Speaker
Description
Dileptons ($l^{+}l^{-}$) are produced in all the stages of the heavy-ion collisions, and escape with minimum interaction with the strongly interacting medium. Thus, $l^{+}l^{-}$ pair measurements play an essential role in the study of hot and dense nuclear matter, created in heavy-ion collisions. Recently, a significant excess of $J/\psi$ yield at very low transverse momentum ($p_{T}<0.3$ GeV/$c$) was reported by the ALICE [1] and STAR collaborations in peripheral A+A collisions. These observations may point to evidence of coherent photoproduction of $J/\psi$ in violent hadronic interactions while traditionally coherent photoproduction is thought to only exist in ultra-peripheral heavy-ion collisions when the traversing nuclei remain intact. It is interesting to investigate the $e^{+}e^{-}$ pair production in a wider invariant mass region ($M_{ee}<4$ GeV/$c^{2}$) at very low $p_{T}$ in heavy-ion collisions for different centrality bins. If the coherent photoproduction mechanism is 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 $e^{+}e^{-}$ pair invariant mass spectra in three $p_{T}$ bins (0-0.15, 0.15-1, and 1-10 GeV/$c$) and $p_{T}$ spectra for $p_{T}<0.3$ GeV/$c$ in three mass regions (0.4-0.76, 1.2-2.6, and 2.8-3.2 GeV/$c^{2}$) in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV and U+U collisions at $\sqrt{s_{NN}}$ = 193 GeV. The structure of $t$ ($t = p_{T}^{2}$) distributions of these three mass regions and comparisons with that in ultra-peripheral collisions will be shown. The centrality dependence of these $e^{+}e^{-}$ pair measurements will be reported, and physics implications will be discussed.
[1] J. Adam $et~al.$ (ALICE Collaboration), Phys. Rev. Lett. 116, 222301 (2016).
| Preferred Track | Electromagnetic Probes |
|---|---|
| Collaboration | STAR |
