13,906 research outputs found
Parton Distributions for Event Generators
In this paper, conventional Global QCD analysis is generalized to produce
parton distributions optimized for use with event generators at the LHC. This
optimization is accomplished by combining the constraints due to existing
hard-scattering experimental data with those from anticipated cross sections
for key representative SM processes at LHC (by the best available theory) as
joint input to the global analyses. The PDFs obtained in these new type of
global analyses using matrix elements calculated in any given order will be
best suited to work with event generators of that order, for predictions at the
LHC. This is most useful for LO event generators at present. Results obtained
from a few candidate PDF sets (labeled as CT09MCS, CT09MC1 and CT09MC2) for LO
event generators produced in this way are compared with those from other
approaches.Comment: 35 pages, 19 figures, and 4 table
Gluon fusion contribution to W+W- + jet production
We describe the computation of the process that contributes
to the production of two -bosons and a jet at the CERN Large Hadron Collider
(LHC). While formally of next-to-next-to-leading order (NNLO) in QCD, this
process can be evaluated separately from the bulk of NNLO QCD corrections
because it is finite and gauge-invariant. It is also enhanced by the large
gluon flux and by selection cuts employed in the Higgs boson searches in the
decay channel , as was first pointed out by Binoth {\it et al.}
in the context of production. For cuts employed by the ATLAS
collaboration, we find that the gluon fusion contribution to
enhances the background by about ten percent and can lead to moderate
distortions of kinematic distributions which are instrumental for the ongoing
Higgs boson searches at the LHC. We also release a public code to compute the
NLO QCD corrections to this process, in the form of an add-on to the package
{\tt MCFM}.Comment: 13 pages, 4 figures, 3 table
Recommended from our members
The timing of the formation and usage of replicase clusters in S-phase nuclei of human diploid fibroblasts
The sites of nascent DNA synthesis were compared
with the distribution of the proliferating cell nuclear antigen (PCNA) in S-phase nuclei of human diploid fibroblasts (HDF) by two in vitro techniques. Firstly, proliferating fibroblasts growing in culture that had been synchronised at S-phase were microinjected with the thymidine analogue biotin-11-dUTP. The sites of incorporation of biotin into injected cells
were compared with the distribution of PCNA by
indirect immunofluorescence microscopy and laser
scanning confocal microscopy (LSCM). In common
with other studies, a progression of patterns for both biotin incorporation and PCNA localisation was observed. However, we did not always observe
coincidence in these patterns, the pattern of biotin incorporation often resembling the expected, preceding distribution of PCNA. In nuclei in which the pattern of biotin incorporation appeared to be identical to the distribution of PCNA, LSCM revealed that not all of the sites of PCNA immunofluorescence were incorporating biotin at the same time. Secondly,
nuclei which had been isolated from quiescent
cultures of HDF were innoculated into cell-free
extracts of Xenopus eggs which support DNA replication in vitro. Following innoculation into these extracts DNA replication was initiated in each nucleus. The sites of DNA synthesis were detected by biotin-11-dUTP incorporation and compared with the distribution of PCNA by indirect immunofluorescence. Only a single pattern of biotin incorporation and PCNA distribution was observed. PCNA accumulated
at multiple discrete spots some 15min before any biotin incorporation was observed. When biotin incorporation did occur, LSCM revealed almost complete coincidence between the sites of DNA synthesis and the sites at which PCNA was localised.Brunel Open Access Publishing Fun
Nucleation- and Emergence-Limited Growth of Ice from Pores
Nucleation of ice from vapor on atmospheric aerosols has been attributed to the condensation and freezing of supercooled water in small pores. Here we use wedge pores on mica to directly observe the growth of ice in confinement prior to the growth of bulk crystals. We report a transition in behavior with a decreasing temperature: At low temperatures, the limiting step is not nucleation but a free energy barrier associated with the growth of ice through a narrow pore mouth to become a bulk phase
b-Initiated processes at the LHC: a reappraisal
Several key processes at the LHC in the standard model and beyond that
involve quarks, such as single-top, Higgs, and weak vector boson associated
production, can be described in QCD either in a 4-flavor or 5-flavor scheme. In
