2,166 research outputs found
The Tevatron Higgs exclusion limits and theoretical uncertainties: a critical appraisal
We examine the exclusion limits set by the CDF and D0 experiments on the
Standard Model Higgs boson mass from their searches at the Tevatron in the
light of large theoretical uncertainties on the signal and background cross
sections. We show that when these uncertainties are consistently taken into
account, the sensitivity of the experiments becomes significantly lower and the
currently excluded mass range -175 GeV would be entirely reopened. The
necessary luminosity required to recover the current sensitivity is found to be
a factor of two higher than the present one.Comment: 11 pages, 5 Figures. Version published in Physics Letter B, including
an erratu
The treatment of the infrared region in perturbative QCD
We discuss the contribution coming from the infrared region to NLO matrix
elements and/or coefficient functions of hard QCD processes. Strictly speaking,
this contribution is not known theoretically, since it is beyond perturbative
QCD. For DGLAP evolution all the infrared contributions are collected in the
phenomenological input parton distribution functions (PDFs), at some relatively
low scale Q_0; functions which are obtained from a fit to the `global' data.
However dimensional regularization sometimes produces a non-zero result coming
from the infrared region. Instead of this conventional regularization
treatment, we argue that the proper procedure is to first subtract from the NLO
matrix element the contribution already generated at the same order in \alpha_s
by the LO DGLAP splitting function convoluted with the LO matrix element. This
prescription eliminates the logarithmic infrared divergence, giving a
well-defined result which is consistent with the original idea that everything
below Q_0 is collected in the PDF input. We quantify the difference between the
proposed treatment and the conventional approach using low-mass Drell-Yan
production and deep inelastic electron-proton scattering as examples; and
discuss the potential impact on the `global' PDF analyses. We present arguments
to show that the difference cannot be regarded as simply the use of an
alternative factorization scheme.Comment: 15 pages, 5 figures, title changed, text considerably modified to
improve presentation, and discussion section enlarge
Perturbative and nonperturbative contributions to the strange quark asymmetry in the nucleon
There are two mechanisms for the generation of an asymmetry between the
strange and anti-strange quark distributions in the nucleon: nonperturbative
contributions originating from nucleons fluctuating into virtual baryon-meson
pairs such as and , and perturbative contributions
arising from gluons splitting into strange and anti-strange quark pairs. While
the nonperturbative contributions are dominant in the large- region, the
perturbative contributions are more significant in the small- region. We
calculate this asymmetry taking into account both nonperturbative and
perturbative contributions, thus giving a more accurate evaluation of this
asymmetry over the whole domain of . We find that the perturbative
contributions are generally a few times larger in magnitude than the
nonperturbative contributions, which suggests that the best region to detect
this asymmetry experimentally is in the region . We find that
the asymmetry may have more than one node, which is an effect that should be
taken into account, e.g. for parameterizations of the strange and anti-strange
quark distributions used in global analysis of parton distributions.Comment: 14 pages, 4 figures, figures comparing theoretical calculations with
NNPDF global analysis added, accepted for publication in EPJ
Nuclear activity and massive star formation in the low luminosity AGN NGC4303: Chandra X-ray observations
We present evidence of the co-existence of either an AGN or an ultraluminous
X-ray source (ULX), together with a young super stellar cluster in the 3
central parsecs of NGC4303. The galaxy contains a low luminosity AGN and hosts
a number of starburst regions in a circumnuclear spiral, as well as in the
nucleus itself. A high spatial resolution Chandra image of this source reveals
that the soft X-ray emission traces the ultraviolet nuclear spiral down to a
core, which is unresolved both in soft and hard X-rays. The astrometry of the
X-ray core coincides with the UV core within the Chandra positioning accuracy.
