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

Update of MRST parton distributions.

We discuss the latest update of the MRST parton distributions in response to the most recent data. We discuss the areas where there are hints of difficulties in the global fit, and compare to some other updated sets of parton distributions, particularly CTEQ6. We briefly discuss the issue of uncertainties associated with partons

MRST global fit update.

We discuss the impact of the most recent data on the MRST global analysis - in particular the new high-ET jet data and their implications for the gluon and the new small x structure function data. In the light of these new data we also consider the uncertainty in predictions for physical quantities depending on parton distributions, concentrating on the W cross-section at hadron colliders

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

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

MRST partons and uncertainties.

We discuss uncertainties in the extraction of parton distributions from global analyses of DIS and related data. We present conservative sets of partons, at both NLO and NNLO, which are stable to x,Q2,W2 cuts on the data. We give the corresponding values of S(M2 Z) and the cross sections for W production at the Tevatron

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

Classification Criteria for Intermediate Uveitis, Non–Pars Planitis Type

Purpose: To determine classification criteria for intermediate uveitis, non-pars planitis type (IU- NPP, also known as undifferentiated intermediate uveitis) / Design: Machine learning of cases with IU-NPP and 4 other intermediate uveitides. / Methods: Cases of intermediate uveitides were collected in an informatics-designed preliminary database, and a final database was constructed of cases achieving supermajority agreement on the diagnosis, using formal consensus techniques. Cases were split into a training set and a validation set. Machine learning using multinomial logistic regression was used on the training set to determine a parsimonious set of criteria that minimized the misclassification rate among the intermediate uveitides. The resulting criteria were evaluated on the validation set. / Results: Five hundred eighty-nine of cases of intermediate uveitides, including 114 cases of IU-NPP, were evaluated by machine learning. The overall accuracy for intermediate uveitides was 99.8% in the training set and 99.3% in the validation set (95% confidence interval 96.1, 99.9). Key criteria for IU-NPP included unilateral or bilateral intermediate uveitis with neither 1) snowballs in the vitreous nor 2) snowbanks on the pars plana. Other key exclusions included: 1) multiple sclerosis, 2) sarcoidosis, and 3) syphilis. The misclassification rates for pars planitis were 0 % in the training set and 0% in the validation set, respectively. / Conclusions: The criteria for IU-NPP had a low misclassification rate and appeared to perform well enough for use in clinical and translational research