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The deconfined state of strongly interacting quarks and gluons, quark–gluon plasma, may be formed in relativistic ion collisions at sufficient temperature and energy density. The signatures of quark–gluon plasma formation were observed in heavy-ion collisions by studying meson production. In small-collision systems, such as , , , and , the volume and lifetime of the produced medium might be insufficient for observation of quark–gluon plasma effects. However various physics mechanisms reflecting initial state of the collision, cold nuclear matter effects, may lead to a collective-like behavior in small-collision systems without quark–gluon plasma formation. The nuclear modified parton distribution functions are considered to be an underlying physics mechanism of cold nuclear matter effects. This paper presents the comparison of meson production in , , , and collisions at GeV at midrapidity ( , measured by PHENIX, to PYTHIA calculations with nuclear modified parton distribution functions EPPS16 and CTEQ15. The meson production in collisions at GeV might be driven by mechanisms additional to nuclear modification of parton distribution functions. [ABSTRACT FROM AUTHOR]

The photoproduction of J/ψ mesons off nuclei at the energies of 8 and 11 GeV, which are close to the threshold for their production on a free nucleon, is considered on the basis of the first-collisionmodel relying on the nuclear spectral function and including incoherent processes of charmonium production in photon-nucleon collisions. The model takes into account the absorption of final J/ψ mesons, the binding of target nucleons, and their Fermi motion, as well as the effect of the scalar J/ψ-nucleus potential (or the inmedium shift of the J/ψ-meson mass) on these processes. Under various assumptions on the magnitude of this shift, the momentum dependences of various observables, such as the relative transparency of carbon and gold nuclei and the J/ψ-meson yields, are calculated for meson production on these nuclei. The observables in question include the relative transparencies of these nuclei to J/ψ mesons and the absolute and relativemeson yields from them. It is shown that, at the energy of 11 GeV, which is above the threshold, the first observable is virtually independent of the in-medium change in the J/ψ-meson mass. At the same time, it is moderately sensitive to this change at the subthreshold energy of 8 GeV. On the other hand, it is shown that, at these photon energies, the absolute and relative values of the differential cross sections, especially for the the heavy nucleus of 197 Au, in the momentum range of 4-6 GeV/с are highly sensitive to the in-medium change in the J/ψ-meson mass. Therefore, the present calculations can be used to determine the unknown shift of the J/ψ-meson mass in nuclei by means of comparing their results with data expected from experiments in Hall C at the Continuous Electron Beam Accelerator Facility (CEBAF, USA) upgraded to the energy of 12 GeV. [ABSTRACT FROM AUTHOR]

On the basis of the first-collision model that relies on the nuclear spectral function and which includes incoherent processes involving charmonium production in proton-nucleon collisions, the photoproduction of J/ ψ mesons on nuclei is considered at energies close to the threshold for their production on a nucleon. The absorption of final J/ ψ mesons, their formation length, and the binding and Fermi motion of target nucleons are taken into account in this model along with the effect of the nuclear potential on these processes. The A dependences of the absolute and relative charmonium yields are calculated together with absolute and relative excitation functions under various assumptions on the magnitude of the cross section for J/ ψN absorption, the J/ ψ-meson formation length, and their inmedium modification. It is shown that, at energies above the threshold, these features are virtually independent of the formation length and the change in the J/ ψ-meson mass in nuclear matter but are rather highly sensitive to the cross section for J/ ψN interaction. The calculations performed in the present study can be used to determine the unknown cross section for J/ ψ-meson absorption in nuclei from a comparison of their results with data expected from experiments in the Hall C of the CEBAF (USA) facility upgraded to the energy of 12 GeV. It is also shown that the absolute and relative excitation functions for J/ ψ mesons in photon-nucleus reactions at subthreshold energies are sensitive to the change in the meson mass and, hence, carry information about the properties of charmonium in nuclear matter. [ABSTRACT FROM AUTHOR]

Interpretations of the most recent experimental data on double J/ ψ production are discussed. It is shown that a significant signal in the region of high invariant masses of a J/ ψ pair may be a piece of evidence not only in favor of the dominance of double parton scattering in this region. The diagrams involving two-gluon exchange in the t channel, which are considered in the present study, make a commensurate contribution to the above production process in the region in question and also to the double production of J/ ψ mesons characterized by a large rapidity difference. [ABSTRACT FROM AUTHOR]