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) 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?   e+e+ e-e-

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

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

Theory prediction of dilepton emission Vaccuum EM correlator Hadronic Many Body theory Dropping Mass Scenario q+q     ee (HTL improved) (q+g  q+    qee not shown) 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 M 2 factor of the EM correlator In the caluculation, partonic photon emission process q+g  q+    qe + e - is not included 1/M  *  ee qq   *  e + e - ≈(M 2 e -E/T ) × 1/M

Virtual photon emission rate at y=0, pt=1.025 GeV/c Vaccuum EM correlator Hadronic Many Body theory Dropping Mass Scenario q+q annihilaiton (HTL improved) 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+  * is not shown; it should be similar size as HMBT at this p T Real photon yield Turbide, Rapp, Gale PRC69,014903(2004) q+g  q+  * ? qq   * ≈M 2 e -E/T

Electron pair measurement in PHENIX 2 central arms: electrons, photons, hadrons –charmonium J/ ,  ’ -> e + e - –vector meson r, w,  -> e + e - –high p T p o, p +, p - –direct photons –open charm –hadron physics Au-Au & p-p spin PC1 PC3 DC magnetic field & tracking detectors e+e+ ee   designed to measure rare probes: + high rate capability & granularity + good mass resolution and particle ID - limited acceptance

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 σ cc = 544 ± 39 ±142 ± 200 μb (stat) (sys) (model) Simultaneous fit of charm and bottom: –σ cc = 518 ± 47 ± 135 ± 190 μb (stat) (sys) (model) –σ bb = 3.9 ± /-2 μb Charm cross section from single electron measurement: –σ cc = 567 ± 57 ± 193 μb Published in PLB670,313(2009) 8

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

pp and AuAu normalized to p 0 Dalitz region (~ same # of particles) p+p: agree with the expected background from hadron decays Au+Au: large Enhancement in GeV/c 2 p+p NORMALIZED TO m ee <100 MeV PLB670,313 (2009)arXiv: PHENIX low mass dielectrons AuAu pp low mass intermediate mass J/  ’’  

Mass Spectra: p T dependency Study p T dependency of the low mass enhancement in Au+Au p+p in agreement with cocktail Au+Au low mass enhancement concentrated at low p T 0 < p T < 8 GeV/c 0 < p T < 0.7 GeV/c 0.7 < p T < 1.5 GeV/c1.5 < p T < 8 GeV/c 11

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/c 2 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  +    +  * or q+g  q+  * Extrapolation to M=0 should give the real photon emission rate.  Talk by Y. Yamaguchi (4C-3) (Data-cocktail) × M ee Excess*M (A.U). Au+Au 200 GeV Vaccuum pt=1.025 GeV/c Drop mass qq

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 p T ?

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

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)