the former, quarks appear only in the final state and are typically
considered massive. In 5-flavor schemes, calculations include quarks in the
initial state, are simpler and allow the resummation of possibly large initial
state logarithms of the type into the
parton distribution function (PDF), being the typical scale of the
hard process. In this work we critically reconsider the rationale for using
5-flavor improved schemes at the LHC. Our motivation stems from the observation
that the effects of initial state logs are rarely very large in hadron
collisions: 4-flavor computations are pertubatively well behaved and a
substantial agreement between predictions in the two schemes is found. We
identify two distinct reasons that explain this behaviour, i.e., the
resummation of the initial state logarithms into the -PDF is relevant only
at large Bjorken and the possibly large ratios 's are
always accompanied by universal phase space suppression factors. Our study
paves the way to using both schemes for the same process so to exploit their
complementary advantages for different observables, such as employing a
5-flavor scheme to accurately predict the total cross section at NNLO and the
corresponding 4-flavor computation at NLO for fully exclusive studies.Comment: Fixed typo in Eq. (A.10) and few typos in Eq. (C.2) and (C.3
MINLO: Multi-scale improved NLO
In the present work we consider the assignment of the factorization and
renormalization scales in hadron collider processes with associated jet
production, at next-to-leading order (NLO) in perturbation theory. We propose a
simple, definite prescription to this end, including Sudakov form factors to
consistently account for the distinct kinematic scales occuring in such
collisions. The scheme yields results that are accurate at NLO and, for a large
class of observables, it resums to all orders the large logarithms that arise
from kinematic configurations involving disparate scales. In practical terms
the method is most simply understood as an NLO extension of the matrix element
reweighting procedure employed in tree level matrix element-parton shower
merging algorithms. By way of a proof-of-concept, we apply the method to Higgs
and Z boson production in association with up to two jets.Comment: 27 pages, 17 figure
Design of Marine Protected Areas on high seas and territorial waters of rockall bank
Fisheries closures are rapidly being developed to protect vulnerable marine ecosystems worldwide. Satellite monitoring of fishing vessel activity indicates that these closures can work effectively with good compliance by international fleets even in remote areas. Here we summarise how remote fisheries closures were designed to protect Lophelia pertusa habitat in a region of the NE Atlantic that straddles the EU fishing zone and the high seas. We show how scientific records, fishers' knowledge and surveillance data on fishing activity can be combined to provide a powerful tool for the design of Marine Protected Areas. © Inter-Research 2009
How to identify when a performance indicator has run its course
The official published version can be found at the link below.Increasing numbers of countries are using indicators to evaluate the quality of clinical care, with some linking payment to achievement. For performance frameworks to remain effective the indicators need to be regularly reviewed. The frameworks cannot cover all clinical areas, and achievement on chosen indicators will eventually reach a ceiling beyond which further improvement is not feasible. However, there has been little work on how to select indictors for replacement. The Department of Health decided in 2008 that it would regularly replace indicators in the national primary care pay for performance scheme, the Quality and Outcomes Framework, making a rigorous approach to removal a priority. We draw on our previous work on pay for performance and our current work advising the National Institute for Health and Clinical Excellence (NICE) on the Quality and Outcomes Framework to suggest what should be considered when planning to remove indicators from a clinical performance framework
t \bar{t} W production and decay at NLO
We present results for the production of a top pair in association with a
W-boson at next-to-leading order. We have implemented this process into the
parton-level integrator MCFM including the decays of both the top quarks and
the W-bosons with full spin correlations. Although the cross section for this
process is small, it is a Standard Model source of same-sign lepton events that
must be accounted for in many new physics searches. For a particular analysis
of same-sign lepton events in which b-quarks are also present, we investigate
the effect of the NLO corrections as a function of the signal region cuts.Comment: 10 pages, 7 figure
- …