Dilepton measurement in H.I.C.

Dilepton measurement in H.I.C. 小沢恭一郎東大・理

Lepton pair measurements pT [GeV/c] φ ω J/y ρ As a function of Energy Species Centrality Rapidity 2 Heavy Flavor Thermal Radiation １ Drell Yan g* １ 2 3 Mass [GeV/c2] ●Au＋Au ●p+p J/y φ ω Heavy Ion Cafe @ Hongo

φ・ω

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 Chiral Symmetry in Hadronic matter 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 Heavy Ion Cafe @ Hongo

f: hadron and lepton decays Main issue: K+K- and e+e- 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 d+Au and p+p data is important as baseline. Heavy Ion Cafe @ Hongo

p+p 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) Heavy Ion Cafe @ Hongo

Spectra of w meson are measured in several collision systems. w spectra • p+p • d+Au • Au+Au • Results are mainly obtained using hadron decays. • Analysis for electron decay mode is underway. Heavy Ion Cafe @ Hongo

Lepton pair continuum(ρ？　Thermal？)

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 Lepton pair continuum • A prediction • R. Rapp, nucl-ex 0204003 Heavy Ion Cafe @ Hongo

@SPS - NA60 • and  : fix yields such as to get, after subtraction, a smoothunderlying 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 > 1.4 GeV/c Heavy Ion Cafe @ Hongo

What’s this? [van Hees+RR ‘06] • absolute norm., meltingr • M>0.9GeV: “4p”→mm; w and f ?! • baryon effects essential Heavy Ion Cafe @ Hongo

submitted to Phys. Rev. Lett arXiv:0706.3034 @RHIC Clear enhancement is observed in the mass region below w. Heavy Ion Cafe @ Hongo

p+p MULTIPLIED by Ncoll pp – AuAu comparison p+p NORMALIZED TO mee<100 MeV Heavy Ion Cafe @ Hongo

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 proportionalto Ncoll Heavy Ion Cafe @ Hongo

photon Heavy Ion Cafe @ Hongo

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 Heavy Ion Cafe @ Hongo

g*direct g g g*incl. p0 p0 e+ g* g e- Compton g q g q can be measured. Idea - g* e+e- 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 e+e- pairs hadron decays and direct photon e+ g* q e- g q *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. Heavy Ion Cafe @ Hongo

g*direct g*incl. gdirect gincl. g*direct / g*inclusive 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. = Heavy Ion Cafe @ Hongo

Direct photon yield Direct photon yield is calculated from the inclusive photon yield. Heavy Ion Cafe @ Hongo

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 Heavy Ion Cafe @ Hongo

Comparison of Rg* btw p+p and Au+Au PHENIX Preliminary Au+Au p+p Heavy Ion Cafe @ Hongo

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 e+e- continuum! Interesting results in Direct photon measurements. Heavy Ion Cafe @ Hongo

Dilepton measurement in H.I.C.