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Dileptons and Photons

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.

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Dileptons and Photons

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  1. Dileptons and Photons Huan Z Huang Department of Physics and Astronomy University of California, Los Angeles Department of Engineering Physics Tsinghua University

  2. Collision Dynamics

  3. Photon Sources in Quark-Gluon Plasma “Naïve” Leading Order Processes: q + q (g) → g (q) + γ q q g [Kapusta etal ’91, Baier etal ’92] But: other contributions toO(αs) collinear enhanced Dg=(t-mD2)-1 ~ 1/αs Bremsstrahlung Pair-ann.+scatt. + ladder resummation (LPM) [Aurenche etal ’00, Arnold,Moore+Yaffe ’01]

  4. p p γ γ p,a1 r p r p p,a1 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

  5. p γ p γ p K K* K K* (ii) wt-Channel p γ Gwrplarge! potentially important … w [Turbide,Gale +RR ’04] r p More Meson Gas Sources (i) Strangeness Contributions: SU(3)F MYM ~25%of pp→ργ ~40%of pr→pγ! (iii) Higher Resonances Ax-Vec:a1,h1→pg,Vec:w,w’,w’’→pgother:p(1300)→pg f1→rg, K1→Kg K*→Kg a2(1320)→pg

  6. Sp r > Sp > 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,RR+Wambach ’98]

  7. HG Emission Rates: Summary • wt-channel (very) important • at high energy • form factor suppression (2-4) • strangeness significant • baryons at low energy mB=220MeV Though EM is well understood, photon production in collisions complicated dynamics !! [Turbide,RR+Gale ’04]

  8. PQCD photons Initial hard production: pp → γX scaling with xT=2pT /√s , + power-law fit[Srivastava ’01]

  9. Au + Au QGP ?! Hadron Gas “Freeze-Out” Naively Thermal Photons  T The higher temperature, the more thermal radiation ! But the relation between initial T and photons is non-trivial !

  10. Predictions for Central Au+Au Collisions at 200 GeV ‘pre-equilibrium’ contribution from parton cascading  major contribution QGP thermal radiation  1-2 GeV (maybe)

  11. Direct g in d+Au PHENIX • p+p and d+Au spectra compared to NLO pQCD • ratio to NLO pQCD • consistent with 1 • No indication for nuclear effects 2 No surprises !

  12. Direct Photons Surely There! p0 suppression  helps Lines Nbinary Scaling

  13. 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

  14. 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 Direct g p0

  15. 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

  16. 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

  17. Quenching = Jet-Plasma interaction. Does this have an EM signature? The plasma mediates a jet-photon conversion Fries, Mueller & Srivastava, PRL 90, 132301 (2003)

  18. Comparison between model and exp

  19. Lattice QCD Polyakov Loop Chiral Condensate • Coincident transitions: • deconfinement and chiral symmetry restoration • it is seen to hold also vs quark mass

  20. 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)....)

  21. 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. 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 e+e- channel is needed! Measurement of p+p- 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 !

  22. 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

  23. Low Masses:Vector Meson Spectral Densities:Hot Meson Gas The spectral density is flattened and broadened Rapp, Gale (99)

  24. Very Difficult to Measure Di-leptons

  25. NA60 Comparison of data to RW, BR and Vacuum  (Broadening vs Shift) Sanja Damjanovic pT dependence

  26. 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

  27. The END

  28. pTassociated varies pTtrigger varies Two-Particle Correlations in d+Au STAR preliminary STAR preliminary Background-subtracted correlations between a high-pT trigger charged particle and an associated charged particle

  29. Photon-hadron correlations STAR preliminary STAR preliminary g+jet correlation in Au+Au in run4? More accurate determination of initial Et

  30. 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.

  31. dE/dx at high pT (62.4 GeV) 2/ndf = 1.5 pT > 3 GeV/c Pion-proton Separation ! rigidity (charge*dE/dx) [keV/cm] negatives positives

  32. 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!

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