Speaker
Summary
Recent ALICE collaboration measurements showed a significant excess in J/$\psi$ yield in the very low transverse momenta ($p_{T}$<0.3 GeV/$c$) in peripheral Pb+Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV. The same behavior of the J/$\psi$ production is observed at STAR in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. These results are interpreted as coherent photoproduction of J/$\psi$ at the moment. The coherent photonproduction scenario may also suggest there is an excess of photoproduced electron-positron production at the very low $p_{T}$ in peripheral collisions. It would be very interesting to investigate the electron-positron pair production in the full mass region ($M_{ee}<4 $GeV/$c^{2}$) at very low $p_{T}$ in heavy-ion collisions in different centrality bins. If the coherent photoproduction mechanism confirmed, the coherently photoproduced $e^{+}e^{-}$ pairs accompanying violent hadronic collisions may provide a novel probe of the hot and dense nuclear matter.
In this talk, we will present $e^{+}e^{-}$ invariant mass spectra in very low $p_{T}$ in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV and U+U collisions at $\sqrt{s_{NN}}$ = 193 GeV. The $p_{T}$ spectra from different mass regions (0.4-0.76, 1.2-2.6, and 2.8-3.2 GeV/$c^{2}$) will be reported. The structure of t (-t = $p_{T}^{2}$ ) distributions of these mass regions and comparisons with that in ultra-peripheral collisions will be shown. The centrality dependence of the $e^{+}e^{-}$ productions will be shown. Physics messages will be discussed.
