47 research outputs found
Specialized minimal PDFs for optimized LHC calculations
We present a methodology for the construction of parton distribution
functions (PDFs) designed to provide an accurate representation of PDF
uncertainties for specific processes or classes of processes with a minimal
number of PDF error sets: specialized minimal PDF sets, or SM-PDFs. We
construct these SM-PDFs in such a way that sets corresponding to different
input processes can be combined without losing information, specifically on
their correlations, and that they are robust upon smooth variations of the
kinematic cuts. The proposed strategy never discards information, so that the
SM-PDF sets can be enlarged by the addition of new processes, until the prior
PDF set is eventually recovered for a large enough set of processes. We
illustrate the method by producing SM-PDFs tailored to Higgs, top quark pair,
and electroweak gauge boson physics, and determine that, when the PDF4LHC15
combined set is used as the prior, around 11, 4 and 11 Hessian eigenvectors
respectively are enough to fully describe the corresponding processes.Comment: 31 pages, 13 figures. Final version published in Eur. Phys. J. C:
typos corrected, discussion of perturbative orders added in sects 2.3 and 3.
Regularising experimental correlations in LHC data: theory and application to a global analysis of parton distributions
An Unbiased Hessian Representation for Monte Carlo PDFs
We develop a methodology for the construction of a Hessian representation of
Monte Carlo sets of parton distributions, based on the use of a subset of the
Monte Carlo PDF replicas as an unbiased linear basis, and of a genetic
algorithm for the determination of the optimal basis. We validate the
methodology by first showing that it faithfully reproduces a native Monte Carlo
PDF set (NNPDF3.0), and then, that if applied to Hessian PDF set (MMHT14) which
was transformed into a Monte Carlo set, it gives back the starting PDFs with
minimal information loss. We then show that, when applied to a large Monte
Carlo PDF set obtained as combination of several underlying sets, the
methodology leads to a Hessian representation in terms of a rather smaller set
of parameters (CMC-H PDFs), thereby providing an alternative implementation of
the recently suggested Meta-PDF idea and a Hessian version of the recently
suggested PDF compression algorithm (CMC-PDFs). The mc2hessian conversion code
is made publicly available together with (through LHAPDF6) a Hessian
representations of the NNPDF3.0 set, and the CMC-H PDF set.Comment: 27 pages, 17 figures; v2: replaced plots in Fig.1 which had a
conflict with google chrome inline viewer; v3: final version, to be published
in EPJC; figs 5,10,11,13 updated with new (more accurate) choice of
parameter
Parton distributions with theory uncertainties: general formalism and first phenomenological studies: NNPDF Collaboration
Why cannot be determined from hadronic processes without simultaneously determining the parton distributions
AbstractWe show that any determination of the strong coupling αs from a process which depends on parton distributions, such as hadronic processes or deep-inelastic scattering, generally does not lead to a correct result unless the parton distributions (PDFs) are determined simultaneously along with αs. We establish the result by first showing an explicit example, and then arguing that the example is representative of a generic situation which we explain using models for the shape of equal χ2 contours in the joint space of αs and the PDF parameters.</jats:p
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Why α s cannot be determined from hadronic processes without simultaneously determining the parton distributions
Abstract: We show that any determination of the strong coupling αs from a process which depends on parton distributions, such as hadronic processes or deep-inelastic scattering, generally does not lead to a correct result unless the parton distributions (PDFs) are determined simultaneously along with αs. We establish the result by first showing an explicit example, and then arguing that the example is representative of a generic situation which we explain using models for the shape of equal χ2 contours in the joint space of αs and the PDF parameters