2,090 research outputs found
Fitting Parton Distribution Data with Multiplicative Normalization Uncertainties
We consider the generic problem of performing a global fit to many
independent data sets each with a different overall multiplicative
normalization uncertainty. We show that the methods in common use to treat
multiplicative uncertainties lead to systematic biases. We develop a method
which is unbiased, based on a self--consistent iterative procedure. We
demonstrate the use of this method by applying it to the determination of
parton distribution functions with the NNPDF methodology, which uses a Monte
Carlo method for uncertainty estimation.Comment: 33 pages, 5 figures: published versio
The treatment of the infrared region in perturbative QCD
We discuss the contribution coming from the infrared region to NLO matrix
elements and/or coefficient functions of hard QCD processes. Strictly speaking,
this contribution is not known theoretically, since it is beyond perturbative
QCD. For DGLAP evolution all the infrared contributions are collected in the
phenomenological input parton distribution functions (PDFs), at some relatively
low scale Q_0; functions which are obtained from a fit to the `global' data.
However dimensional regularization sometimes produces a non-zero result coming
from the infrared region. Instead of this conventional regularization
treatment, we argue that the proper procedure is to first subtract from the NLO
matrix element the contribution already generated at the same order in \alpha_s
by the LO DGLAP splitting function convoluted with the LO matrix element. This
prescription eliminates the logarithmic infrared divergence, giving a
well-defined result which is consistent with the original idea that everything
below Q_0 is collected in the PDF input. We quantify the difference between the
proposed treatment and the conventional approach using low-mass Drell-Yan
production and deep inelastic electron-proton scattering as examples; and
discuss the potential impact on the `global' PDF analyses. We present arguments
to show that the difference cannot be regarded as simply the use of an
alternative factorization scheme.Comment: 15 pages, 5 figures, title changed, text considerably modified to
improve presentation, and discussion section enlarge
Meaningful characterisation of perturbative theoretical uncertainties
We consider the problem of assigning a meaningful degree of belief to
uncertainty estimates of perturbative series. We analyse the assumptions which
are implicit in the conventional estimates made using renormalisation scale
variations. We then formulate a Bayesian model that, given equivalent initial
hypotheses, allows one to characterise a perturbative theoretical uncertainty
in a rigorous way in terms of a credibility interval for the remainder of the
series. We compare its outcome to the conventional uncertainty estimates in the
simple case of the calculation of QCD corrections to the e+e- -> hadrons
process. We find comparable results, but with important conceptual differences.
This work represents a first step in the direction of a more comprehensive and
rigorous handling of theoretical uncertainties in perturbative calculations
used in high energy phenomenology.Comment: 28 pages, 5 figures. Language modified in order to make it more
'bayesian'. No change in results. Version published in JHE
Cross Section Ratios between different CM energies at the LHC: opportunities for precision measurements and BSM sensitivity
The staged increase of the LHC beam energy provides a new class of
interesting observables, namely ratios and double ratios of cross sections of
various hard processes. The large degree of correlation of theoretical
systematics in the cross section calculations at different energies leads to
highly precise predictions for such ratios. We present in this letter few
examples of such ratios, and discuss their possible implications, both in terms
of opportunities for precision measurements and in terms of sensitivity to
Beyond the Standard Model dynamics.Comment: 19 pages, 9 figure
Constraints on the gluon PDF from top quark pair production at hadron colliders
Using the recently derived NNLO cross sections \cite{Czakon:2013goa}, we
provide NNLO+NNLL theoretical predictions for top quark pair production based
on all the available NNLO PDF sets, and compare them with the most precise LHC
and Tevatron data. In this comparison we study in detail the PDF uncertainty
and the scale, and dependence of the theoretical predictions
for each PDF set. Next, we observe that top quark pair production provides a
powerful direct constraint on the gluon PDF at large , and include Tevatron
and LHC top pair data consistently into a global NNLO PDF fit. We then explore
