Parton distributions for LO generators

We present a study of the results obtained combining LO partonic matrix elements with either LO or NLO partons distributions. These are compared to the best prediction using NLO for both matrix elements and parton distributions. The aim is to determine which parton distributions are most appropriate to use in those cases where only LO matrix elements are available, e.g. as in many Monte Carlo generators. Both LO and NLO parton distributions have flaws, sometimes serious, for some processes, so a modified optimal LO set is suggested. We investigate a wide variety of process, and the LO* pdf works at least as well as, and often better than, both LO and NLO pdfs in nearly all cases.The LO* pdf set is now available in the LHAPDF package

PDFs and Top Physics

I present the results from the recent PDF4LHC study, and the resulting new recommendation for combining PDFs sets for LHC calculations. In order to put this into context I summarise continuing developments in PDFs. This includes improvements and recent updates of particular PDF sets due to theory improvements and a variety of new data sets, including most of the up-todate LHC data. I will emphasise particular issues relevant for top physics

Recent progress in parton distributions and implications for LHC physics

I outline some of the most recent developments in the global fit to parton distributions performed by the MRST collaboration

Parton distributions

I discuss our current understanding of parton distributions. I begin with the underlying theoretical framework, and the way in which different data sets constrain different partons, highlighting recent developments. The methods of examining the uncertainties on the distributions and those physical quantities dependent on them is analyzed. Finally I look at the evidence that additional theoretical corrections beyond NLO perturbative QCD may be necessary, what type of corrections are indicated and the impact these may have on the uncertainties.</jats:p

MRST partons generated in a fixed-flavour scheme

We generate fixed three- and four-light-flavour sets of partons using MRST2004 partons as input. We show that it is important to set nf = 3 in the strong coupling, as well as in the splitting and coefficient functions, in order to obtain a consistent set of fixed-flavour partons. We compare the description of data using partons in both variableand fixed-flavour-number-schemes

The role of F-L(x, Q(2)) in parton analyses

We investigate the effect of the structure function FL in global parton analyses of deep inelastic and related hard scattering data. We perform NLO and NNLO analyses which include the reduced cross section HERA data at high y, as well as earlier direct measurements of FL. We find that the NNLO analysis gives a better description of FL at low x than that performed at NLO. Nevertheless the data show evidence of the need of further contributions to FL, which may be of higher-twist origin. We study such corrections both phenomenologically and theoretically via a renormalon approach. The higher-twist corrections extracted from a successful fit to the data are in general agreement with the theoretical expectations, but there is still room for alternative theoretical contributions, particularly at low x and Q2. The importance of future measurements of FL is emphasized

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

Theoretical Procedures and the effect on PDFs and α_S(M_Z^2)

I consider the effect on partons distribution functions (PDFs) of changes in the theoretical procedure used in a PDF fit. I consider using the 3-flavour fixed flavour number scheme instead of the standard general mass variable flavour number scheme used in the MSTW analysis. This results in the light quarks increasing at most x values, the gluon distribution becoming softer at high values of x and larger at small x, and the coupling constant \alpha_S(M_Z^2) falling, particularly at NNLO. The fit quality also deteriorates.I also consider lowering the kinematic cut on W² for DIS data and introducing higher twist terms which are fit to data. This results in much smaller effects on both PDFs and \alpha_S(M_Z^2), with changes generally smaller than uncertainties, except for quarks at very high x. I show that the fixed flavour scheme and variable flavour scheme structure functions differ significantly for x∼0.01 at high Q². I demonstrate that in the fixed flavour scheme there is a slow convergence of large logarithmic terms of the form (\alpha_S\ln(Q^2/m_c^2))^n in this regime. I conclude that some major differences in PDF sets are largely due to the choice of flavour scheme used

Questions on uncertainties in parton distributions

A discussion is presented of the manner in which uncertainties in parton distributions and related quantities are determined. One of the central problems is the criteria used to judge what variation of the parameters describing a set of partons is acceptable within the context of a global fit. Various ways of addressing this question are outlined