Dileptons and Photons

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Dileptons and Photons

• Huan Z Huang
• Department of Physics and Astronomy
• University of California, Los Angeles
• Department of Engineering Physics
• Tsinghua University

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Collision Dynamics
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Photon Sources in Quark-Gluon Plasma
Naïve Leading Order Processes q q (g) ? g
(q) ?
q
g
q
Kapusta etal 91, Baier etal 92
But other contributions to O(as) collinear
enhanced Dg(t-mD2)-1 1/as
Bremsstrahlung Pair-ann.scatt.
ladder resummation (LPM)
Aurenche etal 00, Arnold,MooreYaffe 01
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Direct and Soft Photons
Physics QCD process photons gq ?jet g
quark Bremsstrahlung in QCD medium
thermal radiation QGP photons (Temperature)
hadron spectroscopy
Experimental Approaches 1) photons from
external conversion 2) photons from high
performance calorimeter measurement
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Photon Sources in Hadron Gas

• Photon-producing reactions

mostly at dominant (q0gt0.5GeV)
gauge invariance! q0lt0.5GeV a1-strength
problematic
Hadron Form Factor Important for photon yield
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More Meson Gas Sources
(i) Strangeness Contributions SU(3)F MYM
25 of pp???
40 of pr?p? !
(iii) Higher Resonances Ax-Vec a1,h1?pg,
Vec w,w,w?pg other p(1300)?pg
f1?rg , K1?Kg K?Kg
a2(1320)?pg
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Baryonic Contributions

• use in-medium r spectral funct
• constrained by nucl. g-absorption

g N ? p N,D
gN gA
B,a1,K1...
N,p,K
g N ? B
p-ex
Urban,Buballa,RRWambach 98
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HG Emission Rates Summary

• w t-channel (very) important
• at high energy
• form factor suppression (2-4)
• strangeness significant
• baryons at low energy

mB220MeV
Though EM is well understood, photon production
in collisions complicated dynamics !!
Turbide,RRGale 04
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PQCD photons
Initial hard production pp ? ?X
scaling with xT2pT /vs , power-law fit
Srivastava 01
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Naively Thermal Photons ? T
The higher temperature, the more thermal
radiation ! But the relation between initial T
and photons is non-trivial !
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Predictions for Central AuAu Collisions at 200
GeV
pre-equilibrium contribution from parton
cascading ? major contribution QGP thermal
radiation ? 1-2 GeV (maybe)
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Direct g in dAu
PHENIX

• pp and dAu spectra compared to NLO pQCD

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

2
No surprises !
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Direct Photons Surely There!
p0 suppression ? helps Lines? Nbinary
Scaling
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Direct Photons

• Direct photon spectra over centralities
• Systematic Error 15-20
• Clearly seen that we measured photons over the
  order of 1027!
• See the scale please..
• Again, Thickness-scaled NLO QCD calculation
  describes all the spectra very well
• From Central to Peripheral
• No exception within current errors
• Yellow bands show uncertainty on NLO pQCD
  calculation and thickness function

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Results (RAA)

• Photon RAA is consistent with unity over all the
  centrality.
• the yield follows thickness-scaled hard
  scattering
• p-p reference from NLO pQCD Calculation
• ?0 RAA decreases to 0.2 at Npart320
• Dotted line shows uncertainty of thickness
  function
• Error bars show total error (systematics
  statistical) except thickness function error
• Yellow shows uncertainty on pQCD calculation

Direct g
p0
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Comparison with calculations

• Any of pQCD calculations describe data well
• Adding kT broadening makes factor of 2
  difference
• Around same factor as E706
• Calculation suggests that slopes of the spectra
  at RHIC and E706 are same
• Jet Photon included calculation (Fries et al.,
  PRL 90, 132301 (2003)) is also shown
• Fits very well above 4GeV!
• Assuming existence of hot dense medium
• Prompt partons scatter with thermal partons
• The line approaches to simple pQCD calculation in
  high pT

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Any Hope for QGP Radiation?

• Most realistic calculation
• Including all the contributions
• PHENIX may be able to see QGP contribution in
  1-3GeV/c

PRC69(2004)014903
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Quenching Jet-Plasma interaction. Does this
have an EM signature?
The plasma mediates a jet-photon conversion
Fries, Mueller Srivastava, PRL 90, 132301 (2003)
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Comparison between model and exp
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• Coincident transitions
• deconfinement and chiral symmetry restoration
• it is seen to hold also vs quark mass

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Chiral Symmetry
How Chiral Symmetry has manifested
in nucleus-nucleus collisions?
We must measure vector mesons in both hadronic
and leptonic decay channels! electron PID ? TOF
upgrade HFT reduce conversion BK K-pi PID ?
TOF upgrade h and h ? EMC high
statistics Baryonic resonances
(D,L(1520),S(1385),X(1530)....)
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Vector Meson Mass and Chiral Symmetry

