Measurement of cold nuclear matter effects for inclusive $J/\psi$ in $p$+Au collisions at $\sqrt{s_{_{\mathrm{NN}}}}$ = 200 GeV

The STAR collaboration
Phys.Lett.B 825 (2022) 136865, 2022.

Abstract (data abstract)
Measurement by the STAR experiment at RHIC of the cold nuclear matter (CNM) effects experienced by inclusive \jpsi\ at mid-rapidity in 0-100\% \pAu\ collisions at \sqrtsNN\ = 200 GeV is presented. Such effects are quantified utilizing the nuclear modification factor, \rpa, obtained by taking a ratio of \jpsi\ yield in \pAu\ collisions to that in \pp\ collisions scaled by the number of binary nucleon-nucleon collisions. The differential \jpsi\ yield in both \pp\ and \pAu\ collisions is measured through the dimuon decay channel, taking advantage of the trigger capability provided by the Muon Telescope Detector in the RHIC 2015 run. Consequently, the \jpsi\ \rpa\ is derived within the transverse momentum (\pT) range of 0 to 10 \gev. A suppression of approximately 30\% is observed for $\pT<2$ \gev, while \jpsi\ \rpa\ becomes compatible with unity for \pT\ greater than 3 \gev, indicating the \jpsi\ yield is minimally affected by the CNM effects at high \pT. Comparison to a similar measurement from 0-20\% central \AuAu\ collisions reveals that the observed strong \jpsi\ suppression above 3 \gev\ is mostly due to the hot medium effects, providing strong evidence for the formation of the quark-gluon plasma in these collisions. Several model calculations show qualitative agreement with the measured \jpsi\ \rpa, while their agreement with the \jpsi\ yields in \pp\ and \pAu\ collisions is worse.

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