Transverse momentum broadening of vector bosons in nuclear collisions

We calculate in perturbative QCD the transverse momentum broadening of vector bosons in nuclear collisions. We evaluate the effect of initial-state parton multiple scattering for the production of Drell-Yan and $W/Z$ bosons, and both initial- and final-state multiple scattering effect for heavy quarkonium production. We predict the transverse momentum broadening of $W/Z$ bosons in heavy ion collisions at the LHC energies, and argue that the broadening should be a clean probe of initial-state medium effect [1]. For heavy quarkonium production, we found in both NRQCD and Color Evaporation model that the transverse momentum broadening of heavy quarkonia in hadron-nucleus collision is close to $2C_A/C_F$ times the corresponding Drell-Yan broadening, which is consistent with Fermilab E866 data [1]. We also predict a significant reduction of the transverse momentum broadening for heavy quarkonia in relativistic heavy ion collisions if a dense medium is formed. \newline [1] Z.~B.~Kang and J.~W.~Qiu, arXiv: 0707.0276 [hep-ph]