Sprecher
Beschreibung
An enhancement in the transverse momentum distribution of Drell-Yan pairs was recently observed in $p$-$A$ 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.
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 $q_T \sim Q$ the mass of the dilepton pair, using factorized Transverse Momentum Dependent Parton Distribution Functions (TMDPDFs).
While the case of arbitrary $q_T$, at Next-to-Leading Order and Next-to-Leading-Twist (NLO-NLT), yields hundreds of Feynman diagrams, we show that in the limit where $q_T \ll \Lambda_{QCD}$, only about 26 diagrams dominate.
None of these diagrams involve off-forward distributions, allowing for straightforward 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 $x$ and transverse momentum $k_T$ dependent nuclear shadowing.
[1] - Phys.Rev.D 58 (1998) 036001, Nucl. Phys. B 582 (2000) 537-570, Nucl. Phys. A 879 (2012) 77-106.