Universality in nuclear dependence coefficient $\alpha(q_T)$

Abstract

We derive the nuclear dependence coefficient $\alpha(q_T)$ for Drell-Yan and J/$\psi$ production. We show that at small $q_T$, the $\alpha(q_T)$ is given by an universal functional form: $\alpha(q_T)=a+b q_T^2$, and the parameters $a$ and $b$ are completely determined by either perturbatively calculable or independently measurable quantities. This universal functional form $\alpha(q_T)$ is insensitive to the $A$, and is consistent with existing data.Comment: latex, 4 pages, 3 figures, slightly revise