Observation of Excess $J/\psi$ Yield at Very Low Transverse Momenta in Au+Au Collisions at $\sqrt{s_{\rm{NN}}}$ =200 GeV and U+ U Collisions at $\sqrt{s_{\rm{NN}}}$ =193 GeV

Abstract

We report on the first measurements of $J/\psi$production at very low transverse momentum ($p_T$ < 0.2 GeV/$c$) in hadronic Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 200 GeV and U+U collisions at $\sqrt{s_{\rm{NN}}}$ = 193 GeV. Remarkably, the inferred nuclear modification factor of $J/\psi$ at mid-rapidity in Au+Au (U+U) collisions reaches about 24 (52) for $p_T$ < 0.05 GeV/$c$ in the 60-80% collision centrality class. This noteworthy enhancement cannot be explained by hadronic production accompanied by cold and hot medium effects. In addition, the $dN/dt$ distribution of $J/\psi$ for the very low $p_T$ range is presented for the first time. The distribution is consistent with that expected from the Au nucleus and shows a hint of interference. Comparison of the measurements to theoretical calculations of coherent production shows that the excess yield can be described reasonably well and reveals a partial disruption of coherent production in semi-central collisions, perhaps due to the violent hadronic interactions. Incorporating theoretical calculations, the results strongly suggest that the dramatic enhancement of $J/\psi$ yield observed at extremely low $p_T$ originates from coherent photon-nucleus interactions. In particular, coherently produced $J/\psi$'s in violent hadronic collisions may provide a novel probe of the quark-gluon-plasma.