Speaker
Description
Strong electromagnetic field arising from the Lorentz-contraction and a large number of charges (Z) in the colliding nuclei at ultrarelativistic speeds can generate a large flux of quasi-real photons. Consequent photon-induced interactions could reasonably explain the observed enhancements of $J/\psi$ and $e^{+}e^{-}$ pair productions at very low transverse momenta ($p_{T}$) in peripheral high-energy heavy-ion collisions, via photonuclear ($\propto$ $Z^{2}$) and photon-photon ($\propto$ $Z^{4}$) processes. STAR has collected a large sample of $^{96}_{44}Ru$+$^{96}_{44}Ru$ and $^{96}_{40}Zr$+$^{96}_{40}Zr$ collisions at $\sqrt{s_\mathrm{NN}}$ = 200 GeV in 2018, around two billion good events for each collision system. The isobaric collisions, with different number of charges and same number of nucleons in the colliding nuclei, provide a unique opportunity to test the electromagnetic field dependence of photon-induced production.
In this presentation, we will present the first measurement of the electromagnetic field dependence of $J/\psi$ and $e^{+}e^{-}$ pair production at very low $p_{T}$, via comparisons between the new measurements in isobaric collisions as well as to the published results in Au+Au collisions at $\sqrt{s_\mathrm{NN}}$ = 200 GeV. Physical implications of these results will be discussed.
