Electromagnetic Probes of Strongly Interacting Matter

1
Electromagnetic Probes of Strongly Interacting
Matter

• Joe Kapusta
• University of Minnesota

Hard Probes 2006, Asilomar, California
14 June 2006
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What can we learn from electromagnetic probes?

• We can measure the EM current-current correlation
  function in the medium if we know the dynamical
  evolution of the system.
• We can infer the dynamical evolution of the
  system if we know the EM current-current
  correlation function.
• Theory should reproduce the hadron spectra as
  well as the photon and dilepton spectra.

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Electromagnetic Emission Rates
photons
McLerran Toimela (1985), Weldon (1990), Gale
Kapusta (1991)
dileptons

• The electromagnetic spectra will be direct probes
  of the in-medium
• photon self-energy or electromagnetic
  current-current correlation
• function if we have a dynamical evolution
  scenario over which to
• integrate the rates.

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Vector Meson Dominance
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
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Spectral Densities Shape the Spectra
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Constraints on Spectral Densities
Weinberg sum rules (1967) generalized to finite
temperature by Kapusta and Shuryak (1994) must be
satisfied in the limit of exact chiral symmetry.

• Many possibilities for satisfying the sum rules
• Spectral densities mix (Eletsky-Ioffe)
• ? and a1 masses become degenerate (both go up,
  both go down, or one goes up and the other goes
  down)
• Widths become so large that the vector mesons
  melt away

Coupling of the pion to the longitudinal part of
the axial vector current
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Big discovery by CERES!
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Vector Meson Spectral Densities
Rapp Wambach (1999)
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Vector Meson Spectral Densities
Shuryak (1991) Eletsky Ioffe (1997) Eletsky,
Belkacem, Ellis, Kapusta (2001)
Calculate self-energy from experimental data,
such as resonance masses and widths, phase
shifts, etc.
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Two Approaches Approximately Agree
red Rapp Wambach black Eletsky, Ioffe, Kapusta
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Two Approaches Approximately Agree
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Fold with a Dynamical Evolution Model

• Huovinen, Belkacem,
• Ellis Kapusta (2002)

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Fold with a Dynamical Evolution Model
Rapp Brown-Rho
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NA60!
(see Sanja Damjanovic)
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Does Charm Fill-in the Intermediate MassRegion
Centered Around 2 GeV?
See other talks, especially Gale.
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Thermal Photons from QCD

• Perturbative QCD

Rates diverge
Need HTL resummation
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Hard Thermal Loops
Kapusta, Lichard, Seibert (1991) Baier,
Nakkagawa, Niegawa, Redlich (1992)
Soft radiation Aurenche, Kobes, Gelis,
Petitgirard (1996) Aurenche, Gelis, Kobes,
Zaraket (1998)
Co-linear singularities
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Singularities Can Be Resummed

• Arnold, Moore, and Yaffe (2001)
• Incorporates LPM
• Complete leading order in as
• Inclusive treatment of collinear enhancement,
  photon and gluon emission

Can be expressed in terms of the solution to a
linear integral equation
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Photons Establishing a Baseline
See Peressounko
(preliminary)
Aurenche et al. (1987) consistent with Gordon
Vogelsang
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Direct g in dAu
See Peressounko

• pp and dAu spectra compared to NLO pQCD

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

2
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Huovinen, Belkacem, Ellis, and Kapusta (2002)
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Huovinen, Belkacem, Ellis, and Kapusta (2002)
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Jet Conversion to a Photon
The plasma mediates a jet-photon conversion
Fries, Mueller Srivastava (2003), Jeon talk
Novel features!

• More jet-photon conversion where medium is
  thicker
• v2 for these photons is negative
• Turbide, Gale, Fries (2005), Heinz talk
• Can separate them from other sources
  thermal/prompt

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Calculations by Gale, Rapp TurbideSee Isobe
for PHENIX
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Interesting Theoretical Approaches

• Spectral densities from lattice QCD (see S. Gupta
  talk)
• Spectral densities from AdS/CFT (see Kovtun talk)

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Conclusion

• Solid results are being obtained, both
  theoretically and experimentally, about many-body
  physics at high energy density, such as
  modification of vector spectral densities and QCD
  processes at high energy.
• Ask all the following speakers to clearly
  separate the correlation/response functions
  characterizing a system in thermal equilibrium
  from the space-time evolution characterizing a
  heavy ion collision.

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Finite-Temperature Field TheoryPrinciples and
ApplicationsJoseph Kapusta and Charles Gale

• 1. Review of quantum statistical mechanics
• 2. Functional integral representation of the
  partition function
• 3. Interactions and diagrammatic techniques
• 4. Renormalization
• 5. Quantum electrodynamics
• 6. Linear response theory
• 7. Spontaneous symmetry breaking and restoration
• 8. Quantum chromodynamics

• Resummation and hard thermal loops
• Lattice gauge theory
• Dense nuclear matter
• Hot hadronic matter
• Nucleation theory
• Heavy ion collisions
• Weak interactions
• Astrophysics and cosmology
• Conclusion
• Appendix