4,913 research outputs found
Reweighting NNPDFs: the W lepton asymmetry
We present a method for incorporating the information contained in new
datasets into an existing set of parton distribution functions without the need
for refitting. The method involves reweighting the ensemble of parton densities
through the computation of the chi-square to the new dataset. We explain how
reweighting may be used to assess the impact of any new data or pseudodata on
parton densities and thus on their predictions. We show that the method works
by considering the addition of inclusive jet data to a DIS+DY fit, and
comparing to the refitted distribution. We then use reweighting to determine
the impact of recent high statistics lepton asymmetry data from the D0
experiment on the NNPDF2.0 parton set. We find that the D0 inclusive muon and
electron data are perfectly compatible with the rest of the data included in
the NNPDF2.0 analysis and impose additional constraints on the large-x d/u
ratio. The more exclusive D0 electron datasets are however inconsistent both
with the other datasets and among themselves, suggesting that here the
experimental uncertainties have been underestimated.Comment: 36 pages, 22 figures: errors in Eqns.12,36,37 corrected and parts of
Figs.1,6,10,13,15,19 replace
Parton distributions with threshold resummation
We construct a set of parton distribution functions (PDFs) in which
fixed-order NLO and NNLO calculations are supplemented with soft-gluon
(threshold) resummation up to NLL and NNLL accuracy respectively, suitable for
use in conjunction with any QCD calculation in which threshold resummation is
included at the level of partonic cross sections. These resummed PDF sets,
based on the NNPDF3.0 analysis, are extracted from deep-inelastic scattering,
Drell-Yan, and top quark pair production data, for which resummed calculations
can be consistently used. We find that, close to threshold, the inclusion of
resummed PDFs can partially compensate the enhancement in resummed matrix
elements, leading to resummed hadronic cross-sections closer to the fixed-order
calculation. On the other hand, far from threshold, resummed PDFs reduce to
their fixed-order counterparts. Our results demonstrate the need for a
consistent use of resummed PDFs in resummed calculations.Comment: 43 pages, 17 figures, accepted for publication in JHE
Parton distributions with LHC data
We present the first determination of parton distributions of the nucleon at
NLO and NNLO based on a global data set which includes LHC data: NNPDF2.3. Our
data set includes, besides the deep inelastic, Drell-Yan, gauge boson
production and jet data already used in previous global PDF determinations, all
the relevant LHC data for which experimental systematic uncertainties are
currently available: ATLAS and LHCb W and Z lepton rapidity distributions from
the 2010 run, CMS W electron asymmetry data from the 2011 run, and ATLAS
inclusive jet cross-sections from the 2010 run. We introduce an improved
implementation of the FastKernel method which allows us to fit to this extended
data set, and also to adopt a more effective minimization methodology. We
present the NNPDF2.3 PDF sets, and compare them to the NNPDF2.1 sets to assess
the impact of the LHC data. We find that all the LHC data are broadly
consistent with each other and with all the older data sets included in the
fit. We present predictions for various standard candle cross-sections, and
compare them to those obtained previously using NNPDF2.1, and specifically
discuss the impact of ATLAS electroweak data on the determination of the
strangeness fraction of the proton. We also present collider PDF sets,
constructed using only data from HERA, Tevatron and LHC, but find that this
data set is neither precise nor complete enough for a competitive PDF
determination.Comment: 56 pages, 30 figures. LHCb dataset updated, all tables and plots
recomputed accordingly (results essentially unchanged). Several typos
corrected, several small textual improvements and clarification
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
Hadamard matrices and 1-factorizations of complete graphs
We discuss 1-factorizations of complete graphs that âmatchâ a given Hadamard matrix. We prove the existence of these factorizations for two families of Hadamard matrices: Walsh matrices and certain Paley matrices
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