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Register a new userPhotoproduction of $J/psi$ in p-p ultra-peripheral collisions at LHC energy
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$J/psi$ production in p-p ultra-peripheral collisions through the elastic and inelastic photoproduction processes, where the virtual photons emitted from the projectile interact with the target, are studied. The comparisions between the exact treatment results and the ones of equivalent photon approximation are expressed as $Q^{2}$ (virtuality of photon), $z$ and $p_{T}$ distributions, and the total cross sections are also estimated. The method developed by Martin and Ryskin is employed to avoid double counting when the different production mechanism are considered simultaneously. The numerical results indicate that, the equivalent photon approximation can be only applied to the coherent or elastic electromagnetic process, the improper choice of $Q^{2}_{mathrm{max}}$ and $y_{mathrm{max}}$ will cause obvious errors. And the exact treatment is needed to deal accurately with the $J/psi$ photoproduction.
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The production mechanism of quarkonia states in hadronic collisions is still to be understood by the scientific community. In high-multiplicity $p+p$ collisions, Underlying Event (UE) observables are of major interest. The Multi-Parton Interactions (MPI) is a UE observable, where several interactions occur at the partonic level in a single $p+p$ event. This leads to dependence of particle production on event multiplicity. If the MPI occurs in a harder scale, there will be a correlation between the yield of quarkonia and total charged particle multiplicity. The ALICE experiment at the Large Hadron Collider (LHC) in $p+p$ collisions at $sqrt{s}$ = 7 and 13 TeV has observed an approximate linear increase of relative $J/psi$ yield ($frac{dN_{J/psi}/dy}{<dN_{J/psi}/dy>}$) with relative charged particle multiplicity density ($frac{dN_{ch}/dy}{<dN_{ch}/dy>}$). In our present work we have performed a comprehensive study of the production of charmonia as a function of charged particle multiplicity in $p+p$ collisions at LHC energies using pQCD-inspired multiparton interaction model, PYTHIA8 tune 4C, with and without Color Reconnection (CR) scheme. A detail multiplicity and energy dependent study is performed to understand the effects of MPI on $J/psi$ production. The ratio of $psi(2S)$ to $J/psi$ is also studied as a function of charged particle multiplicity at LHC energies.
Gluon density and its distributions inside nuclei and the parton modification of bounded nucleons inside a nucleus, are some of the main standing problems in nuclear and particle physics. In recent years, ultra-peripheral collisions (UPC) of heavy ions have provided a new way of probing the gluon density, which is based on coherent diffractive vector-meson productions, e.g., $J/psi$ meson. For heavy ions, e.g., Pb, the gluon density is found to be significantly suppressed through the UPC $J/psi$ measurement, suggesting a strong gluon shadowing effect in heavy nuclei. In this analysis, we aim to look at a unique set of data taken by the STAR experiment, where $J/psi$ mesons are photoproduced off the deuteron target with no other particle produced, except for the deuteron or its breakup products. The Zero Degree Calorimeter response with respect to the deuteron dissociation by detecting a beam-rapidity neutron is also investigated and provides additional information about the underlying physics process. The cross section of $J/psi$ photoproduction in the photon-deuteron system is measured at the photon-nucleon center-of-mass energy $Wsim25~rm{GeV}$, as well as the momentum transfer $t$ dependence cross section, $dsigma/dt$. Data suggests a wider gluon density distribution than the Hulthen charge density distribution in deuteron.
By generalizing the statistical model for particle production to the spin degree of freedom of initially produced J/psi, we study the spin projection J_y of J/psi perpendicular to the reaction plane in peripheral heavy ion collisions at the LHC energy that leads to a strong, albeit of short duration, magnetic field. We find that for J/psis produced directly from charm and anticharm quarks in the color singlet state, like that in the Color-Singlet Model, their yield in the presence of the magnetic field is larger for J_y=0 than for J_y=1 or -1. This leads to a spin asymmetry of finally produced J/psi even after including their final-state scattering in the produced quark-gluon plasma.
We suggest inclusive hadron production in ultra-peripheral proton-nucleus collisions (UPCs) $p^uparrow A to h AX$ as a new channel to investigate single spin asymmetries (SSAs), in particular, to test the assumed dominance of the contribution from twist-three fragmentation functions. The UPC cross sections are obtained by considering the photoproduction limit of semi-inclusive deep inelastic scattering (SIDIS). In particular, we find simple formulas for the polarized UPC cross sections in the collinear twist-three framework. We then numerically calculate the fragmentation contribution to SSA in $p^uparrow A to pi A X$ at $sqrt{s} = 200$ GeV and find a few percent asymmetry in the forward region.
The PHENIX experiment at the Relativistic Heavy Ion Collider has measured the differential cross section, mean transverse momentum, mean transverse momentum squared of inclusive $J/psi$ and cross-section ratio of $psi(2S)$ to $J/psi$ at forward rapidity in pp collisions at sqrts = 510 GeV via the dimuon decay channel. Comparison is made to inclusive $J/psi$ cross sections measured at sqrts = 200 GeV and 2.76--13 TeV. The result is also compared to leading-order nonrelativistic QCD calculations coupled to a color-glass-condensate description of the low-$x$ gluons in the proton at low transverse momentum ($p_T$) and to next-to-leading order nonrelativistic QCD calculations for the rest of the $p_T$ range. These calculations overestimate the data at low $p_T$. While consistent with the data within uncertainties above $approx3$ GeV/$c$, the calculations are systematically below the data. The total cross section times the branching ratio is BR $dsigma^{J/psi}_{pp}/dy (1.2<|y|<2.2, 0<p_T<10~mbox{GeV/$c$}) =$ 54.3 $pm$ 0.5 (stat) $pm$ 5.5 (syst) nb.
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