321 research outputs found
The Proton Spin and Flavor Structure in the Chiral Quark Model
After a pedagogical review of the simple constituent quark model and deep
inelastic sum rules, we describe how a quark sea as produced by the emission of
internal Goldstone bosons by the valence quarks can account for the observed
features of proton spin and flavor structures. Some issues concerning the
strange quark content of the nucleon are also discussed.Comment: 59 pages with table of contents, Lecture delivered at the Schladming
Winter School (March 1997), to be published by Springer-Verlag under the
title "Computing Particle Properties" (eds. C. B. Lang and H. Gausterer
Handling misclassified stratification variables in the analysis of randomised trials with continuous outcomes
Many trials use stratified randomisation, where participants are randomised within strata defined by one or more baseline covariates. While it is important to adjust for stratification variables in the analysis, the appropriate method of adjustment is unclear when stratification variables are affected by misclassification and hence some participants are randomised in the incorrect stratum. We conducted a simulation study to compare methods of adjusting for stratification variables affected by misclassification in the analysis of continuous outcomes when all or only some stratification errors are discovered, and when the treatment effect or treatment-by-covariate interaction effect is of interest. The data were analysed using linear regression with no adjustment, adjustment for the strata used to perform the randomisation (randomisation strata), adjustment for the strata if all errors are corrected (true strata), and adjustment for the strata after some errors are discovered and corrected (updated strata). The unadjusted model performed poorly in all settings. Adjusting for the true strata was optimal, while the relative performance of adjusting for the randomisation strata or the updated strata varied depending on the setting. As the true strata are unlikely to be known with certainty in practice, we recommend using the updated strata for adjustment and performing subgroup analyses, provided the discovery of errors is unlikely to depend on treatment group, as expected in blinded trials. Greater transparency is needed in the reporting of stratification errors and how they were addressed in the analysis.Lisa N. Yelland, Jennie Louise, Brennan C. Kahan, Tim P. Morris, Katherine J. Lee, Thomas R. Sulliva
Gravitational collapse of a Hagedorn fluid in Vaidya geometry
The gravitational collapse of a high-density null charged matter fluid,
satisfying the Hagedorn equation of state, is considered in the framework of
the Vaidya geometry. The general solution of the gravitational field equations
can be obtained in an exact parametric form. The conditions for the formation
of a naked singularity, as a result of the collapse of the compact object, are
also investigated. For an appropriate choice of the arbitrary integration
functions the null radial outgoing geodesic, originating from the shell
focussing central singularity, admits one or more positive roots. Hence a
collapsing Hagedorn fluid could end either as a black hole, or as a naked
singularity. A possible astrophysical application of the model, to describe the
energy source of gamma-ray bursts, is also considered.Comment: 14 pages, 2 figures, to appear in Phys. Rev.
L-infinity algebra connections and applications to String- and Chern-Simons n-transport
We give a generalization of the notion of a Cartan-Ehresmann connection from
Lie algebras to L-infinity algebras and use it to study the obstruction theory
of lifts through higher String-like extensions of Lie algebras. We find
(generalized) Chern-Simons and BF-theory functionals this way and describe
aspects of their parallel transport and quantization.
