790 research outputs found
Electroweak top-quark pair production at the LHC with bosons to NLO QCD in POWHEG
We present the calculation of the NLO QCD corrections to the electroweak
production of top-antitop pairs at the CERN LHC in the presence of a new
neutral gauge boson. The corrections are implemented in the parton shower Monte
Carlo program POWHEG. Standard Model (SM) and new physics interference effects
are properly taken into account. QED singularities, first appearing at this
order, are consistently subtracted. Numerical results are presented for SM and
total cross sections and distributions in invariant mass, transverse
momentum, azimuthal angle and rapidity of the top-quark pair. The remaining
theoretical uncertainty from scale and PDF variations is estimated, and the
potential of the charge asymmetry to distinguish between new physics models is
investigated for the Sequential SM and a leptophobic topcolor model.Comment: 33 pages, 15 figures, 1 table. Discussion added on uncertainties from
parton showers and NLL contribution
Can new heavy gauge bosons be observed in ultra-high energy cosmic neutrino events?
A wide range of models beyond the Standard Model predict charged and neutral
resonances, generically called - and -bosons, respectively. In this
paper we study the impact of such resonances on the deep inelastic scattering
of ultra-high energy neutrinos as well as on the resonant charged current
scattering (Glashow resonance). We find that the effects of
such resonances can not be observed with the Pierre Auger Observatory or any
foreseeable upgrade of it.Comment: 5 pages, 2 figure
nCTEQ15 - Global analysis of nuclear parton distributions with uncertainties in the CTEQ framework
We present the new nCTEQ15 set of nuclear parton distribution functions with
uncertainties. This fit extends the CTEQ proton PDFs to include the nuclear
dependence using data on nuclei all the way up to 208^Pb. The uncertainties are
determined using the Hessian method with an optimal rescaling of the
eigenvectors to accurately represent the uncertainties for the chosen tolerance
criteria. In addition to the Deep Inelastic Scattering (DIS) and Drell-Yan (DY)
processes, we also include inclusive pion production data from RHIC to help
constrain the nuclear gluon PDF. Furthermore, we investigate the correlation of
the data sets with specific nPDF flavor components, and asses the impact of
individual experiments. We also provide comparisons of the nCTEQ15 set with
recent fits from other groups.Comment: 35 page
NLO+NLL limits on W' and Z' gauge boson masses in general extensions of the Standard Model
QCD resummation predictions for the production of charged (W') and neutral
(Z') heavy gauge bosons decaying leptonically are presented. The results of our
resummation code at next-to-leading order and next-to-leading logarithmic
(NLO+NLL) accuracy are compared to Monte Carlo predictions obtained with PYTHIA
at leading order (LO) supplemented with parton showers (PS) and FEWZ at NLO and
next-to-next-to-leading order (NNLO) for the -differential and total cross
sections in the Sequential Standard Model (SSM) and general SU(2)xSU(2)xU(1)
models. The LO+PS Monte Carlo and NNLO fixed-order predictions are shown to
agree approximately with those at NLO+NLL at small and intermediate ,
respectively, and the importance of resummation for total cross sections is
shown to increase with the gauge boson mass. The theoretical uncertainties are
estimated by variations of the renormalisation/factorisation scales and of the
parton densities, the former being significantly reduced by the resummation
procedure. New limits at NLO+NLL on W' and Z' boson masses are obtained by
reinterpreting the latest ATLAS and CMS results in general extensions of the
Standard Model.Comment: 26 pages, 10 figures, 6 table
A Review of the Intrinsic Heavy Quark Content of the Nucleon
We present a review of the state of the art of our understanding of the intrinsic charm and bottom content of the nucleon. We discuss theoretical calculations, constraints from global analyses, and collider observables sensitive to the intrinsic heavy quark distributions. A particular emphasis is put on the potential of a high energy and high luminosity fixed target experiment using the LHC beams (AFTER@LHC) to search for intrinsic charm
Attempts to detect retrotransposition and de novo deletion of Alus and other dispersed repeats at specific loci in the human genome
