Dielectron measurements in Au+Au collisions at 200 GeV by PHENIX

Dielectron production is a key electromagnetic probe of the space-time evolution of the Quark-Gluon Plasma (QGP) and hadronic matter produced in ultra-relativistic heavy-ion collisions. Since such pairs do not interact strongly and are emitted during all stages of the collisions, the dielectron continuum carries direct information on the thermal radiation from the QGP, hadronic phase decays, and semi-leptonic heavy-flavor meson decays. In the intermediate mass region (m_φ < m_ee < m_J/ψ), the dielectron yield contains a mixture of thermal QGP radiation and open-charm/bottom decays. Disentangling these contributions is crucial for quantifying dynamics of thermal radiation and constraining theoretical models of the QCD matter.

We report the first PHENIX measurement of the dielectron invariant mass spectrum at mid-rapidity Au+Au collisions at 200 GeV using the Silicon Vertex Detector. A good signal-to-background ratio is achieved using a boosted decision tree classifier for electron/hadron separation, along with precise hit-to-track matching and photon conversion rejection. The measured invariant mass spectrum agrees with expectations from hadronic decays based on previous PHENIX results and shows excess contributions consistent with expectations from heavy-flavor decays and/or thermal production. The achieved resolution in pair transverse distance of closest approach, which reflects the spatial separation between e⁺ and e⁻ tracks, suggests the feasibility of statistically separating prompt thermal radiation from heavy-flavor decay sources.