11th International Conference on Hard and Electromagnetic Probes of High-Energy Nuclear Collisions

We present a new high-statistics measurement of inclusive $J/\psi$ production versus event multiplicity in $p$+$p$ collisions at $\sqrt{s} = 500$ GeV with the STAR experiment at RHIC. At mid-rapidity, calorimeter-triggered events are selected for candidate $J/\psi$ detection in the dielectron decay channel. Existing measurements at both $\sqrt{s} = 200$ GeV from STAR and $\sqrt{s} = 7$ TeV from ALICE have shown a faster-than-linear rise as a function of mid-rapidity multiplicity. Potential dependence on collision energy is examined, and measurements are made separately for several intervals over a broad $J/\psi$ transverse momentum range. Proposed explanatory mechanisms, including multi-parton interactions, string screening, and high gluon radiation are discussed, along with the guidance this measurement and related probes provide to model calculations.