Drell-Yan process at forward rapidity at the LHC

Abstract

We analyze the Drell-Yan lepton pair production at forward rapidity at the Large Hadron Collider. Using the dipole framework for the computation of the cross section we find a significant suppression in comparison to the collinear factorization formula due to saturation effects in the dipole cross section. We develop a twist expansion in powers of Q 2 s /M 2 where Qs is the saturation scale and M the invariant mass of the produced lepton pair. For the nominal LHC energy the leading twist description is sufficient down to masses of 6 GeV. Below that value the higher twist terms give a significant contribution.

Twist expansion of Drell-Yan structure functions in color dipole approach

“…All structure functions T i may be decomposed into twist-series using the Operator Product Expansion (OPE), in which the leading twist-two contributions may be computed using standard parton densities and the (unknown) higher twist terms are suppressed by negative powers of the process hard scale. This power suppression, however, may be compensated in the region of moderate scale and very small-x by rapidly growing higher twist matrix elements, so that the higher twist contributions are expected to become important below µ 2 ∼ 30 GeV 2 [7]. 1 Hence the four DY structure functions carry enriched information on higher twists: the higher twist hadronic matrix elements are coupled to four different coefficient functions.…”

“…[24], further developed in ref. [25] and then applied to the total forward DY cross-section [7] and diffractive DIS [26]. In this framework the twist contributions are related to complex singularities (poles or branch points) in the Mellin plane.…”

“…In this framework the twist contributions are related to complex singularities (poles or branch points) in the Mellin plane. We follow the technique of the total DY cross-section twist decomposition proposed in [7], but extend the results to differential cross-sections in the lepton angles and the DY pair transverse momentum q T . In more detail we compute the Mellin transforms of all the forward DY impact factors at given q T and combine the results with the color dipole cross-section to get q T -dependent twist decomposition of all the four DY structure functions.…”

“…This may be done with a technique described in refs. [7,24,25], based on analytic structure in s of the Mellin representation. One closes the contour C of the inverse Mellin transform (3.30) with a semicircle at complex infinity for Re s > 0.…”

“…The dipole cross-section is constrained by the HERA data, and the fit of results may be applied to predict the DY cross-sections including the higher twist effects [7,[17][18][19][20][21][22], see also ref. [23].…”