Abstract
We study the Drell-Yan cross section differential with respect to the transverse momentum of the produced lepton pair. We consider data with moderate invariant mass Q of the lepton pair, between 4.5 GeV and 13.5 GeV, and similar (although slightly smaller) values of the transverse momentum q_T. We approach the problem by deriving predictions based on standard collinear factorization, which are expected to be valid toward the high-q_T end of the spectrum and to which any description of the spectrum at lower q_T using transverse-momentum dependent parton distributions ultimately needs to be matched. We find that the collinear framework predicts cross sections that in most cases are significantly below available data at high q_T. We discuss additional perturbative and possible non-perturbative effects that increase the predicted cross section, but not by a sufficient amount.
Highlights
The Drell-Yan (DY) process [1] is one of the main sources of information about the internal structure of the nucleon
Factorization theorems were first established for DY [3], and global extractions of parton distribution functions (PDFs) heavily rely on measurements of the DY cross section differential in the rapidity of the produced boson
We have shown that theoretical predictions based on fixed-order perturbation theory fail to describe Drell-Yan data from Fermilab and CERN ISR at large values qT ∼ Q of the transverse momentum of the lepton pair, the experimental cross sections being significantly larger than the theoretical ones
Summary
The Drell-Yan (DY) process [1] is one of the main sources of information about the internal structure of the nucleon (for a recent review, see [2]). The cross section should be well described by a collinear factorization framework in terms of collinear PDFs convoluted with a partonic hard scattering calculated up to a fixed order in αs. We investigate possible sources of uncertainty in the predictions based on collinear factorization, and two extensions of the collinear framework: the resummation of high-qT threshold logarithms, and transverse-momentum smearing. None of these appear to lead to a satisfactory agreement with the data. The discrepancies we report here arguably appear more serious since the calculation of the Drell-Yan cross section relies on the very well constrained PDFs, while SIDIS is sensitive to the comparably more poorly known fragmentation functions
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