. Phys. J. (1998)

Li and Gale, PRC (1998)

R. Rapp & E. Shuryak, PLB (2000)

na50 data cont nd
NA50 Data (cont’nd)

I. Kvasnikova, C. Gale, and

D. K. Srivastava, PRC 2002

  • In agreement with multiplicity dependence
  • Includes detector acceptance & efficiency
    • (O. Drapier, NA50)


NA60 IMR analysis: weighted offset fits (R. Shahoyan)

Extract prompts by fixing Open Charm contribution

Fix Charm contribution to “world average” value


Fix Charm contribution to NA50 p-A expected value

Fit always requires ~2 times more Prompts

low and intermediate masses partial summary
Low and Intermediate masses: partial summary
  • Thermal sources shine in the LMR and IMR. No great sensitivity to the QGP
  • The data is precise enough to consider a differentiation of space-time models
  • DY? At low M, medium-enhanced multiple parton scatterings might be large (Qiu, Zhang (02), Fries, Schaefer, Stein, Mueller (00). pA measurement.)
theory homework
(Theory) Homework
  • Unite (standardize?) space-time modeling [nD hydro, fireballs, transport approaches…]. Rapidity dependence of photon signal: a probe of stopping (Renk, PRC (05))
  • The power of the data is only fully realized if a general-purpose acceptance filter exists.
electromagnetic radiation from qcd
Electromagnetic radiation from QCD

First approaches

McLerran, Toimela (1986); Kajantie, Kapusta, McLerran, Mekjian (1986)

Baier, Pire, Schiff (1988); Altherr, Ruuskanen (1992)

Rates diverge:



HTL program: Klimov (1981), Weldon (1982)

Braaten & Pisarski (1990); Frenkel & Taylor (1990)

Kapusta, Lichard, Seibert (1991)

Baier, Nakkagawa, Niegawa, Redlich (1992)

Going to two loops: Aurenche, Kobes, Gelis, Petitgirard (1996)

Aurenche, Gelis, Kobes, Zaraket (1998)

Co-linear singularities:

singularities can be re summed
Singularities can be re-summed

Arnold, Moore, and Yaffe

JHEP 12, 009 (2001); JHEP 11, 057 (2001)

  • Incorporates LPM
  • Complete leading order in αs
  • Inclusive treatment of collinear enhancement, photon and gluon


Can be expressed in terms of the solution to a linear integral equation

how big small is this
How big (small) is this?

Turbide, Rapp & Gale PRC (2004)


Pedestal&flow subtracted

Azimuthal correlation

  • Shows the absence of “away-side” jet.
quenching jet plasma interaction does this have an em signature
Quenching = Jet-Plasma interaction. Does this have an EM signature?

The plasma mediates a jet-photon conversion

Fries, Mueller & Srivastava, PRL 90, 132301 (2003)

photon sources
Photon sources
  • Hard direct photons
  • EM bremsstrahlung
  • Thermal photons from hot medium
  • Jet-photon conversion
  • Jet in-medium bremsstrahlung
energy loss in the jet photon conversion jet bremsstrahlung
Energy loss in the jet-photon conversion? Jet bremsstrahlung?

Use the approach of Arnold, Moore, and Yaffe

JHEP 12, 009 (2001); JHEP 11, 057 (2001)

  • Incorporates LPM
  • Complete leading order in S
  • Inclusive treatment of collinear enhancement, photon and gluon


Can be expressed in terms of the solution to a linear integral equation

time evolution of quark distribution

The entire

distribution is

evolved by the

collision Kernel(s)

Of the FP equation

Turbide, Gale, Jeon, and Moore (2004)

Time-evolution of quark distribution
photons establishing a baseline
Photons: establishing a baseline


Aurenche et al., NPB 286, 553 (1987)

Consistent with Gordon & Vogelsang

direct g in d au
p+p and d+Au spectra compared to NLO pQCDDirect g in d+Au

(S. Bathe)

  • ratio to NLO pQCD
  • consistent with 1
  • No indication for nuclear effects


Poster H. Torii

Poster D. Peressounko

new preliminary phenix data bathe buesching




140-200 MeV





New (preliminary) PHENIX Data(Bathe, Buesching)
new preliminary phenix data bathe buesching1
New (preliminary) PHENIX Data(Bathe, Buesching)

A prediction: all source

Sizes fixed prior to QM

other signature of jet photon conversion
Other signature of jet-photon conversion?
  • Jet-plasma photons will come out of the

hadron-blind region. “Optical” v2 < 0

Turbide, Gale, Fries (05)


If photons can be detected in coincidence with hadrons

The jet-plasma photons can be easier to isolate (Cole)

jet plasma interactions measurable em signatures
Jet-plasma interactions: measurable EM signatures!
  • RHIC:
    • Jet-plasma interaction is a large source of photons up to pT ~ 6 GeV.
    • Conclusions include energy-loss considerations
    • True also in the dilepton channel: signal competes with Drell-Yan (NLO)
  • LHC:
    • Jet-plasma photon signal is important
    • Large mass lepton pairs dominate over Drell-Yan emission.

Towards a consistent treatment of jets & EM radiation

summary conclusions open issues
Summary, Conclusions, Open Issues
  • Low and mass dileptons: NA60 data can distinguish between models
  • IMR: More homework to be done (Higher twist…)
  • Space-time evolution models
  • RHIC: There are measurable electromagnetic signatures of jet-plasma interaction: those constitute complementary observables to signal the existence of conditions suitable for jet-quenching
  • Photon v2, a revealing probe
  • RHIC dileptons: systematic errors still too large to permit source identification (A. Toia, PHENIX)
  • EM radiation and hard probes: the start of a beautiful friendship…


Simon Turbide, McGill University

Rainer Fries, University of Minnesota

R. Rapp, Texas A&M

Dinesh Srivastava, VECC, Calcutta