the phenomenological consequences of the reduced gluon PDF uncertainties, by
showing how they can improve predictions for Beyond the Standard Model
processes at the LHC. Finally, we update to full NNLO+NNLL the theoretical
predictions for the ratio of top quark cross sections between different LHC
center of mass energies, as well as the cross sections for hypothetical heavy
fourth-generation quark production at the LHC.Comment: 29 pages, 13 figures, accepted for publication in JHE
Theoretical Uncertainties in Electroweak Boson Production Cross Sections at 7, 10, and 14 TeV at the LHC
We present an updated study of the systematic errors in the measurements of
the electroweak boson cross-sections at the LHC for various experimental cuts
for a center of mass energy of 7, 10 and 14 TeV. The size of both electroweak
and NNLO QCD contributions are estimated, together with the systematic error
from the parton distributions. The effects of new versions of the MSTW, CTEQ,
and NNPDF PDFs are considered.Comment: PDFLatex with JHEP3.cls. 22 pages, 43 figures. Version 2 adds the
CT10W PDF set to analysis and updates the final systematic error table and
conclusions, plus several citations and minor wording changes. Version 3 adds
some references on electroweak and mixed QED/QCD corrections. Version 4 adds
more references and acknowledgement
The QCD Coupling Constant
This paper presents a summary of the current status of determinations of the
strong coupling constant alpha_s. A detailed description of the definition,
scale dependence and inherent theoretical ambiguities is given. The various
physical processes that can be used to determine alpha_s are reviewed and
attention is given to the uncertainties, both theoretical and experimental.Comment: 56 page
Neural Network Parameterizations of Electromagnetic Nucleon Form Factors
The electromagnetic nucleon form-factors data are studied with artificial
feed forward neural networks. As a result the unbiased model-independent
form-factor parametrizations are evaluated together with uncertainties. The
Bayesian approach for the neural networks is adapted for chi2 error-like
function and applied to the data analysis. The sequence of the feed forward
neural networks with one hidden layer of units is considered. The given neural
network represents a particular form-factor parametrization. The so-called
evidence (the measure of how much the data favor given statistical model) is
computed with the Bayesian framework and it is used to determine the best form
factor parametrization.Comment: The revised version is divided into 4 sections. The discussion of the
prior assumptions is added. The manuscript contains 4 new figures and 2 new
tables (32 pages, 15 figures, 2 tables
Inclusive top-pair production phenomenology with TOPIXS
We discuss various aspects of inclusive top-quark pair production based on
TOPIXS, a new, flexible program that computes the production cross section at
the Tevatron and LHC at next-to-next-to-leading logarithmic accuracy in soft
and Coulomb resummation, including bound-state effects and the complete
next-to-next-to-leading order result in the q-qbar channel, which has recently
become available. We present the calculation of the top-pair cross section in
pp collisions at 8 TeV centre-of-mass energy, as well as the cross sections for
hypothetical heavy quarks in extensions of the standard model. The dependence
on the parton distribution input is studied. Further we investigate the impact
of LHC top cross section measurements at sqrt(s)=7 TeV on global fits of the
gluon distribution using the NNPDF re-weighting method.Comment: 27 pages, 5 figures; v2: corrected typos in Eqs. (2.8) and (6.2) and
the text, added footnote on page 4, matches published versio
aMCfast: automation of fast NLO computations for PDF fits
We present the interface between MadGraph5 aMC@NLO, a self-contained program that calculates cross sections up to next-to-leading order accuracy in an automated manner, and APPLgrid, a code that parametrises such cross sections in the form of look-up tables which can be used for the fast computations needed in the context of PDF fits. The main characteristic of this interface, which we dub aMCfast, is its being fully automated as well, which removes the need to extract manually the process-specific information for additional physics processes, as is the case with other matrix-element calculators, and renders it straightforward to include any new process in the PDF fits. We demonstrate this by studying several cases which are easily measured at the LHC, have a good constraining power on PDFs, and some of which were previously unavailable in the form of a fast interface
- âŠ