Quark Matter 2017

Coherent very low transverse momentum e+e− pair production in hadronic Au+Au collisions at sNN = 200 GeV and U+U collisions at sNN = 193 GeV at STAR

Not scheduled

2h 30m

Hyatt Regency Chicago

Board: H04

Poster Poster Session

Speaker

Shuai Yang (Brookhaven National Laboratory)

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 $etal.$ (ALICE Collaboration), Phys. Rev. Lett. 116, 222301 (2016).

Presentation materials

QM17poster_lowPt_dielectron_v2.pdf