• mesons No significant width change or mass
  shift has been
• observed.
• Measurement of both KK- and ee- channels.
• r mesons Some kind of mass shift has been
  observed in STAR, but
• the interpretation of the shift is not clear!
• Measurement of r mesons in ee- channel is
  needed!
• Measurement of pp- invariant mass and the
  residual distribution
• after combinatorial background subtraction
• --- r and s mesons
• --- the nature of s mesons (q-qbar or
  four-quark)
• Electron measurement will be possible with the
  TPC and TOF
• -- remove photon conversion and Dalitz
  background !

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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

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Low MassesVector Meson Spectral DensitiesHot
Meson Gas
The spectral density is flattened and broadened
Rapp, Gale (99)
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Very Difficult to Measure Di-leptons
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PHENIX low mass dielectrons
pp NORMALIZED TO meelt100 MeV
pp and AuAu normalized to p0 region pp follows
the cocktail AuAu large Enhancement in
0.15-0.75
low mass
AuAu
intermediate mass
pp
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PHENIX Preliminary Data
10-20
0-10
-- data very puzzling !
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pT dependency
arXiv 0706.3034
arXiv 0802.0050
AuAu
pp
0ltpTlt8.0 GeV/c
0ltpTlt0.7 GeV/c
0.7ltpTlt1.5 GeV/c
1.5ltpTlt8 GeV/c
pp follows the cocktail AuAu enhancement
concentrated at low pT
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PHENIX Dilepton Data

• First measurements of dielectron continuum at
  RHIC

• pp
• Low mass
• Excellent agreement with cocktail
• Intermediate mass
• Extract charm and bottom
• sc 544 39 (stat) 142 (sys) 200 (model)
  mb
• sb 3.9 2.4 (stat) 3/-2 (sys) mb

• AuAu
• Low mass
• Enhancement above the cocktail expectations
  3.40.2(stat.) 1.3(syst.)0.7(model)
• Centrality dependency increase faster than Npart
• pT dependency enhancement concentrated at low pT
• Intermediate mass
• Agreement with PYTHIA coincidence?

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Dileptons and Photons are related
PHENIX Preliminary
PHENIX Preliminary

• mlt2p only Dalitz contributions
• pp no enhancement at low pT
• AuAu large enhancement at low pT
• Any source of real g produces virtual g with
  very low mass
• Assuming internal conversion of direct photon ?
  extract the fraction of direct photon
• pp follows pQCD
• AuAu clear excess above pQCD ? signal of
  thermal photons?

T.Dahms Session XV Y.Yamaguchi Poster 125
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NA60 Comparison of data to RW, BR and Vacuum ?
(Broadening vs Shift)
Sanja Damjanovic
pT dependence
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Different Sources of dimuons of Mlt1.0 and Mgt1.0
GeV/c2 ?
Dimuons of Mlt1.0 GeV/c2 participate in hadronic
rescattering !
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(No Transcript)
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Quark-Gluon Fluid
Chiral Properties at Tc -- quasi-particles --
mass shift -- width broadening Dilepton
Measurement -- in the low mass region 0.5-2
GeV/c2 -- very difficult
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The END
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Two-Particle Correlations in dAu

STAR preliminary
STAR preliminary
Background-subtracted correlations between a
high-pT trigger charged particle and an
associated charged particle
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Photon-hadron correlations
gjet correlation in AuAu in run4?
More accurate determination of initial Et
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Results for p-p
Bands represent systematic errors.
(Subtraction)
Errors on the backgrounds result in enlarged
errors on the signal, especially at low-pT
region.

• NLO-pQCD calculation
• CTEQ6M PDF.
• Gluon Compton scattering
• fragmentation photon
• Set Renormalization scale
• and factorization scale pT/2,pT,2pT
• Systematic Error
• 20(high pT)-45(low pT)

The theory calculation shows a good agreement
with our result.
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dE/dx at high pT (62.4 GeV)
?2/ndf 1.5
pT gt 3 GeV/c Pion-proton Separation !
rigidity (chargedE/dx) keV/cm
positives
negatives
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Jet Photon overwhelms QGP?

• Break-up of Fries prediction
• Jet Photons overwhelms all the other
  contributions below 7GeV/c
• Jet production rate calculated by LO pQCD with K
  factor compensation of 2.5
• pQCD photon calculation from LO with no K factor
• Fitting too good!
• In Peripheral, the calculation should fit the
  data as well
• RAA and spectra themselves tell you what happens
• Calculation is assuming existence of hot dense
  medium, which is not the case in peripheral!