Dilepton measurement in H.I.C.

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Dilepton measurement in H.I.C.

• ?????
• ???

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Lepton pair measurements
pT GeV/c
f
?
J/y
?
As a function of Energy Species Centrality
Rapidity
2
Heavy Flavor
Thermal Radiation
1
Drell Yan
g
1
2
3
Mass GeV/c2
? AuAu ? pp
J/y
f
?
3
f?
4
Chiral Symmetry in Hadronic matter

• Mass shift and/or modification of vector mesons
  are expected
• Lepton decays become good probes
• Not interacting strongly.
• However, we only see integrated information from
  all stages of collisions.
• Energy and Centrality dependence

T.Hatsuda and S.Lee (Phys.Rev.C46-R34-38, 1992)

• Line shape
• Direct measurements
• Yield
• Q value of f ? KK is small

After Freeze Out
Hadron gas
QGP

• Integrated information

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f hadron and lepton decays
Main issue KK- and ee- seem not consistent
Many efforts are on-going to understand the
difference btw K and e.
We have to prove that the difference is not made
artificially. Analysis in dAu and pp data is
important as baseline.
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pp
omega
phi
ds/dy Extracted by integration OMEGA 3.50037 /-
0.175942 (stat) /- 0.875 (sys) 4.37554 (upper
sys) 2.62521 (lower sys) PHI 0.369182 /-
0.0195987 (stat) /- 0.0894 (sys) 0.458564
(upper sys) 0.2798 (lower sys)
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w spectra

• Spectra of w meson are measured in several
  collision systems.

• pp

• AuAu

• dAu

• Results are mainly obtained using hadron decays.
• Analysis for electron decay mode is underway.

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Lepton pair continuum(?? Thermal?)
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Lepton pair continuum

• A prediction

• Broadening of r meson makes continuum on the
  invariant mass of electron-positron pair
• In addition, thermal radiation by the system via
  quark anti-quark annihilation also makes
  continuum and it carries direct information from
  the matter.
• Matter is formed
• Deconfinement
• Thermalized
• Experimentally, combinatorial background is very
  large and must be subtracted properly.
• Also, useful for charm measurements

• R. Rapp, nucl-ex 0204003

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_at_SPS - NA60

• ? and ? fix yields such as to get, after
  subtraction, a smooth underlying continuum
• ?
• (?) set upper limit, defined by saturating the
  measured yield in the mass region close to 0.2
  GeV/c2 (lower limit for excess).
• (?) use yield measured for pT gt 1.4 GeV/c

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Whats this?
van HeesRR 06

• absolute norm., melting r
• Mgt0.9GeV 4p?mm w and f ?!

• baryon effects essential

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_at_RHIC

• Clear enhancement is observed in the mass region
  below w.

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pp AuAu comparison
pp NORMALIZED TO meelt100 MeV
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Centrality Dependence
LOW MASS

• p0 production scales with Npart
• If in-medium enhancement from pp or qq
  annihilation
• ?yield should increase faster than proportional
  to Npart

INTERMEDIATE MASS

• p0 production scales with Npart
• charm follows binary scaling? yield should
  increase proportional to Ncoll

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photon
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Thermal photon
Turbide, Rapp, Gale, Phys. Rev. C 69 (014903),
2004

• Direct photon
• Directly emitted from the medium
• Not from hadron decays
• Hard photon
• Initial pQCD
• Thermal photon
• Hadron gas
• QGP

• After subtraction of large background from hadron
  decays,

• Window for thermal photons from QGP in this
  calculation pT 1 - 3 GeV/c
• pQCD calculation should be confirmed in High pT
  region

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Idea - g ? ee-
Any source of real g emits virtual g with very
low mass
Dalitz decay
Use lepton pairs to measure virtual g and
extrapolate to real g
PHENIX can measure low mass ee- pairs hadron
decays and direct photon
In the analysis, Mee distribution for Dalitz and
direct photon is calculated theoretically.
Therefore, the ratio of Nee in two different mass
region is converted to the ratio between direct
and inclusive photons. The systematic error is
reduced.
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gdirect / ginclusive
Significant 10 excess of
very-low-mass virtual direct photons
Results agrees well with Conventional result
One can extract the direct photon yield from the
inclusive photon yield.

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Direct photon yield
Direct photon yield is calculated from the
inclusive photon yield.
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Direct photon yield
Direct photon yield is calculated from the
inclusive photon yield.
Compare to NLO pQCD L.E.Gordon and W.
Vogelsang Phys. Rev. D48, 3136 (1993)
Excess above the calculation
It might include contribution from thermal photons
Reference measurement needed pp and dAu Using
same analysis method
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Comparison of Rg btw pp and AuAu
PHENIX Preliminary
AuAu
pp
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Summary

• So many interesting analysis topics are on-going
  at PHENIX
• Such as, Heavy flavor measurements, J/y, f, and
  lepton pair continuum
• Measurement for chiral symmetry is underway.
• First result will publish soon.
• Enhancement is observed in ee- continuum!
• Interesting results in Direct photon measurements.