Publications (12)0 Total impact
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ABSTRACT: We study several sources of theoretical uncertainty in the determination of
parton distributions (PDFs) which may affect current PDF sets used for
precision physics at the Large Hadron Collider, and explain discrepancies
between them. We consider in particular the use of fixed-flavor versus
variable-flavor number renormalization schemes, higher twist corrections, and
nuclear corrections. We perform our study in the framework of the NNPDF2.3
global PDF determination, by quantifying in each case the impact of different
theoretical assumptions on the output PDFs. We also study in each case the
implications for benchmark cross sections at the LHC. We find that the impact
in a global fit of a fixed-flavor number scheme is substantial, the impact of
higher twists is negligible, and the impact of nuclear corrections is moderate
and circumscribed.
03/2013;
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ABSTRACT: Heavy quark structure functions from HERA provide a direct handle on the
medium and small-x gluon PDF. In this contribution, we discuss ongoing progress
on the implementation of the FONLL General-Mass scheme with running heavy quark
masses, and of its benchmarking with the HOPPET and OpenQCDrad codes, and then
present the impact of the recently released combined HERA charm production
cross sections in the NNPDF2.3 analysis. We find that the combined charm data
contribute to constraining the gluon and quarks at small values of Bjorken-x.
12/2012;
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Richard D. Ball, Valerio Bertone,
Stefano Carrazza,
Christopher S. Deans,
Luigi Del Debbio,
Stefano Forte,
Alberto Guffanti,
Nathan P. Hartland,
Jose I. Latorre,
Juan Rojo,
Maria Ubiali
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ABSTRACT: 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.
07/2012;
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ABSTRACT: We determine the strong coupling alpha_s at NNLO in perturbative QCD using
the global dataset input to the NNPDF2.1 NNLO parton fit: data from neutral and
charged current deep-inelastic scattering, Drell-Yan, vector boson production
and inclusive jets. We find alpha_s(M_Z)=0.1173+- 0.0007 (stat), where the
statistical uncertainty comes from the underlying data and uncertainties due to
the analysis procedure are negligible. We show that the distribution of alpha_s
values preferred by different experiments in the global fit is statistically
consistent, without need for rescaling uncertainties by a "tolerance" factor.
We show that if deep-inelastic data only are used, the best-fit value of
alpha_s is somewhat lower, but consistent within one sigma with the global
determination. We estimate the dominant theoretical uncertainty, from higher
orders corrections, to be Delta alpha_s (pert) ~ 0.0009.
10/2011;
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ABSTRACT: We discuss the statistical properties of parton distributions within the
framework of the NNPDF methodology. We present various tests of statistical
consistency, in particular that the distribution of results does not depend on
the underlying parametrization and that it behaves according to Bayes' theorem
upon the addition of new data. We then study the dependence of results on
consistent or inconsistent datasets and present tools to assess the consistency
of new data. Finally we estimate the relative size of the PDF uncertainty due
to data uncertainties, and that due to the need to infer a functional form from
a finite set of data.
10/2011;
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NNPDF Collaboration,
Richard D. Ball, Valerio Bertone,
Francesco Cerutti,
Luigi Del Debbio,
Stefano Forte,
Alberto Guffanti,
Nathan P. Hartland,
Jose I. Latorre,
Juan Rojo,
Maria Ubiali
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ABSTRACT: We develop in more detail our reweighting method for incorporating new
datasets in parton fits based on a Monte Carlo representation of PDFs. After
revisiting the derivation of the reweighting formula, we show how to construct
an unweighted PDF replica set which is statistically equivalent to a given
reweighted set. We then use reweighting followed by unweighting to test the
consistency of the method, specifically by verifying that results do not depend
on the order in which new data are included in the fit via reweighting. We
apply the reweighting method to study the impact of LHC W lepton asymmetry data
on the NNPDF2.1 set. We show how these data reduce the PDF uncertainties of
light quarks in the medium and small x region, providing the first solid
constraints on PDFs from LHC data.
