36 research outputs found
A Mathematica interface to NNPDFs
We present a Mathematica interface for handling the parton distribution
functions of the NNDPF Collaboration, available from the NNPDF hepforge website
http://nnpdf.hepforge.org/. As a case study we briefly summarise the first PDF
set which includes all relevant LHC data, NNPDF2.3, and demonstrate the use of
our new Mathematica interface.Comment: 4 pages, 5 figures, to appear in the proceedings of the 16th
International Conference in Quantum Chromodynamics (QCD2012), Montpellier
July 201
APFELgrid: a high performance tool for parton density determinations
We present a new software package designed to reduce the computational burden
of hadron collider measurements in Parton Distribution Function (PDF) fits. The
APFELgrid package converts interpolated weight tables provided by APPLgrid
files into a more efficient format for PDF fitting by the combination with PDF
and evolution factors provided by APFEL. This combination
significantly reduces the number of operations required to perform the
calculation of hadronic observables in PDF fits and simplifies the structure of
the calculation into a readily optimised scalar product. We demonstrate that
our technique can lead to a substantial speed improvement when compared to
existing methods without any reduction in numerical accuracy.Comment: 13 pages, 2 figures. Submitted to CPC. Code available from
https://github.com/nhartland/APFELgri
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
A determination of the fragmentation functions of pions, kaons, and protons with faithful uncertainties
We present NNFF1.0, a new determination of the fragmentation functions (FFs)
of charged pions, charged kaons, and protons/antiprotons from an analysis of
single-inclusive hadron production data in electron-positron annihilation. This
determination, performed at leading, next-to-leading, and
next-to-next-to-leading order in perturbative QCD, is based on the NNPDF
methodology, a fitting framework designed to provide a statistically sound
representation of FF uncertainties and to minimise any procedural bias. We
discuss novel aspects of the methodology used in this analysis, namely an
optimised parametrisation of FFs and a more efficient minimisation
strategy, and validate the FF fitting procedure by means of closure tests. We
then present the NNFF1.0 sets, and discuss their fit quality, their
perturbative convergence, and their stability upon variations of the kinematic
cuts and the fitted dataset. We find that the systematic inclusion of
higher-order QCD corrections significantly improves the description of the
data, especially in the small- region. We compare the NNFF1.0 sets to other
recent sets of FFs, finding in general a reasonable agreement, but also
important differences. Together with existing sets of unpolarised and polarised
parton distribution functions (PDFs), FFs and PDFs are now available from a
common fitting framework for the first time.Comment: 50 pages, 22 figures, 5 table
Proton structure at the LHC
A determination of Parton Distribution Functions (PDFs) from a global fit to
a dataset including measurements from the LHC has been performed for the
first time. The determinations have been performed according to the NNPDF
methodology, leading to a fit relatively free of parametrisation bias and with an
accurate account of PDF uncertainty.
In this thesis the importance of QCD measurements at the LHC to PDF
extraction are discussed, and we summarise some of the technical difficulties in
their inclusion into PDF fits. A number of methods are presented that permit
the efficient inclusion of these observables into PDF determinations.
Firstly a Bayesian reweighting procedure taking advantage of the Monte Carlo
representation of PDF uncertainties in NNPDF sets is discussed. The utility of
the Bayesian reweighting method is demonstrated by a study of the impact of
early W production asymmetry measurements from ATLAS, CMS and LHCb
upon an earlier PDF set.
A package for the fast computation of observables in an automated NLO
framework is presented, providing an interface between Monte Carlo event
generators and NLO interpolation tools.
Finally, a new method of combining PDF evolution with interpolating codes
for hadronic observable computation is also described. This method largely
overcomes the computational difficulties in performing fast perturbative QCD
predictions for collider observables. The method has been applied to the
determination of PDFs from a global dataset including electroweak vector boson
production data from LHCb, ATLAS and CMS along with inclusive jet data from
ATLAS. The resulting set, NNPDF2.3 provides the most accurate determination
of parton distributions via the NNPDF methodology to date.
Finally, the method of closure testing is introduced, and the method is applied
to the study of the NNPDF methodology. A number of improvements are found in
the minimisation and stopping procedures, which are adopted for the development
of the next NNPDF release, NNPDF3.0. Alongside the sounder methodological
basis, the NNPDF3.0 PDF set will provide a determination based upon an
expanded datfits
Boosting Higgs pair production in the final state with multivariate techniques
The measurement of Higgs pair production will be a cornerstone of the LHC
program in the coming years. Double Higgs production provides a crucial window
upon the mechanism of electroweak symmetry breaking and has a unique
sensitivity to the Higgs trilinear coupling. We study the feasibility of a
measurement of Higgs pair production in the final state at
the LHC. Our analysis is based on a combination of traditional cut-based
methods with state-of-the-art multivariate techniques. We account for all
relevant backgrounds, including the contributions from light and charm jet
mis-identification, which are ultimately comparable in size to the irreducible
QCD background. We demonstrate the robustness of our analysis strategy in
a high pileup environment. For an integrated luminosity of
ab, a signal significance of is obtained,
indicating that the final state alone could allow for the
observation of double Higgs production at the High-Luminosity LHC.Comment: 47 pages, 22 figures. v2: updated references, added comparison of
post-MVA kinematic distributions. v3: matches published version in EPJ
MCgrid:projecting cross section calculations on grids
MCgrid is a software package that provides access to the APPLgrid
interpolation tool for Monte Carlo event generator codes, allowing for fast and
flexible variations of scales, coupling parameters and PDFs in cutting edge
leading and next-to-leading-order QCD calculations. This is achieved by
providing additional tools to the Rivet analysis system for the construction of
MCgrid enhanced Rivet analyses. The interface is based around a one-to-one
correspondence between a Rivet histogram class and a wrapper for an APPLgrid
interpolation grid. The Rivet system provides all of the analysis tools
required to project a Monte Carlo weight upon an observable bin, and the MCgrid
package provides the correct conversion of the event weight to an APPLgrid fill
call. MCgrid has been tested and designed for use with the SHERPA event
generator, however as with Rivet the package is suitable for use with any code
which can produce events in the HepMC event record format.Comment: 35 pages, 11 figure
A Monte Carlo global analysis of the Standard Model Effective Field Theory: the top quark sector
We present a novel framework for carrying out global analyses of the Standard
Model Effective Field Theory (SMEFT) at dimension-six: SMEFiT. This approach is
based on the Monte Carlo replica method for deriving a faithful estimate of the
experimental and theoretical uncertainties and enables one to construct the
probability distribution in the space of the SMEFT degrees of freedom. As a
proof of concept of the SMEFiT methodology, we present a first study of the
constraints on the SMEFT provided by top quark production measurements from the
LHC. Our analysis includes more than 30 independent measurements from 10
different processes at 8 and 13 TeV such as inclusive top-quark pair and
single-top production and the associated production of top quarks with weak
vector bosons and the Higgs boson. State-of-the-art theoretical calculations
are adopted both for the Standard Model and for the SMEFT contributions, where
in the latter case NLO QCD corrections are included for the majority of
processes. We derive bounds for the 34 degrees of freedom relevant for the
interpretation of the LHC top quark data and compare these bounds with
previously reported constraints. Our study illustrates the significant
potential of LHC precision measurements to constrain physics beyond the
Standard Model in a model-independent way, and paves the way towards a global
analysis of the SMEFT.Comment: 76 pages, 24 figures, version accepted for publication in JHE