The total X-ray luminosity of the core, 1.5*10^{39} erg/s, is similar to that
from some LINERs or from the weakest Seyferts detected so far. The soft X-rays
in both the core and the extended structure surrounding it can be well
reproduced by evolutionary synthesis models (which include the emission
expected from single stars, the hot diffuse gas, supernova remnants and binary
systems), consistent with the properties of the young stellar clusters
identified in the UV. The hard X-ray tail detected in the core spectrum,
however, most likely requires the presence of an additional source. This
additional source could either be a weak active nucleus black hole or an
ultraluminous X-ray object. The implications of these results are discussed.Comment: 37 pages, 7 figures, ApJ accepte
W boson production at hadron colliders: the lepton charge asymmetry in NNLO QCD
We consider the production of W bosons in hadron collisions, and the
subsequent leptonic decay W->lnu_l. We study the asymmetry between the rapidity
distributions of the charged leptons, and we present its computation up to the
next-to-next-to-leading order (NNLO) in QCD perturbation theory. Our
calculation includes the dependence on the lepton kinematical cuts that are
necessarily applied to select W-> lnu_l events in actual experimental analyses
at hadron colliders. We illustrate the main differences between the W and
lepton charge asymmetry, and we discuss their physical origin and the effect of
the QCD radiative corrections. We show detailed numerical results on the charge
asymmetry in ppbar collisions at the Tevatron, and we discuss the comparison
with some of the available data. Some illustrative results on the lepton charge
asymmetry in pp collisions at LHC energies are presented.Comment: 37 pages, 21 figure
Inclusive top-pair production phenomenology with TOPIXS
We discuss various aspects of inclusive top-quark pair production based on
TOPIXS, a new, flexible program that computes the production cross section at
the Tevatron and LHC at next-to-next-to-leading logarithmic accuracy in soft
and Coulomb resummation, including bound-state effects and the complete
next-to-next-to-leading order result in the q-qbar channel, which has recently
become available. We present the calculation of the top-pair cross section in
pp collisions at 8 TeV centre-of-mass energy, as well as the cross sections for
hypothetical heavy quarks in extensions of the standard model. The dependence
on the parton distribution input is studied. Further we investigate the impact
of LHC top cross section measurements at sqrt(s)=7 TeV on global fits of the
gluon distribution using the NNPDF re-weighting method.Comment: 27 pages, 5 figures; v2: corrected typos in Eqs. (2.8) and (6.2) and
the text, added footnote on page 4, matches published versio
Magnetic moment of the pentaquark with light-cone QCD sum rules
In this article, we study the magnetic moment of the pentaquark state as diquark-diquark-antiquark () state in the
framework of the light-cone QCD sum rules approach. The numerical results
indicate the magnetic moment of the pentaquark state is about
.Comment: 10 pages, 1 figure. The main contents of this article is included in
hep-ph/0503007, this article will not be submitted to a journal for
publicatio
QCD factorization for forward hadron scattering at high energies
We consider the QCD factorization of DIS structure functions at small x and
amplitudes of 2->2 -hadronic forward scattering at high energy. We show that
both collinear and k_T-factorization for these processes can be obtained
approximately as reductions of a more general (totally unintegrated) form of
the factorization. The requirement of ultraviolet and infrared stability of the
factorization convolutions allows us to obtain restrictions on the fits for the
parton distributions in k_T- and collinear factorization.Comment: 18 pages, 10 figures In the present version misprints found in the
prevcious version are corrected and some more details are explaine
Advanced cell-based modeling of the royal disease: characterization of the mutated F9 mRNA
Essentials The Royal disease (RD) is a form of hemophilia B predicted to be caused by a splicing mutation. We generated an iPSC-based model of the disease allowing mechanistic studies at the RNA level. F9 mRNA analysis in iPSC-derived hepatocyte-like cells showed the predicted abnormal splicing. Mutated F9 mRNA level was very low but we also found traces of wild type transcripts.
SUMMARY:
Background The royal disease is a form of hemophilia B (HB) that affected many descendants of Queen Victoria in the 19th and 20th centuries. It was found to be caused by the mutation F9 c.278-3A>G. Objective To generate a physiological cell model of the disease and to study F9 expression at the RNA level. Methods Using fibroblasts from skin biopsies of a previously identified hemophilic patient bearing the F9 c.278-3A>G mutation and his mother, we generated induced pluripotent stem cells (iPSCs). Both the patient's and mother's iPSCs were differentiated into hepatocyte-like cells (HLCs) and their F9 mRNA was analyzed using next-generation sequencing (NGS). Results and Conclusion We demonstrated the previously predicted aberrant splicing of the F9 transcript as a result of an intronic nucleotide substitution leading to a frameshift and the generation of a premature termination codon (PTC). The F9 mRNA level in the patient's HLCs was significantly reduced compared with that of his mother, suggesting that mutated transcripts undergo nonsense-mediated decay (NMD), a cellular mechanism that degrades PTC-containing mRNAs. We also detected small proportions of correctly spliced transcripts in the patient's HLCs, which, combined with genetic variability in splicing and NMD machineries, could partially explain some clinical variability among affected members of the European royal families who had lifespans above the average. This work allowed the demonstration of the pathologic consequences of an intronic mutation in the F9 gene and represents the first bona fide cellular model of HB allowing the study of rare mutations at the RNA level
- …