Dispersed repeat elements contribute to genome instability by de novo insertion and unequal recombination between repeats. To study the dynamics of these processes, we have developed single DNA molecule approaches to detect de novo insertions at a single locus and Alu-mediated deletions at two different loci in human genomic DNA. Validation experiments showed these approaches could detect insertions and deletions at frequencies below 10(-6) per cell. However, bulk analysis of germline (sperm) and somatic DNA showed no evidence for genuine mutant molecules, placing an upper limit of insertion and deletion rates of 2 x 10(-7) and 3 x 10(-7), respectively, in the individuals tested. Such re-arrangements at these loci therefore occur at a rate lower than that detectable by the most sensitive methods currently available
Enhanced magnetic moment and conductive behavior in NiFe2O4 spinel ultrathin films
Bulk NiFe2O4 is an insulating ferrimagnet. Here, we report on the epitaxial
growth of spinel NiFe2O4 ultrathin films onto SrTiO3 single-crystals. We will
show that - under appropriate growth conditions - epitaxial stabilization leads
to the formation of a spinel phase with magnetic and electrical properties that
radically differ from those of the bulk material : an enhanced magnetic moment
(Ms) - about 250% larger - and a metallic character. A systematic study of the
thickness dependence of Ms allows to conclude that its enhanced value is due to
an anomalous distribution of the Fe and Ni cations among the A and B sites of
the spinel structure resulting from the off-equilibrium growth conditions and
to interface effects. The relevance of these findings for spinel- and, more
generally, oxide-based heterostructures is discussed. We will argue that this
novel material could be an alternative ferromagetic-metallic electrode in
magnetic tunnel junctions.Comment: accepted for publication in Phys. Rev.
Impact of LHC vector boson production in heavy ion collisions on strange PDFs
The extraction of the strange quark parton distribution function (PDF) poses a long-standing puzzle. Measurements from neutrino-nucleus deep inelastic scattering (DIS) experiments suggest the strange quark is suppressed compared to the light sea quarks, while recent studies of W^\pm /Z boson production at the LHC imply a larger strange component at small x values. As the parton flavor determination in the proton depends on nuclear corrections, e.g. from heavy-target DIS, LHC heavy ion measurements can provide a distinct perspective to help clarify this situation. In this investigation we extend the nCTEQ nPDFs to study the impact of the LHC proton-lead W^\pm /Z production data on both the flavor differentiation and nuclear corrections. This complementary data set provides new insights on both the LHC W^\pm /Z proton analyses and the neutrino-nucleus DIS data. We identify these new nPDFs as nCTEQWZ. Our calculations are performed using a new implementation of the nCTEQ code (nCTEQ++) based on C++ which enables us to easily interface to external programs such as HOPPET, APPLgrid and MCFM. Our results indicate that, as suggested by the proton data, the small x nuclear strange sea appears larger than previously expected, even when the normalization of the W^{\pm }/Z data is accommodated in the fit. Extending the nCTEQ analysis to include LHC W^\pm /Z data represents an important step as we advance toward the next generation of nPDFs
Impact of LHC vector boson production in heavy ion collisions on strange PDFs
The extraction of the strange quark parton distribution function (PDF) poses a long-standing puzzle. Measurements from neutrino-nucleus deep inelastic scattering (DIS) experiments suggest the strange quark is suppressed compared to the light sea quarks, while recent studies of W^\pm /Z boson production at the LHC imply a larger strange component at small x values. As the parton flavor determination in the proton depends on nuclear corrections, e.g. from heavy-target DIS, LHC heavy ion measurements can provide a distinct perspective to help clarify this situation. In this investigation we extend the nCTEQ nPDFs to study the impact of the LHC proton-lead W^\pm /Z production data on both the flavor differentiation and nuclear corrections. This complementary data set provides new insights on both the LHC W^\pm /Z proton analyses and the neutrino-nucleus DIS data. We identify these new nPDFs as nCTEQWZ. Our calculations are performed using a new implementation of the nCTEQ code (nCTEQ++) based on C++ which enables us to easily interface to external programs such as HOPPET, APPLgrid and MCFM. Our results indicate that, as suggested by the proton data, the small x nuclear strange sea appears larger than previously expected, even when the normalization of the W^{\pm }/Z data is accommodated in the fit. Extending the nCTEQ analysis to include LHC W^\pm /Z data represents an important step as we advance toward the next generation of nPDFs
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