§­p Collision Drell-yan Process The Color Dipole Model

We study Drell-Yan(DY) dilepton production in -proton collisions within the light-cone(LC) color dipole picture, which is described in the target rest frame. It was pointed out that in the target rest frame, DY dilepton production looks like bremsstrahlung, rather than parton annihilation. A quark (or an antiquark) from the projectile hadron radiates a virtual photon on impact on the target, the photon subsequently decays into a lepton pair. The virtual of this model is that the high energy collision is studied in the target rest frame, so it is not only convenient to compare with the experiment data, but also easy to understand the physic mechanic.The basic blocks in the color dipole method are the LC wavefunction and the dipole cross section. The LC wavefunction is calculated from perturbation theory. We have shown that the LC wavefunction for the y'q configuration plays the role of a weight for the different transverse separation p contribution to the process. Namely, small transverse separations are selected by both the transverse and longitudinal pieces. However, the transverse one can select non-negligible large sizes p. In addition, larger invariant mass M scans smaller y'q separations. Moreover, the longitudinal piece is higher twist (suppressed by a factorI/A/2).The dipole cross section is largely unknown, only at small distance pit can be expressed in terms of gluon density. However, several parameterizations exist in the literature, describing the whole function r(x,p), without explicitly taking into account the QCD evolution ofthe density. The saturation model provided by Golec-Biernat and Wiisthoff is used here with very economical parameterization.Estimates for n-p Drell-Yan differential cross section atRHIC(= 200Ge )and LHC( = 5.57e )are presented. We also study the transverse momentum distribution of DY pairs for n - p collision atthe energies of RHIC and LHC.