151 research outputs found
MMHT2014 PDFs - HERA I+II data and other updates
We will cover a number of topics: some clarifications on MMHT PDFs with varying quark
masses; a review of the fit to final HERA data, with consequences for MMHT PDFs, and a study
of the fit quality; a brief introduction to topics just starting investigation, mainly the inclusion of
new LHC data sets
An investigation of the αS and heavy quark mass dependence in the MSHT20 global PDF analysis
We investigate the MSHT20 global PDF sets, demonstrating the effects of varying the strong coupling αS(M2Z) and the masses of the charm and bottom quarks. We determine the preferred value, and accompanying uncertainties, when we allow αS(M2Z) to be a free parameter in the MSHT20 global analyses of deep-inelastic and related hard scattering data, at both NLO and NNLO in QCD perturbation theory. We also study the constraints on αS(M2Z) which come from the individual data sets in the global fit by repeating the NNLO and NLO global analyses at various fixed values of αS(M2Z), spanning the range αS(M2Z)=0.108 to 0.130 in units of 0.001. We make all resulting PDFs sets available. We find that the best fit values are αS(M2Z)=0.1203±0.0015 and 0.1174±0.0013 at NLO and NNLO respectively. We investigate the relationship between the variations in αS(M2Z) and the uncertainties on the PDFs, and illustrate this by calculating the cross sections for key processes at the LHC. We also perform fits where we allow the heavy quark masses mc and mb to vary away from their default values and make PDF sets available in steps of Δmc=0.05 GeV and Δmb=0.25 GeV, using the pole mass definition of the quark masses. As for varying αS(M2Z) values, we present the variation in the PDFs and in the predictions. We examine the comparison to data, particularly the HERA data on charm and bottom cross sections and note that our default values are very largely compatible with best fits to data. We provide PDF sets with 3 and 4 active quark flavours, as well as the standard value of 5 flavours
On the consistent use of scale variations in PDF fits and predictions
We present an investigation of the theoretical uncertainties in parton distribution functions (PDFs) due to missing higher-order corrections in the perturbative predictions used in the fit, and their relationship to the uncertainties in subsequent predictions made using the PDFs. We consider in particular the standard approach of factorization and renormalization scale variation, and derive general results for the consistent application of these at the PDF fit stage. To do this, we use the fact that a PDF fit may be recast in a physical basis, where the PDFs themselves are bypassed entirely, and one instead relates measured observables to predicted ones. In the case of factorization scale variation we find that in various situations there is a high degree of effective correlation between the variation in the fit and in predicted observables. In particular, including such a variation in both cases can lead to an exaggerated theoretical uncertainty. More generally, a careful treatment of this correlation appears mandatory, at least within the standard scale variation paradigm. For the renormalization scale, the situation is less straightforward, but again we highlight the potential for correlations between related processes in the fit and predictions to enter at the same level as between processes in the fit or prediction alone
Updates of the MMHT2014 PDFs
We briefly discuss some of the developments since the publication of the MMHT14 parton distributions. In particular we explore the impact of recent LHC data for W±, Z and tt¯production, and perform a preliminary new analysis including these data. In this re-fit (which we tentatively call ‘MMHT16’) there are few changes of significance in the central values of the PDFs, but some
data reduce the uncertainties, mainly in the strange and valence quark distributions. We find that an extended d¯− u¯ parametrization only leads to minor changes, with the difference going to zero as x → 0. We comment on the determination of the photon PDF
Updates of the MMHT2014 PDFs
We briefly discuss some of the developments since the publication of the MMHT14 parton distributions. In particular we explore the impact of recent LHC data for W±, Z and tt¯production, and perform a preliminary new analysis including these data. In this re-fit (which we tentatively call ‘MMHT16’) there are few changes of significance in the central values of the PDFs, but some
data reduce the uncertainties, mainly in the strange and valence quark distributions. We find that an extended d¯− u¯ parametrization only leads to minor changes, with the difference going to zero as x → 0. We comment on the determination of the photon PDF
Contact Manifolds, Contact Instantons, and Twistor Geometry
Recently, Kallen and Zabzine computed the partition function of a twisted
supersymmetric Yang-Mills theory on the five-dimensional sphere using
localisation techniques. Key to their construction is a five-dimensional
generalisation of the instanton equation to which they refer as the contact
instanton equation. Subject of this article is the twistor construction of this
equation when formulated on K-contact manifolds and the discussion of its
integrability properties. We also present certain extensions to higher
dimensions and supersymmetric generalisations.Comment: v3: 28 pages, clarifications and references added, version to appear
in JHE
Instantons and Killing spinors
We investigate instantons on manifolds with Killing spinors and their cones.
Examples of manifolds with Killing spinors include nearly Kaehler 6-manifolds,
nearly parallel G_2-manifolds in dimension 7, Sasaki-Einstein manifolds, and
3-Sasakian manifolds. We construct a connection on the tangent bundle over
these manifolds which solves the instanton equation, and also show that the
instanton equation implies the Yang-Mills equation, despite the presence of
torsion. We then construct instantons on the cones over these manifolds, and
lift them to solutions of heterotic supergravity. Amongst our solutions are new
instantons on even-dimensional Euclidean spaces, as well as the well-known
BPST, quaternionic and octonionic instantons.Comment: 40 pages, 2 figures v2: author email addresses and affiliations adde
Normal levels of p27Xic1 are necessary for somite segmentation and determining pronephric organ size
The Xenopus laevis cyclin dependent kinase inhibitor p27Xic1 has been shown to be involved in exit from the cell cycle and differentiation of cells into a quiescent state in the nervous system, muscle tissue, heart and retina. We show that p27Xic1 is expressed in the developing kidney in the nephrostomal regions. Using over-expression and morpholino oligonucleotide (MO) knock-down approaches we show normal levels of p27Xic1 regulate pronephros organ size by regulating cell cycle exit. Knock-down of p27Xic1 expression using a MO prevented myogenesis, as previously reported; an effect that subsequently inhibits pronephrogenesis. Furthermore, we show that normal levels of p27Xic1 are required for somite segmentation also through its cell cycle control function. Finally, we provide evidence to suggest correct paraxial mesoderm segmentation is not necessary for pronephric induction in the intermediate mesoderm. These results indicate novel developmental roles for p27Xic1, and reveal its differentiation function is not universally utilised in all developing tissues
Yang-Mills instantons and dyons on homogeneous G_2-manifolds
We consider Lie G-valued Yang-Mills fields on the space R x G/H, where G/H is
a compact nearly K"ahler six-dimensional homogeneous space, and the manifold R
x G/H carries a G_2-structure. After imposing a general G-invariance condition,
Yang-Mills theory with torsion on R x G/H is reduced to Newtonian mechanics of
a particle moving in R^6, R^4 or R^2 under the influence of an inverted
double-well-type potential for the cases G/H = SU(3)/U(1)xU(1),
Sp(2)/Sp(1)xU(1) or G_2/SU(3), respectively. We analyze all critical points and
present analytical and numerical kink- and bounce-type solutions, which yield
G-invariant instanton configurations on those cosets. Periodic solutions on S^1
x G/H and dyons on iR x G/H are also given.Comment: 1+26 pages, 14 figures, 6 miniplot
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