Discrepancy in low transverse momentum dileptons from relativistic heavy-ion collisions

The dilepton transverse momentum spectra and invariant mass spectra for low p(T) < 0.15 GeV/c in Au+Au collisions of different centralities at root s(NN) = 200 GeV are studied within the parton-hadron-string dynamics (PHSD) transport approach. The PHSD describes the whole evolution of the system on a microscopic basis and incorporates hadronic and partonic degrees of freedom, the dynamical hadronization of partons, and hadronic rescattering. For dilepton production in p + p, p + A, and A + A reactions, the PHSD incorporates the leading hadronic and partonic channels (also for heavy flavors) and includes in-medium effects such as a broadening of the vector-meson spectral functions in hadronic matter and a modification of initial heavy-flavor correlations by interactions with the partonic and hadronic medium. The transport calculations reproduce well the momentum integrated invariant mass spectra from the STAR Collaboration for minimum bias Au+Au collisions at root s(NN) = 200 GeV, while the description of the STAR data, when gating on low p(T) < 0.15 GeV/c, gets worse when going from central to peripheral collisions. An analysis of the transverse momentum spectra shows that the data for peripheral (60-80%) collisions are well reproduced for p(T) > 0.2 GeV/c, while the strong peak at low p(T) < 0.15 GeV/c that shows up in the experimental data for the mass bins (0.4 < M < 0.7 GeV and 1.2 < M < 2.6 GeV) is fully missed by the PHSD and cannot be explained by the standard in-medium effects. This provides a new puzzle for microscopic descriptions of low p T dilepton data from the STAR Collaboration.