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Dilepton Radiation Measured in PHENIX probing the Strongly Interacting Matter Created at RHIC

Dilepton Radiation Measured in PHENIX probing the Strongly Interacting Matter Created at RHIC. Y. Akiba (RIKEN Nishina Center) for PHENIX Collaboration Quark Matter 2009 Knoxville, TN, USA April 2 nd , 2009. Electromagentic probes (photon and lepton pairs). e +. e -. g*. g.

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Dilepton Radiation Measured in PHENIX probing the Strongly Interacting Matter Created at RHIC

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  1. Dilepton Radiation Measured in PHENIX probing the Strongly Interacting Matter Created at RHIC Y. Akiba (RIKEN Nishina Center) for PHENIX Collaboration Quark Matter 2009 Knoxville, TN, USA April 2nd, 2009

  2. Electromagentic probes (photon and lepton pairs) e+ e- g* g • Photons and lepton pairs are cleanest probes of the dense matter formed at RHIC • These probes have little interaction with the matter so they carry information deep inside of the matter • Temperature? • Hadrons inside the matter? • Matter properties?

  3. What we can learn from lepton pair emission q q Emission rate of dilepton per volume e.g. Rapp, Wambach Adv.Nucl.Phys 25 (2000) g*ee decay Boltzmann factor temperature EM correlator Medium property Medium modification of meson Chiral restoration Hadronic contribution Vector Meson Dominance qq annihilation Thermal radiation from partonic phase (QGP) From emission rate of dilepton, the medium effect on the EM correlator as well as temperature of the medium can be decoded.

  4. Relation between dilepton and virtual photon Emission rate of dilepton per volume Emission rate of (virtual) photon per volume Prob. g*l+l- Relation between them This relation holds for the yield after space-time integral Dilepton virtual photon Virtual photon emission rate can be determined from dilepton emission rate M ×dNee/dM gives virtual photon yield For M0, ng*  ng(real); real photon emission rate can also be determined

  5. Theory prediction of dilepton emission Theory calculation by Ralf Rapp at y=0, pt=1.025 GeV/c Usually the dilepton emission is measured and compared as dN/dptdM The mass spectrum at low pT is distorted by the virtual photonee decay factor 1/M, which causes a steep rise near M=0 qq annihilaiton contribution is negligible in the low mass region due to the M2 factor of the EM correlator In the caluculation, partonic photon emission process q+gq+g*qe+e- is not included Vaccuum EM correlator Hadronic Many Body theory Dropping Mass Scenario q+qg*ee (HTL improved) (q+gq+g*qee not shown) 1/M g*ee qqg*e+e- ≈(M2e-E/T)×1/M

  6. Virtual photon emission rate Real photon yield Turbide, Rapp, Gale PRC69,014903(2004) at y=0, pt=1.025 GeV/c The same calculation, but shown as the virtual photon emission rate. The steep raise at M=0 is gone, and the virtual photon emission rate is more directly related to the underlying EM correlator. When extrapolated to M=0, the real photon emission rate is determined. q+gq+g* is not shown; it should be similar size as HMBTat this pT Vaccuum EM correlator Hadronic Many Body theory Dropping Mass Scenario q+q annihilaiton (HTL improved) q+g  q+g* ? qqg* ≈M2e-E/T

  7. Electron pair measurement in PHENIX g p DC e+ e- PC1 magnetic field & tracking detectors PC3 designed to measure rare probes:+ high rate capability & granularity + good mass resolution and particle ID - limited acceptance • 2 central arms: electrons, photons, hadrons • charmonium J/, ’ -> e+e- • vector meson r, w,  -> e+e- • high pTpo, p+, p- • direct photons • open charm • hadron physics Au-Au & p-p spin

  8. p+p results 2.25pb-1 of triggered p+p data Data absolutely normalized Excellent agreement with Cocktail Filtered in PHENIX acceptance Light hadron contributions subtracted Heavy Quark Cross Sections: Charm: integration after cocktail subtraction σcc = 544 ± 39 ±142 ± 200 μb (stat) (sys) (model) Simultaneous fit of charm and bottom: σcc = 518 ± 47 ± 135 ± 190μb (stat) (sys) (model) σbb = 3.9 ± 2.4 +3/-2 μb Charm cross section from single electron measurement: σcc = 567 ± 57 ± 193 μb Published in PLB670,313(2009)

  9. Charm and bottom cross sections CHARM BOTTOM Dilepton measurement in agreement with measurement from e-h correlation and with FONLL (upper end) Dilepton measurement in agreement with single electron, single muon, and with FONLL (upper end) Di-electron:PLB670,313(2009) e-h corr: arXiv:0903.4851 First measurements of bottom cross section at RHIC energies!

  10. arXiv:0706.3034 PLB670,313 (2009) PHENIX low mass dielectrons p+p NORMALIZED TO mee<100 MeV low mass w AuAu f pp and AuAu normalized to p0 Dalitz region (~ same # of particles) p+p: agree with the expected background from hadron decays Au+Au: large Enhancement in 0.15-0.75 GeV/c2 J/y intermediate mass y’ pp

  11. Mass Spectra: pT dependency 0 < pT < 8 GeV/c 0 < pT < 0.7 GeV/c 0.7 < pT < 1.5 GeV/c 1.5 < pT < 8 GeV/c • p+p in agreement with cocktail • Au+Au low mass enhancement concentrated at low pT Study pT dependency of the low mass enhancement in Au+Au

  12. Excess of virtual photon The excess of electron pairs over the cocktail is almost constant level at high pT. The excess is converted to the excess of virtual photon by divided by 1/M shape coming from the virtual photon decay. The distribution is ~flat over half GeV/c2 No indication of strong modification of EM correlator at this high pT region (presumably the virtual photon emission is dominated by hadronic scattering process like p+rp+g* or q+gq+g* Extrapolation to M=0 should give the real photon emission rate.  Talk by Y. Yamaguchi (4C-3) Au+Au 200 GeV Vaccuum HMBT @ pt=1.025 GeV/c Drop mass qq (Data-cocktail)× Mee Excess*M (A.U).

  13. What is the Source of the huge excess? A very large excess at low pT and low mass in Au+Au The shape of the excess seems to be incompatible with a constant virtual photon emission rate. Large enhancement of EM correlator at low mass, low pT?

  14. Dilepton Spectra p+p Au+Au 0.3<Mee<1GeV • p+p • Agreement with cocktail • Au+Au • pT<1GeV/c: large excess for 0.3<M<1 GeV • Low temperature component with strong modification of EM correlator?

  15. Summary • EM probes (photons and dileptons) are ideal “penedtrating probes” of dense partonic matter created at RHIC • Double differential measurement of dilepton emission rates can provide • Temperature of the matter • Medium modification of EM spectral function • PHENIX measured dilepton continuum in p+p and Au+Au • pp good agreement with cocktail heavy flavor xsection is deduced • Au+Au Much larger enhancement, strong dependence on pT hint of modification of the spectral function? • Higher statistics data with Hadron Blind Detector (HBD) allows more precise measurements ( HBD poster by J.M.Durham (#908)) • Photon emission can be deduced from dilepton  Y.Yamaguchi (4C-3)

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