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 root S-NN = 200 GeV and U + U collisions at ONN = 193 GeV. Remarkably, the inferred nuclear modification factor of J/psi at midrapidity 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 semicentral 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 in violent hadronic collisions may provide a novel probe of the quark-gluon plasma.