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

We report on the first measurements of $J/\psi$ production at very low transverse momentum ($p_T<0.2\mathrm{GeV}/c$) in hadronic $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ and $\mathrm{U}+\mathrm{U}$ collisions at $\sqrt{s_{NN}}=193\mathrm{GeV}$. Remarkably, the inferred nuclear modification factor of $J/\psi$ at midrapidity in $\mathrm{Au}+\mathrm{Au}$ ($\mathrm{U}+\mathrm{U}$) collisions reaches about 24 (52) for $p_T<0.05\mathrm{GeV}/c$ 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 $J/\psi$’s in violent hadronic collisions may provide a novel probe of the quark-gluon plasma.

Figure 1

The dielectron invariant mass spectrum for the 40%–80% centrality class in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ (a) and in $\mathrm{U}+\mathrm{U}$ collisions at $\sqrt{s_{NN}}=193\mathrm{GeV}$ (b). The error bars are the statistical uncertainties. The fitting curves are described in the text.

Figure 2

The $J/\psi$ invariant yields for $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ and $\mathrm{U}+\mathrm{U}$ collisions at $\sqrt{s_{NN}}=193\mathrm{GeV}$ as a function of $p_T$ for different centralities at midrapidity ($|y|<1$). The error bars depict the statistical errors while the boxes represent the systematic uncertainties. The data points with $p_T<0.2\mathrm{GeV}/c$ have been slightly shifted along the horizontal axis to facilitate viewing of the data. The solid lines are the fits to data points in the range of $p_T>0.2\mathrm{GeV}/c$, while the dashed lines are the extrapolations of the fits.

Figure 3

The $J/\psi R_{\mathrm{AA}}$ as a function $p_T$ in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ and $\mathrm{U}+\mathrm{U}$ collisions at $\sqrt{s_{NN}}=193\mathrm{GeV}$. The error bars represent the statistical uncertainties, while the boxes represent the systematic uncertainties. The shaded bands at unity indicate the uncertainties on the $p+p$ reference [38] and $⟨N_{\mathrm{coll}}⟩$.

Figure 4

The $J/\psi$ yield with the expected hadronic contribution subtracted as a function of the negative momentum transfer squared $-t$ ($-t\sim p_T^2$) for the 40%–80% collision centrality class in $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{U}+\mathrm{U}$ collisions. The error bars represent the statistical uncertainties, while the boxes represent the systematic uncertainties. The black solid line is an exponential fit to the $\mathrm{Au}+\mathrm{Au}$ data points in the range of $0.002–0.015(\mathrm{GeV}/c{)}^2$. The dashed black lines are extrapolations of the fit. The blue dash-dotted line is a fit to the $\mathrm{Au}+\mathrm{Au}$ data points in the range of $0–0.015(\mathrm{GeV}/c{)}^2$ with the interference shape from Ref. [47].

Figure 5

The $p_T$-integrated $J/\psi$ yields ($p_T<0.1\mathrm{GeV}/c$) with the expected hadronic contribution subtracted as a function of $N_{\mathrm{part}}$ for 30%–80% $\mathrm{Au}+\mathrm{Au}$ collisions and 40%–80% $\mathrm{U}+\mathrm{U}$ collisions. The expected hadronic contributions for $\mathrm{Au}+\mathrm{Au}$ collisions are also shown. The error bars represent the statistical uncertainties, while the boxes represent the systematic uncertainties. The lines are model calculations of coherent photoproduction with different scenarios for photon and Pomeron emitters [47].