QCD evolution equations can be recast in terms of parton branching processes. We present a new numerical solution of the equations. We show that this parton-branching solution can be applied to ...analyze infrared contributions to evolution, order-by-order in the strong coupling αs, as a function of the soft-gluon resolution scale parameter. We examine the cases of transverse-momentum ordering and angular ordering. We illustrate that this approach can be used to treat distributions which depend both on longitudinal and on transverse momenta.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
A
bstract
We study parton-branching solutions of QCD evolution equations and present a method to construct both collinear and transverse momentum dependent (TMD) parton densities from this approach. ...We work with next-to-leading-order (NLO) accuracy in the strong coupling. Using the unitarity picture in terms of resolvable and non-resolvable branchings, we analyze the role of the soft-gluon resolution scale in the evolution equations. For longitudinal momentum distributions, we find agreement of our numerical calculations with existing evolution programs at the level of better than 1% over a range of five orders of magnitude both in evolution scale and in longitudinal momentum fraction. We make predictions for the evolution of transverse momentum distributions. We perform fits to the high-precision deep inelastic scattering (DIS) structure function measurements, and we present a set of NLO TMD distributions based on the parton branching approach.
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, μ2 and kt, 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 αs. This is taken to be either the evolution scale μ or the transverse momentum qt. While both choices yield similarly good χ2 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.
Full text
Available for:
CMK, CTK, FMFMET, IJS, NUK, PNG, UM
HERAFitter Alekhin, S.; Behnke, O.; Belov, P. ...
The European physical journal. C, Particles and fields,
07/2015, Volume:
75, Issue:
7
Journal Article
Peer reviewed
Open access
HERAFitter is an open-source package that provides a framework for the determination of the parton distribution functions (PDFs) of the proton and for many different kinds of analyses in Quantum ...Chromodynamics (QCD). It encodes results from a wide range of experimental measurements in lepton–proton deep inelastic scattering and proton–proton (proton–antiproton) collisions at hadron colliders. These are complemented with a variety of theoretical options for calculating PDF-dependent cross section predictions corresponding to the measurements. The framework covers a large number of the existing methods and schemes used for PDF determination. The data and theoretical predictions are brought together through numerous methodological options for carrying out PDF fits and plotting tools to help to visualise the results. While primarily based on the approach of collinear factorisation, HERAFitter also provides facilities for fits of dipole models and transverse-momentum dependent PDFs. The package can be used to study the impact of new precise measurements from hadron colliders. This paper describes the general structure of HERAFitter and its wide choice of options.
Full text
Available for:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Achieving the highest precision for theoretical predictions at the LHC requires the calculation of hard-scattering cross sections that include perturbative QCD corrections up to (N)NNLO and ...electroweak (EW) corrections up to NLO. Parton distribution functions (PDFs) need to be provided with matching accuracy, which in the case of QED effects involves introducing the photon parton distribution of the proton,
x
γ
(
x
,
Q
2
)
. In this work a determination of the photon PDF from fits to recent ATLAS measurements of high-mass Drell–Yan dilepton production at
s
=
8
TeV is presented. This analysis is based on the xFitter framework, and has required improvements both in the APFEL program, to account for NLO QED effects, and in the aMCfast interface to account for the photon-initiated contributions in the EW calculations within MadGraph5_aMC@NLO. The results are compared with other recent QED fits and determinations of the photon PDF, consistent results are found.
Full text
Available for:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We investigate the impact of displaced heavy-quark matching scales in a global fit. The heavy-quark matching scale
μ
m
determines at which energy scale
μ
the QCD theory transitions from
N
F
to
N
F
+
...1
in the variable flavor number scheme (VFNS) for the evolution of the parton distribution functions (PDFs) and strong coupling
α
S
(
μ
)
. We study the variation of the matching scales, and their impact on a global PDF fit of the combined HERA data. As the choice of the matching scale
μ
m
effectively is a choice of scheme, this represents a theoretical uncertainty; ideally, we would like to see minimal dependence on this parameter. For the transition across the charm quark (from
N
F
=
3
to 4), we find a large
μ
m
=
μ
c
dependence of the global fit
χ
2
at NLO, but this is significantly reduced at NNLO. For the transition across the bottom quark (from
N
F
=
4
to 5), we have a reduced
μ
m
=
μ
b
dependence of the
χ
2
at both NLO and NNLO as compared to the charm. This feature is now implemented in xFitter 2.0.0, an open source QCD fit framework.
Full text
Available for:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A study of the parameterization uncertainty at low Bjorken x⩽0.1 for the parton distribution functions of the proton is presented. The study is based on the HERA I combined data using a flexible ...parameterization form based on Chebyshev polynomials with and without an additional regularization constraint. The accuracy of the data allows to determine the gluon density in the kinematic range of 0.0005⩽x⩽0.05 with a small parameterization uncertainty. An additional regularization prior leads to a significantly reduced uncertainty for x⩽0.0005.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
We present a new measurement of the difference between the nucleon strange and antistrange quark distributions from dimuon events recorded by the NuTeV experiment at Fermilab. This analysis is the ...first to use a complete next to leading order QCD description of charm production from neutrino scattering. Dimuon events in neutrino deep inelastic scattering allow direct and independent study of the strange and antistrange content of the nucleon. We find a positive strange asymmetry with a significance of 1.6sigma. We also report a new measurement of the charm mass.
Full text
Available for:
CMK, CTK, FMFMET, IJS, NUK, PNG, UM
Sets of parton distribution functions (PDFs) of the proton are reported for the leading (LO), next-to-leading (NLO) and next-to-next-to-leading-order (NNLO) QCD calculations. The parton distribution ...functions are determined with the HERAFitter program using the data from the HERA experiments and preserving correlations between uncertainties for the LO, NLO and NNLO PDF sets. The sets are used to study cross-section ratios and their uncertainties when calculated at different orders in QCD. A reduction of the overall theoretical uncertainty is observed if correlations between the PDF sets are taken into account for the ratio of
W
W
di-boson to
Z
boson production cross sections at the LHC.
Full text
Available for:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
PDF
