Nuclear dependence in transverse momentum distribution for Drell-Yan pair

Recently, it was found experimentally that the transverse momentum distribution of Drell-Yan pair in hadron-nucleus collisions shows nuclear dependence not linearly proportional to A, the atomic number of the target [1]. In addition, the average of squared transverse momentum, 〈q T 〉, grows approximately as A 1/3 [1,2]. This observed non-linear nuclear dependence indicates that multiple-scattering is important in hadron-nucleus collisions. In this talk, I present a calculation of double-scattering in QCD perturbation theory for nuclear dependence of Drell-Yan q T spectrum, dσ/dQ 2 dq T . Although double-scattering is in principle a power correction (or known as a high twist effect) to the leading single scattering, it is not necessary small because of A enhancement from large nuclear size. In terms of QCD factorization generalized to higher twist [3], Luo, Qiu and Sterman (LQS) developed a consistent perturbative treatment of multiple scattering at parton level [4]. Since Drell-Yan pair does not interact strongly once produced, the observed non-linear nuclear dependence is a result of multiple scattering between the incoming beam parton and nuclear matter before the pair was produced. Drell-Yan dσ/dQ dq T has two observed physical scales: Q 2 and q T . When Q 2 and q T are both large, dσ/dQ dq T is effectively an one-scale process. Method developed by LQS [4] for double scattering can be naturally applied. However, when q T ≪ Q , a resummation of large ln(Q/q T ) is necessary for a reliable prediction of Drell-Yan q 2 T spectrum [5].