STAR focus: Measurements of the transverse-momentum-dependent cross sections of $J/\psi$ production at mid-rapidity in proton+proton collisions at $\sqrt{s}$ = 510 and 500 GeV
 The STAR Collaboration has recently published "Measurements of the transverse-momentum-dependent cross sections of $J/\psi$ production at mid-rapidity in proton+proton collisions at $\sqrt{s}$ = 510 and 500 GeV with the STAR detector" in Physical Review D 100, 052009 (2019). The $J/\psi$ meson is a subatomic particle, a bound state of charm and anticharm quarks, which was discovered in 1974 at Brookhaven National Laboratory (BNL) and Standford Linear Accelerator Center. However, the $J/\psi$ production mechanism is still not yet fully understood after several decades of the discovery. Therefore, it is important to continue confronting the models that incorporate the most current understanding with new data. Relativistic Heavy-Ion Collider (RHIC) at BNL provides a unique collision energy to study the detailed production mechanism of the $J/\psi$. In addition, in 2013-2014 the STAR detector has implemented a new subdetector dedicated for muon detection, the Muon Telescope Detector (MTD), and this provides us an opportunity to probe the low-pT region of the $J/\psi$ production. This publication presents the $J/\psi$ meson production cross sections in proton+proton collisions at center-of-mass energies of 510 and 500 GeV using the μ+μ- and e+e- decay channels to probe low-pT and high-pT regions, respectively.
 FIG: (a) The $J/\psi$ differential full production cross sections as a function of $p_{T}^{J/\psi}$ in proton+proton collisions at $\sqrt{s}$ = 510 and 500 GeV measured through the $\mu^{+}\mu^{-}$ (blue stars) and $e^{+}e^{-}$ decay channels (red circles). The shaded region around the data points denotes the polarization envelope and the green curve is the estimation of the B-hadron feed-down from FONLL.(b, c, d) Ratios of data and different model calculations to the Levy fit function.
 Two different measurements of the inclusive $J/\psi$ production cross section have been presented. The first is a fidjucial cross section measurement utilizing only a restricted phase space as defined by detector acceptance. It is independent of the assumptions regarding polarization which results in a large systematic uncertainty at low pT. This allows direct comparisons between measurements and theoretical calculations in the future, for more discriminating tests of the models. The second is a full cross section measurement, as shown in the figure, accessing the full $J/\psi$ decay phase space, depending highly on assumptions regarding polarization. The calculations from Color Glass Condensate (CGC), Next-to-leading oder None-Relativistic QCD formalism (NLO NRQCD) and Improved Color Evaporation Model (ICEM), which cover low, high and both pT regions respectively, give a reasonable description for the data within the polarization envelope. The results presented in this paper, together with cross section measurements at other energies, and measurements of the polarization, contribute to the goal of better understanding the production of heavy quarkonium in hadronic collisions. Model calculations, which incorporate different approaches toward the $J/\psi$ production mechanism, are compared with experimental results and show reasonable agreement within uncertainties. Posted Oct 24, 2019 More STAR Focus Features

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