176 research outputs found

### PDFs, $\alpha_s$, and quark masses from global fits

The strong coupling constant $\alpha_s$ and the heavy-quark masses, $m_c$,
$m_b$, $m_t$ are extracted simultaneosly with the parton distribution functions
(PDFs) in the updated ABM12 fit including recent data from CERN-SPS, HERA,
Tevatron, and the LHC. The values of \begin{eqnarray} \nonumber
\alpha_s(M_Z)&=&0.1147\pm0.0008~({\rm exp.)},\\ \nonumber m_c(m_c)&=&1.252\pm
0.018~({\rm exp.})~{\rm GeV},\\ \nonumber m_b(m_b)&=&3.83\pm0.12~({\rm
exp.})~{\rm GeV},\\ \nonumber m_t(m_t)&=&160.9\pm1.1~({\rm exp.})~{\rm GeV}
\end{eqnarray} are obtained with the $\overline{MS}$ heavy-quark mass
definition being employed throughout the analysis.Comment: 7 pages, 4 figures; preprint number correcte

### Iso-spin asymmetry of quark distributions and implications for single top-quark production at the LHC

We present an improved determination of the up- and down-quark distributions
in the proton using recent data on charged lepton asymmetries from $W^\pm$
gauge-boson production at the LHC and Tevatron. The analysis is performed in
the framework of a global fit of parton distribution functions. The fit results
are consistent with a non-zero iso-spin asymmetry of the sea, $x(\bar d - \bar
u)$, at small values of Bjorken $x\sim 10^{-4}$ indicating a delayed onset of
the Regge asymptotics of a vanishing $(\bar d - \bar u)$-asymmetry at
small-$x$. We compare with up- and down-quark distributions available in the
literature and provide accurate predictions for the production of single
top-quarks at the LHC, a process which can serve as a standard candle for the
light quark flavor content of the proton.Comment: 21 pages, 13 figure

### HECTOR 1.00 - A program for the calculation of QED, QCD and electroweak corrections to ep and lN deep inelastic neutral and charged current scattering

A description of the Fortran program HECTOR for a variety of semi-analytical
calculations of radiative QED, QCD, and electroweak corrections to the
double-differential cross sections of NC and CC deep inelastic charged lepton
proton (or lepton deuteron) scattering is presented. HECTOR originates from the
substantially improved and extended earlier programs HELIOS and TERAD91. It is
mainly intended for applications at HERA or LEPxLHC, but may be used also for
muon scattering in fixed target experiments. The QED corrections may be
calculated in different sets of variables: leptonic, hadronic, mixed,
Jaquet-Blondel, double angle etc. Besides the leading logarithmic approximation
up to order O(alpha^2), exact order O(alpha) corrections and inclusive soft
photon exponentiation are taken into account. The photoproduction region is
also covered.Comment: 74 pages, LaTex, 14 figures, 7 tables, a uuencoded file containing
the latex file and figures is available from: http://www.ifh.de/theory/ or on
request from e-mail: [email protected]

### Heavy-flavor PDF evolution and variable-flavor number scheme uncertainties in deep-inelastic scattering

We consider a detailed account on the construction of the heavy-quark parton
distribution functions for charm and bottom, starting from $n_f=3$ light
flavors in the fixed-flavor number (FFN) scheme and by using the standard
decoupling relations for heavy quarks in QCD. We also account for two-mass
effects. Furthermore, different implementations of the variable-flavor-number
(VFN) scheme in deep-inelastic scattering (DIS) are studied, with the
particular focus on the resummation of large logarithms in $Q^2/m_h^2$, the
ratio the virtuality of the exchanged gauge-boson $Q^2$ to the heavy-quark mass
squared $m_h^2$. A little impact of resummation effects if found in the
kinematic range of the existing data on the DIS charm-quark production so that
they can be described very well within the FFN scheme. Finally, we study the
theoretical uncertainties associated to the VFN scheme, which manifest
predominantly at small $Q^2$.Comment: 20 pages, 12 figure

### Heavy flavour contributions to the deep inelastic sum rules

We have calculated the first and second order corrections to several deep
inelastic sum rules which are due to heavy flavour contributions. A comparison
is made with the existing perturbation series which has been computed up to
third order for massless quarks only. In general it turns out that the effects
of heavy quarks are very small except when $Q \sim m$ or $Q \gg m$. Here $Q$
and $m$ denote the virtual mass of the vector boson and the mass of the heavy
quark, respectively. For $Q \gg m$ the radiative corrections reveal large
logarithms of the type $\ln Q^2/m^2$ which have to be absorbed in the running
coupling constant so that the number of light flavours $n_f$ is enhanced by one
unit. However this has to happen at much larger values of Q i.e. $Q \sim 6.5~
m$ than one usually assumes for the flavour thresholds which appear in the
running coupling constant. An alternative description for the heavy flavour
dependence of the running coupling constant in the context of the MOM-scheme is
discussed.Comment: 12 pages Latex, all compressed by uufile

### Resummation of large logarithms in the VFN scheme for DIS heavy-quark production

We consider the impact of the resummation of large logarithms, which appear
in the QCD evolution of the heavy-quark distributions, on the phenomenology of
deep-inelastic heavy-quark production. The heavy-quark PDFs are derived using
the fixed-order matching conditions as a boundary for the QCD evolution and the
result obtained is compared to the distributions defined by the matching
conditions at all scales. With such an approach, the effect of heavy-quark PDF
evolution is found to be sizable at LO and dramatically reduces at NLO. The
NNLO evolved distributions are not very different from the NLO ones at large
scales, however, show substantial differences at low virtualities, i.e. where
the additional large logarithms are numerically not important, while a mismatch
between the NLO accuracy of the matching conditions and the NNLO accuracy in
the evolution kernels causes a substantial excess in the heavy-quark
distributions. This excess propagates into the variable flavor number (VFN)
scheme predictions for the deep-inelastic structure functions and has to be
compensated by a decrease in the small-$x$ gluon distribution determined from
PDF fits based on the VFN scheme, which should be considered as a theoretical
uncertainty in VFN PDF fits and reaches $\sim 30\%$ for the small-$x$ gluon
distribution extracted from the data on deep-inelastic charm-quark production.Comment: 6 pages, 3 figures, proceedings of XXVII International Workshop on
Deep-Inelastic Scattering and Related Subjects (DIS2018), 8-12 Apr, Torino
(Italy

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