Photoproduction Processes And Quark Wigner Distribution Of The Pion Meson

Photoproduction processes are the essential part of high energy heavy-ion collision-s,it’s the crucial means for making precision measurements.Since dilepton,photon and the dileptontic decay channel of light vector mesons do not participate in strong inter-actions directly,they can be used to improve our understanding of strong interactions in the high energy region.Besides,understanding the internal structures of hadrons is one of the main goals of QCD and hadronic physics.The Wigner distributions,compared with traditional PDFs,can provide the complete description for the parton structure of hadron,and the unique information that can not be gained from the conventional PDFs.At present,the Wigner distribution of the pion meson was less studied in the literature,therefore it can provide complimentary information on the multidimensional description of the partonic structure of hadrons.We propose an exact treatment for the photoproduction of dilepton and photon in p-p collision at LHC energies,in which the photon radiated from nucleus or its con-stituents is off mass shell and no longer transversely polarized,and for avoiding double counting,the squared form factor F_E~2（Q~2）is used as the coherent probability.The com-parisons between the exact treatment results and the EPA ones are expressed as the Q~2（the virtuality of photon）and p _Tdistributions.The numerical results indicate that,EPA is only effective in small Q~2region and can be used for coh.pho with proper choice of Q~2_max,but can not be used for incoh.pho.The exact treatment is needed to deal accurately with the photoproduction of large p _Tdilepton and photon.We extend our exact treatment to deal with the photoproduction processes in p-p,p-Pb and Pb-Pb collisions,and the production of light vector mesons are also calculated.We take into account the contributions of fragmentation processes and magnetic form factor F_M（Q~2）,and give the Q~2,p _Tand y_r（rapidity）dependent differential cross sec-tions.The numerical results indicate that,EPA is sensitive to the values of kinematical variables and its errors are much larger in p-Pb and Pb-Pb collisions.The variable y（rel-ative energy loss of photon emitter）is limited by Q~2,thus it’s maximum can not be set as y_max=1 directly.Otherwise,coherence condition（Q~2<1/R_A~2,R_Ais the nuclear radius）is the good choice for coh.pho and OIC.pho,it cut the EPA errors naturally.But for incoh.pho and UIC.pho,EPA is still inapplicable.Besides,it is worth mentioning that the fragmentation processes dominate the photoproduction processes,its contribution is even larger than the initial parton hard scattering in p-Pb collision.We study the Quark Wigner distributions of the pion meson which are the phase-space distributions containing the most general one-parton information.Using the pion wavefunctions deduced from a light-cone quark model,we calculate the Wigner dis-tributions of the unpolarized,longitudinally polarized and transversely polarized quarks inside the pion meson via the Fock-state overlap representation,respectively.We present the numerical results of the transverse Wigner distributions,and the mixed Wigner dis-tributions which is the functions of b_yand k_x.As a intermediate step,we provide the analytic results for the generalized transverse momentum dependent distributions of the pion meson in the same model.