Abstract
We present the first measurements of [Formula: see text] pair production at very low transverse momentum ([Formula: see text] 0.15 GeV/[Formula: see text]) in Au + Au collisions at [Formula: see text] = 200 GeV and U + U collisions at [Formula: see text] = 193 GeV using the STAR detector at the Relativistic Heavy Ion Collider. A significant excess, with respect to known hadronic contributions, is observed in 60-80% central heavy-ion collisions over the whole [Formula: see text] range. Remarkably, the excess almost entirely happens below [Formula: see text] 0.15 GeV/[Formula: see text], and can not be explained by a theoretical model calculation incorporating in-medium broadened [Formula: see text] spectral function. Moreover, the observed excess yield has no significant centrality dependence. In addition, the steepness of [Formula: see text] distribution exhibits mild invariant mass and collision species dependence.
Highlights
Dileptons play an essential role in studying the hot, dense, and strongly interacting matter, created in Relativistic Heavy-Ion Collider (RHIC),[1] because they are generated at all stages of the collisions and escape with minimum interaction with strongly interacting medium
Previous dilepton measurements at Super Proton Synchrotron (SPS)[2,3] and RHIC4–6 showed a significant enhancement with respect to the known hadronic sources in the mass region below ∼700 MeV/c2, which can be consistently described by an in-medium broadened ρ model calculation.[7]
The measurements of NA60 collaboration in the mass region Mφ < Mll < MJ/ψ provided the first indication of thermal dileptons from a partonic source,[3] which is expected to become significant at top RHIC energy.[8]
Summary
Dileptons play an essential role in studying the hot, dense, and strongly interacting matter, created in Relativistic Heavy-Ion Collider (RHIC),[1] because they are generated at all stages of the collisions and escape with minimum interaction with strongly interacting medium. Previous dilepton measurements at Super Proton Synchrotron (SPS)[2,3] and RHIC4–6 showed a significant enhancement with respect to the known hadronic sources (cocktail) in the mass region below ∼700 MeV/c2, which can be consistently described by an in-medium broadened ρ model calculation.[7] The measurements of NA60 collaboration in the mass region Mφ < Mll < MJ/ψ provided the first indication of thermal dileptons from a partonic source,[3] which is expected to become significant at top RHIC energy.[8] This is an Open Access article published by World Scientific Publishing Company. A model calculation based on an in-medium broadened ρ spectral function is compared with the measurements
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