Collinear and TMD parton densities from fits to precision DIS measurements in the parton branching method

Abstract

Collinear and transverse-momentum-dependent (TMD) parton densities are obtained from fits to precision measurements of deep-inelastic scattering (DIS) cross sections at HERA. The parton densities are evolved by Dokshitzer-Gribov-Lipatov-Altarelli-Parisi evolution with next-to-leading-order (NLO) splitting functions using the parton branching method, allowing one to determine simultaneously collinear and TMD densities for all flavors over a wide range in x, μ 2 and k t , relevant for predictions at the LHC. The DIS cross section is computed from the parton densities using perturbative NLO coefficient functions. Parton densities satisfying angular ordering conditions are presented. Two sets of parton densities are obtained, differing in the renormalization scale choice for the argument in the strong coupling α s. This is taken to be either the evolution scale μ or the transverse momentum q t. While both choices yield similarly good χ 2 values for the fit to DIS measurements, the gluon density especially turns out to differ between the two sets. The TMD densities are used to predict the transverse momentum spectrum of Z bosons at the LHC.

Production of Z bosons in the parton branching method

“…The PB method is described in detail in Refs. [33,34,38]. In particular, the TMD parton density distributions are obtained from convoluting the perturbative evolution kernel K with the non-perturbative starting distribution A 0,b (x , k 2 t,0 , µ 2 0 ), as described in [38],…”

“…Collinear and TMD parton distributions were obtained in [38] from fits of the parameters of the starting distribution f 0,b (x, µ 2 0 ) to the inclusive-DIS precision measurements from HERA, after QCD evolution and convolution with the coefficient functions at NLO. Two different sets of parton distributions were obtained: Set 1, which corresponds at collinear level to HERAPDF2.0NLO [43], and Set 2, which differs by the choice of the scale used in the running coupling α s , namely, it uses the transverse momentum (instead of the evolution scale), corresponding to the angular-ordering approach.…”

“…In Ref. [38] leading order (LO) matrix elements for the calculation of Z production in pp collisions at the LHC are used. A rather good description was obtained for the small-p T region.…”

“…In this article we apply the PB approach to Z-boson DY production at the LHC. We use the TMDs obtained within the PB method and fitted [38] to inclusive deep-inelastic scattering (DIS) precision data from HERA together with a next-to-leading-order (NLO) calculation of the DY process. All parameters are kept as obtained from the fits to HERA DIS data, and no further adjustment is performed.…”

Drell-Yan plus jet production and BFKL evolution

“…The Monte Carlo implementation of the all loop CCFM branching scheme was done in [45][46][47]. Further extensions of this implementation and subsequent analyses were presented in [48][49][50][51].…”