Speaker
Description
An enhancement in the transverse momentum distribution of Drell-Yan pairs was recently observed in $p$-$Au$ collisions, in the proton going direction, by the PHENIX experiment at RHIC. This observation, along with the absence of any enhancement in the nucleus going direction is consistent with the Cronin effect, and with the observed enhancement in a series of fixed target experiments at Fermilab, carried out over several decades (R209, E288, E537, E605, E615, E772, E866, NuSea etc.). Hard partons from the proton undergo pre-scattering in a large nucleus, prior to the hard process where a dilepton pair with mass $Q$ and transverse momentum $q_T$ is formed. While earlier studies focussed on the angular distribution, the energy loss of low $q_T$ Drell-Yan pairs, as well as the $q_T$ distribution in the collinear limit [1], we specifically focus on large transverse momentum pairs, where $q_T \sim Q$, using factorized Transverse Momentum Dependent Parton Distribution Functions (TMDPDFs). From the hundreds of Feynman diagrams contributing at Next-to-Leading Order and Next-to-Leading-Twist (NLO-NLT), for the case of arbitrary $q_T$, we show that only about 24 diagrams dominate in the limit where $q_T \gg \Lambda_{QCD}$. None of these diagrams involve off-forward distributions, allowing for a constrained evaluation using known parameterizations of TMDPDFs. Evaluating these diagrams, we illustrate the dependence of the differential spectrum, as a function of $q_T$, on the excess transverse momentum imparted by the pre-scattering. Comparing numerical results to data, we illustrate, for the first time, the possibility of momentum fraction ($x$) and intrinsic transverse momentum ($k_T$) dependence in nuclear shadowing. These contributions are also present in hadron and jet production in $p$-$A$ collisions, where convolution with final state interactions obfuscates the actual effect of initial state scattering.
| Is this an experimental talk? | No |
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| Is this on behalf of a collaboration? | No |
| Are you willing to present as a poster if it is not selected for oral presentation? | No |