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 [Kapusta etal ’91, Baier etal ’92] O But: other contributions to O(α s ) collinear enhanced D g =(t-m D 2 ) -1 ~ 1/α s [Aurenche etal ’00, Arnold,Moore+Yaffe ’01] Bremsstrahlung Pair-ann.+scatt. + ladder resummation (LPM) q gq

Photon Sources in Hadron Gas γ    γ    a1a1 a1a1 Photon-producing reactions: mostly at dominant (q 0 >0.5GeV) gauge invariance! q 0 <0.5GeV a 1 -strength problematic Hadron Form Factor Important for photon yield

More Meson Gas Sources (i) Strangeness Contributions: SU(3) F MYM (iii) Higher Resonances Ax-Vec: a 1,h 1 → , Vec: ,  ’,  ’’→  other:  (1300)→  f 1 → , K 1 →K  K * →K  a 2 (1320)→  γ  KK K γ  K*K* K  ~25% of   →  ~40% of   →  (ii)  t-Channel γ     G  large! potentially important … [Turbide,Gale +RR ’04]

Baryonic Contributions use in-medium  –spectral funct: constrained by nucl.  -absorption: > >    B *,a 1,K 1... N, ,K …  N →  N,   N →   NANA  -ex [Urban,Buballa,RR+Wambach ’98]

HG Emission Rates: Summary  B =220MeV [Turbide,RR+Gale ’04]  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 x T =2p T /√s, + power-law fit [Srivastava ’01] PQCD photons

Naively Thermal Photons  T 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 contribution QGP 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 nuclear effects 2 PHENIX No surprises !

Direct Photons Surely There!  0 suppression  helps Lines  N binary Scaling

Direct Photons Direct photon spectra over centralities Systematic Error: ~15-20% Clearly seen that we measured photons over the order of ! –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

Results (R AA ) Photon R AA is consistent with unity over all the centrality. –the yield follows thickness-scaled hard scattering –p-p reference from NLO pQCD Calculation –  0 R AA 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 Direct  00

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, (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 PRC69(2004)014903

Quenching = Jet-Plasma interaction. Does this have an EM signature? The plasma mediates a jet-photon conversion Fries, Mueller & Srivastava, PRL 90, (2003)

Comparison between model and exp

Lattice QCD Chiral CondensatePolyakov Loop Coincident transitions: deconfinement and chiral symmetry restoration it is seen to hold also vs quark mass

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  and  ’  EMC + high statistics Baryonic resonances (  )....)

Vector Meson Mass and Chiral Symmetry  mesons: No significant width change or mass shift has been observed. Measurement of both K + K - and e + e  channels.  mesons: Some kind of mass shift has been observed in STAR, but the interpretation of the shift is not clear! Measurement of  mesons in e + e  channel is needed! Measurement of     invariant mass and the residual distribution after 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 and broadened Rapp, Gale (99)

Very Difficult to Measure Di-leptons

NA60 Comparison of data to RW, BR and Vacuum  (Broadening vs Shift) p T dependence Sanja Damjanovic

Quark-Gluon Fluid Chiral Properties at T c -- quasi-particles -- mass shift -- width broadening Dilepton Measurement -- in the low mass region GeV/c 2 -- very difficult

The END

p T associated variesp T trigger varies STAR preliminary Two-Particle Correlations in d+Au Background-subtracted correlations between a high-p T trigger charged particle and an associated charged particle

Photon-hadron correlations STAR preliminary  +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 p T (62.4 GeV) rigidity (charge*dE/dx) [keV/cm] positivesnegatives  2 /ndf = 1.5 pT > 3 GeV/c Pion-proton Separation !

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 R AA and spectra themselves tell you what happens –Calculation is assuming existence of hot dense medium, which is not the case in peripheral!