08/2011;
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ABSTRACT: We present a determination of the parton distributions of the nucleon from a
global set of hard scattering data using the NNPDF methodology at LO and NNLO
in perturbative QCD, thereby generalizing to these orders the NNPDF2.1 NLO
parton set. Heavy quark masses are included using the so-called FONLL method,
which is benchmarked here at NNLO. We demonstrate the stability of PDFs upon
inclusion of NNLO corrections, and we investigate the convergence of the
perturbative expansion by comparing LO, NLO and NNLO results. We show that the
momentum sum rule can be tested with increasing accuracy at LO, NLO and NNLO.
We discuss the impact of NNLO corrections on collider phenomenology,
specifically by comparing to recent LHC data. We present PDF determinations
using a range of values of alpha_s, m_c and m_b. We also present PDF
determinations based on various subsets of the global dataset, show that they
generally lead to less accurate phenomenology, and discuss the possibility of
future PDF determinations based on collider data only.
07/2011;
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ABSTRACT: We determine the strong coupling alpha_s from a next-to-leading order
analysis of processes used for the NNPDF2.1 parton determination, which
includes data from neutral and charged current deep-inelastic scattering,
Drell-Yan and inclusive jet production. We find alpha_s(M_Z)=0.1191+-0.0006
(exp), where the uncertainty includes all statistical and systematic
experimental uncertainties, but not purely theoretical uncertainties. We study
the dependence of the results on the dataset, by providing further
determinations based respectively on deep-inelastic data only, and on HERA data
only. The deep-inelastic fit gives the consistent result
alpha_s(M_Z)=0.1177+-0.0009(exp), but the result of the HERA-only fit is only
marginally consistent. We provide evidence that individual data subsets can
have runaway directions due to poorly determined PDFs, thus suggesting that a
global dataset is necessary for a reliable determination.
03/2011;
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ABSTRACT: We discuss the impact of the treatment of NMC structure function data on
parton distributions in the context of the NNPDF2.1 global PDF determination at
NLO and NNLO. We show that the way these data are treated, and even their
complete removal, has no effect on parton distributions at NLO, and at NNLO has
an effect which is below one sigma. In particular, the Higgs production
cross-section in the gluon fusion channel is very stable.
02/2011;
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ABSTRACT: We present a determination of the parton distributions of the nucleon from a
global set of hard scattering data using the NNPDF methodology including heavy
quark mass effects: NNPDF2.1. In comparison to the previous NNPDF2.0 parton
determination, the dataset is enlarged to include deep--inelastic charm
structure function data. We implement the FONLL-A general-mass scheme in the
FastKernel framework and assess its accuracy by comparison to the Les Houches
heavy quark benchmarks. We discuss the impact on parton distributions of the
treatment of the heavy quark masses, and we provide a determination of the
uncertainty in the parton distributions due to uncertainty in the masses. We
assess the impact of these uncertainties on LHC observables by providing parton
sets with different values of the charm and bottom quark masses. Finally, we
construct and discuss parton sets with a fixed number of flavours.
01/2011;
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Sergey Alekhin,
Simone Alioli,
Richard D. Ball, Valerio Bertone,
Johannes Blumlein,
Michiel Botje,
Jon Butterworth,
Francesco Cerutti,
Amanda Cooper-Sarkar,
Albert De Roeck, [......],
Voica Radescu,
Juan Rojo,
Torbjorn Sjostrand,
W. J. Stirling,
Daniel Stump,
Robert S. Thorne,
Maria Ubiali,
Alessandro Vicini,
Graeme Watt,
C. -P. Yuan
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ABSTRACT: This document is intended as a study of benchmark cross sections at the LHC
(at 7 TeV) at NLO using modern parton distribution functions currently
available from the 6 PDF fitting groups that have participated in this
exercise. It also contains a succinct user guide to the computation of PDFs,
uncertainties and correlations using available PDF sets.
A companion note, also submitted to the archive, provides an interim summary
of the current recommendations of the PDF4LHC working group for the use of
parton distribution functions and of PDF uncertainties at the LHC, for cross
section and cross section uncertainty calculations.
01/2011;
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ABSTRACT: 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.
12/2010;
