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 qT . We approach the problem by deriving predictions based on standard collinear factorization, which are expected to be valid toward the high-qT end of the spectrum and to which any description of the spectrum at lower qT 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 qT . We discuss additional perturbative and possible non-perturbative effects that increase the predicted cross section, but not by a sufficient amount. PACS numbers: 12.38.Bx, 12.39.St, 13.85.Qk I. INTRODUCTIONThe 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]). 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 (see, e.g., [4,5] and references therein). DY processes also offer the possibility to access transverse momentum distributions (TMDs) [6][7][8][9][10][11][12][13][14][15], if the cross section is kept differential in the transverse momentum of the produced boson.Considering the invariant mass of the produced boson, Q, its transverse momentum, q T , and a typical QCD scale, Λ QCD , we can distinguish a region of "high transverse momentum" 1 where Λ QCD q T ∼ Q and a region of "low transverse momentum" where q T Q. In the first region, 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 α s . This calculation is nowadays possible even up to order α 3 s (NNLO) [17], but most of the phenomenology is carried out at order α 2 s (NLO) [18][19][20][21][22][23][24][25] or even only order α s (LO). In the low transverse momentum region, the cross section should be described in the framework of TMD factorization, which also incorporates the effects of the resummation of large logarithms in q T /Q. The all-order corrections dominating the cross section in this region are embodied in the so-called "W term" of the Collins-Soper-Sterman formalism [26]. The matching of the collinear formalism at high-q T with the TMD resummation at low-q T is usually performed through the introduction of the so-called "Y term", i.e., the difference of the fixed-order perturbative result and the asymptotic expansion of the resummed result. In the low-q T region, the asymptotic piece and the fixed-order one ideally cancel each other, leaving only the W term. In the high-q T region, on the other hand, the cancellation takes pla...

We present a comprehensive comparison of the available experimental data for the Drell-Yan lepton angular coefficients λ and ν to calculations at leading and next-to-leading order of perturbative QCD. To obtain the next-to-leading order corrections, we make use of publicly available numerical codes that allow us to compute the Drell-Yan cross section at second order in perturbation theory and from which the contributions we need can be extracted. Our comparisons reveal that perturbative QCD is able to describe the experimental data overall rather well, especially at colliders, but also in the fixed-target regime. On the basis of the angular coefficients alone, there appears to be little (if any) convincing evidence for effects that go beyond fixed-order collinear factorized perturbation theory, although the presence of such effects is not ruled out.

We present a comprehensive comparison of the available experimental data for the Drell-Yan lepton angular coefficients λ and ν to calculations at leading and next-to-leading order of perturbative QCD. To obtain the next-to-leading order corrections, we make use of publicly available numerical codes that allow us to compute the Drell-Yan cross section at second order in perturbation theory and from which the contributions we need can be extracted. Our comparisons reveal that perturbative QCD is able to describe the experimental data overall rather well, especially at colliders, but also in the fixed-target regime. On the basis of the angular coefficients alone, there appears to be little (if any) convincing evidence for effects that go beyond fixed-order collinear factorized perturbation theory, although the presence of such effects is not ruled out.

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 based on 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.

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