Progress in Parton Distribution Functions and Implications for LHC

Parton distribution functions (pdfs) are an important ingredient for LHC phenomenology. Recent progress in determining pdfs from global analyses is reviewed, and some of the most important outstanding issues are highlighted. Particular attention is paid to the precision with which predictions for LHC ‘standard-candle’ cross sections can be made, and also to new information that LHC can provide on pdfs

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

Double parton distributions incorporating perturbative QCD evolution and momentum and quark number sum rules

It is anticipated that hard double parton scatterings will occur frequently in the collisions of the LHC, producing interesting signals and significant backgrounds to certain single scattering processes. For double scattering processes in which the same hard scale t = ln(Q2) is involved in both collisions, we require the double parton distributions (dPDFs) Dj1j2 h (x1, x2; t) in order to make theoretical predictions of their rates and properties. We describe the development of a new set of leading order dPDFs that represents an improvement on approaches used previously. First, we derive momentum and number sum rules that the dPDFs must satisfy. The fact that these must be obeyed at any scale is used to construct improved dPDFs at the input scale Q0, for a particular choice of input scale (Q20 = 1 GeV2) and corresponding single PDFs (the MSTW2008LO set). We then describe a novel program which uses a direct x−space method to numerically integrate the LO DGLAP equation for the dPDFs, and which may be used to evolve the input dPDFs to any other scale. This program has been used along with the improved input dPDFs to produce a set of publicly available dPDF grids covering the ranges 10−6 < x1 < 1, 10−6 < x2 < 1, and 1 < Q2 < 109 GeV2

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

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

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

Central exclusive chi (c) meson production at the Tevatron revisited

Motivated by the recent experimental observation of exclusive c events at the Tevatron, we revisit earlier studies of central exclusive scalar c0 meson production, before generalising the existing formalism to include c1 and c2 mesons. Although c0 production was previously assumed to be dominant, we find that the c1 and c2 rates for the experimentally considered c ! J/ ! μ+μ− decay process are in fact comparable to the c0 rate. We have developed a new Monte Carlo event generator, SuperCHIC, which models the central exclusive production of the three c states via this decay chain, and have explored possible ways of distinguishing them, given that their mass differences are not resolvable within the current experimental set-up. Although we find that the severity of current experimental cuts appears to preclude this, the acceptance does not change crucially between the three states and so our conclusions regarding the overall rates remain unchanged. This therefore raises the interesting possibility that exclusive c1 and c2 production has already been observed at the Tevatron

Semileptonic decays of the standard Higgs boson

The Higgs boson decay into a pair of real or virtual W bosons, with one of them decaying leptonically, is predicted within the Standard Model to have the largest branching fraction of all Higgs decays that involve an isolated electron or muon, for M_h > 120 GeV. We compute analytically the fully-differential width for this h -> l \nu jj decay at tree level, and then explore some multi-dimensional cuts that preserve the region of large signal. Future searches for semileptonic decays at the Tevatron and LHC, employing fully-differential information as outlined here, may be essential for ruling out or in the Higgs boson and for characterizing a Higgs signal.Comment: 17 pages, 5 .eps figure

Pseudo-aneurysm of the anterior tibial artery, a rare cause of ankle swelling following a sports injury

BACKGROUND: Ankle pain and swelling following sports injuries are common presenting complaints to the accident and emergency department. Frequently these are diagnosed as musculoskeletal injuries, even when no definitive cause is found. Vascular injuries following trauma are uncommon and are an extremely rare cause of ankle swelling and pain. These injuries may however be limb threatening and are important to diagnose early, in order that appropriate treatment can be delivered. We highlight the steps to diagnosis of these injuries, and methods of managing these injuries. It is important for clinicians to be aware of the potential for this injury in patients with seemingly innocuous trauma from sports injuries, who have significant ankle pain and swelling. CASE PRESENTATION: A young, professional sportsman presented with a swollen, painful ankle after an innocuous hyper-plantar flexion injury whilst playing football, which was initially diagnosed as a ligamentous injury after no bony injury was revealed on X-Ray. He returned 2 days later with a large ulcer at the lateral malleolus and further investigation by duplex ultrasound and transfemoral arteriogram revealed a Pseudo-Aneurysm of the Anterior Tibial Artery. This was initially managed with percutaneous injection of thrombin, and later open surgery to ligate the feeding vessel. The patient recovered fully and was able to return to recreational sport. CONCLUSION: Vascular injuries remain a rare cause of ankle pain and swelling following sports injuries, however it is important to consider these injuries when no definite musculo-skeletal cause is found. Ultrasound duplex and Transfemoral arteriogram are appropriate, sensitive modalities for investigation, and may allow novel treatment to be directed percutaneously. Early diagnosis and intervention are essential for the successful outcome in these patients

Predicting climate change impacts on polar bear litter size

Predicting the ecological impacts of climate warming is critical for species conservation. Incorporating future warming into population models, however, is challenging because reproduction and survival cannot be measured for yet unobserved environmental conditions. In this study, we use mechanistic energy budget models and data obtainable under current conditions to predict polar bear litter size under future conditions. In western Hudson Bay, we predict climate warming-induced litter size declines that jeopardize population viability: ∼28% of pregnant females failed to reproduce for energetic reasons during the early 1990s, but 40–73% could fail if spring sea ice break-up occurs 1 month earlier than during the 1990s, and 55–100% if break-up occurs 2 months earlier. Simultaneously, mean litter size would decrease by 22–67% and 44–100%, respectively. The expected timeline for these declines varies with climate-model-specific sea ice predictions. Similar litter size declines may occur in over one-third of the global polar bear population