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

Abstract

The dilepton transverse momentum spectra and invariant mass spectra for low pT < 0.15 GeV/c in Au+Au collisions of different centralities at √ sNN = 200 GeV are studied within the partonhadron-string dynamics (PHSD) transport approach. The PHSD describes the whole evolution of the system on a microscopic basis, 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 √ sNN = 200 GeV, while the description of the STAR data -when gating on low pT < 0.15 GeV/c -is getting 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 pT > 0.2 GeV/c while the strong peak at low pT < 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 pT dilepton data from the STAR Collaboration.

Dielectron production at low transverse momentum in Pb-Pb collisions at $ \sqrt{s_{\mathrm{NN}}}=5.02 $ TeV with ALICE

“…Indications for dilepton production via photon-photon interactions in hadronic heavy-ion collisions were reported at RHIC and the LHC. The STAR collaboration found an excess with respect to the hadronic expectation of low-p T,ee dielectrons in Au-Au and U-U collisions, which is compatible with calculations for coherent photo-production [1][2][3]6] and which cannot be linked to hadronic sources [7]. The ATLAS collaboration observes photoproduced dimuons in Pb-Pb collisions, which exhibit a centrality dependent broadening effect of their angular pair correlations that seems not to be present in UPC [8,9].…”

Traces of nonequilibrium effects, initial condition, bulk dynamics, and elementary collisions in the charm observables

“…It has been shown that the scattering cross sections reproduce the spatial diffusion coefficient of heavy quarks from lQCD calculations and the experimental data on D mesons. Even more, also single electrons as well as dileptons are in agreement with experiment from the beam energy scan energies at RHIC to LHC energies [11,12,[26][27][28].…”

Photon induced processes: from ultraperipheral to semicentral heavy ion collisions

“…While thermal radiation (and the hadronic decay "cocktail") dominate the yield for P T >0.2 GeV at all centralities, while the γ γ -fusion gives rise to the pronounced enhancement at P T < 0.15 GeV. Note that a recent transport calculation [81] confirmed that hadronic sources (thermal emission plus cocktail) cannot explain the low-P T excess observed by STAR in peripheral Au-Au collisions. No photoproduction processes were considered in that study.…”