Observation of Excess J/ Yield at Very Low Transverse Momenta in Au+Au Collisions at sqrt[s_{NN}]=200GeV and U+U Collisions at sqrt[s_{NN}]=193GeV.
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We report on the first measurements of J/ production at very low transverse momentum (p_{T}<0.2GeV/c) in hadronic Au+Au collisions at sqrt[s_{NN}]=200GeV and U+U collisions at sqrt[s_{NN}]=193GeV. Remarkably, the inferred nuclear modification factor of J/ at midrapidity in Au+Au (U+U) collisions reaches about 24 (52) for p_{T}<0.05GeV/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/ 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/ yield observed at extremely low p_{T} originates from coherent photon-nucleus interactions. In particular, coherently produced J/'s in violent hadronic collisions may provide a novel probe of the quark-gluon plasma.