Dileptons and Photons. Huan Z Huang Department of Physics and Astronomy University of California, Los Angeles Department of Engineering Physics Tsinghua University. Collision Dynamics. Photon Sources in Quark-Gluon Plasma. “Naïve” Leading Order Processes: q + q (g) → g (q) + γ. q. q. - PowerPoint PPT Presentation
Dileptons and Photons
Huan Z HuangDepartment of Physics and AstronomyUniversity of California, Los Angeles
Department of Engineering PhysicsTsinghua University
Photon Sources in Quark-Gluon Plasma
“Naïve” Leading Order Processes: q + q (g) → g (q) + γ
Ts 0/q-2230 4
[Kapusta etal ’91, Baier etal ’92]
But: other contributions to OO(αs)
collinear enhanced Dg=(t-mD2)-1 ~ 1/αs
[Aurenche etal ’00, Arnold,Moore+Yaffe ’01]
Bremsstrahlung Pair-ann.+scatt. + ladder resummation (LPM)
Photon Sources in Hadron Gas
• Photon-producing reactions:
mostly at dominant (q0>0.5GeV) gauge invariance! q0<0.5GeV a1-strength problematic
Hadron Form Factor Important for photon yield
More Meson Gas Sources
(i) Strangeness Contributions: SU(3)F MYM
(iii) Higher ResonancesAx-Vec: a1,h1→, Vec: ,’,’’→ other: (1300)→ f1→ , K1→K K*→K a2(1320)→
(ii) t-Channel γ
potentially important …[Turbide,Gale +RR ’04]
• use in-medium–spectral funct:
• constrained by nucl. -absorption:
mIm med 02
N → N,
HG Emission Rates: Summary
• t-channel (very) important at high energy
• form factor suppression (2-4)
• strangeness significant
• baryons at low energy
Though EM is well understood, photon production in collisions complicated dynamics !!
Initial hard production: pp → γX
scaling with xT=2pT /√s , + power-law fit [Srivastava ’01]
Naively Thermal Photons T
“Freeze-Out” Hadron Gas QGP ?!
Au + Au
The higher temperature, the more thermal radiation !
But the relation between initial T and photons is non-trivial !
Predictions for Central Au+Au Collisions at 200 GeV
‘pre-equilibrium’ contribution from parton cascading major contributionQGP thermal radiation 1-2 GeV (maybe)
Direct in d+Au
• p+p and d+Au spectra compared to NLO pQCD
• ratio to NLO pQCD• consistent with 1• No indication for
No surprises !
Direct Photons Surely There!
• 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
• Yellow bands show uncertainty on NLO pQCD calculation and thickness function
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 Npart=320
• Dotted line shows uncertainty of thickness function– Error bars show total error (systematics + statistical) except
thickness function error– Yellow shows uncertainty on pQCD calculation
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
Any Hope for QGP Radiation?
• Most realistic calculation– Including all the contributions
PHENIX may be able to see QGP contribution in 1-3GeV/c
Quenching = Jet-Plasma interaction. Does this have an EM signature?
The plasma mediates a jet-photon conversion
Fries, Mueller & Srivastava, PRL 90, 132301 (2003)
Comparison between model and exp
Chiral CondensatePolyakov Loop
• Coincident transitions: deconfinement and chiral symmetry restoration it is seen to hold also vs quark mass
How Chiral Symmetry has manifested innucleus-nucleus collisions?
We must measure vector mesons in both hadronic and leptonic decay channels!
electron PID TOF upgradeHFT – reduce conversion BK
K-pi PID TOF upgradeand ’ EMC + high statistics
Baryonic resonances ()....)
Vector Meson Mass and Chiral Symmetry
mesons: No significant width change or mass shift has beenobserved. Measurement of both K+K- and e+e channels.
mesons: Some kind of mass shift has been observed in STAR, butthe interpretation of the shift is not clear! Measurement of mesons in e+e channel is needed!
Measurement of invariant mass and the residual distributionafter combinatorial background subtraction
--- and mesons--- the nature of mesons (q-qbar or four-quark)
Electron measurement will be possible with the TPC and TOF-- remove photon conversion and Dalitz background !
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
Low Masses:Vector Meson Spectral Densities:Hot Meson Gas
The spectral density is flattened
Rapp, Gale (99)
Very Difficult to Measure Di-leptons
NA60 Comparison of data to RW, BR and NA60 Comparison of data to RW, BR and Vacuum Vacuum (Broadening vs Shift) (Broadening vs Shift)
Chiral Properties at Tc
-- quasi-particles-- mass shift-- width broadening
Dilepton Measurement-- in the low mass region 0.5-2 GeV/c2
-- very difficult
pTassociated varies pT
Two-Particle Correlations in d+Au
Background-subtracted correlations between a high-pT trigger charged particle and an associated charged particle
+jet correlation in Au+Au in run4?
More accurate determination of initial Et
Results for p-p
• 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.
(Subtraction)Bands represent systematic errors.
Errors on the backgrounds result in enlarged errors on the signal,especially at low-pT region.
dE/dx at high pT (62.4 GeV)
rigidity (charge*dE/dx) [keV/cm]
2/ndf = 1.5
pT > 3 GeV/cPion-protonSeparation !
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!