It is known that over a D-brane the Kalb-Ramond background field of the
string restricts to a 2-bundle with connection (a gerbe) which can be seen as
the obstruction to lifting the PU(H)-bundle on the D-brane to a U(H)-bundle. We
discuss how this phenomenon generalizes from the ordinary central extension
U(1) -> U(H) -> PU(H) to higher categorical central extensions, like the
String-extension BU(1) -> String(G) -> G. Here the obstruction to the lift is a
3-bundle with connection (a 2-gerbe): the Chern-Simons 3-bundle classified by
the first Pontrjagin class. For G = Spin(n) this obstructs the existence of a
String-structure. We discuss how to describe this obstruction problem in terms
of Lie n-algebras and their corresponding categorified Cartan-Ehresmann
connections. Generalizations even beyond String-extensions are then
straightforward. For G = Spin(n) the next step is "Fivebrane structures" whose
existence is obstructed by certain generalized Chern-Simons 7-bundles
classified by the second Pontrjagin class.Comment: 100 pages, references and clarifications added; correction to section
5.1 and further example to 9.3.1 adde
Light flavor asymmetry of nucleon sea
The light flavor antiquark distributions of the nucleon sea are calculated in
the effective chiral quark model and compared with experimental results. The
contributions of the flavor-symmetric sea-quark distributions and the nuclear
EMC effect are taken into account to obtain the ratio of Drell-Yan cross
sections , which can match well
with the results measured in the FermiLab E866/NuSea experiment. The calculated
results also match the measured from different
experiments, but unmatch the behavior of derived
indirectly from the measurable quantity
by the FermiLab E866/NuSea
Collaboration at large . We suggest to measure again
at large from precision experiments with careful experimental data
treatment. We also propose an alternative procedure for experimental data
treatment.Comment: 10 pages, 8 figures, final version to appear in EPJ
The risk of deterioration in GCS13-15 patients with traumatic brain injury identified by CT imaging : a systematic review and meta-analysis
The optimal management of mild traumatic brain injury (TBI) patients with injuries identified by computed tomography (CT) brain scan is unclear. Some guidelines recommend hospital admission for an observation period of at least 24 h. Others argue that selected lower-risk patients can be discharged from the Emergency Department (ED). The objective of our review and meta-analysis was to estimate the risk of death, neurosurgical intervention, and clinical deterioration in mild TBI patients with injuries identified by CT brain scan, and assess which patient factors affect the risk of these outcomes. A systematic review and meta-analysis adhering to PRISMA standards of protocol and reporting were conducted. Study selection was performed by two independent reviewers. Meta-analysis using a random effects model was undertaken to estimate pooled risks for: clinical deterioration, neurosurgical intervention, and death. Meta-regression was used to explore between-study variation in outcome estimates using study population characteristics. Forty-nine primary studies and five reviews were identified that met the inclusion criteria. The estimated pooled risk for the outcomes of interest were: clinical deterioration 11.7% (95% confidence interval [CI]: 11.7%-15.8%), neurosurgical intervention 3.5% (95% CI: 2.2%-4.9%), and death 1.4% (95% CI: 0.8%-2.2%). Twenty-one studies presented within-study estimates of the effect of patient factors. Meta-regression of study characteristics and pooling of within-study estimates of risk factor effect found the following factors significantly affected the risk for adverse outcomes: age, initial Glasgow Coma Scale (GCS), type of injury, and anti-coagulation. The generalizability of many studies was limited due to population selection. Mild TBI patients with injuries identified by CT brain scan have a small but clinically important risk for serious adverse outcomes. This review has identified several prognostic factors; research is needed to derive and validate a usable clinical decision rule so that low-risk patients can be safely discharged from the ED
Measurement of dijet photoproduction for events with a leading neutron at HERA
Differential cross sections for dijet photoproduction and this process in
association with a leading neutron, e+ + p -> e+ + jet + jet + X (+ n), have
been measured with the ZEUS detector at HERA using an integrated luminosity of
40 pb-1. The fraction of dijet events with a leading neutron was studied as a
function of different jet and event variables. Single- and double-differential
cross sections are presented as a function of the longitudinal fraction of the
proton momentum carried by the leading neutron, xL, and of its transverse
momentum squared, pT^2. The dijet data are compared to inclusive DIS and
photoproduction results; they are all consistent with a simple pion-exchange
model. The neutron yield as a function of xL was found to depend only on the
fraction of the proton beam energy going into the forward region, independent
of the hard process. No firm conclusion can be drawn on the presence of
rescattering effects.Comment: 40 pages, 18 figure
Collider aspects of flavour physics at high Q
This review presents flavour related issues in the production and decays of
heavy states at LHC, both from the experimental side and from the theoretical
side. We review top quark physics and discuss flavour aspects of several
extensions of the Standard Model, such as supersymmetry, little Higgs model or
models with extra dimensions. This includes discovery aspects as well as
measurement of several properties of these heavy states. We also present public
available computational tools related to this topic.Comment: Report of Working Group 1 of the CERN Workshop ``Flavour in the era
of the LHC'', Geneva, Switzerland, November 2005 -- March 200
Combination of searches for Higgs boson pairs in pp collisions at \sqrts = 13 TeV with the ATLAS detector
This letter presents a combination of searches for Higgs boson pair production using up to 36.1 fb(-1) of proton-proton collision data at a centre-of-mass energy root s = 13 TeV recorded with the ATLAS detector at the LHC. The combination is performed using six analyses searching for Higgs boson pairs decaying into the b (b) over barb (b) over bar, b (b) over barW(+)W(-), b (b) over bar tau(+)tau(-), W+W-W+W-, b (b) over bar gamma gamma and W+W-gamma gamma final states. Results are presented for non-resonant and resonant Higgs boson pair production modes. No statistically significant excess in data above the Standard Model predictions is found. The combined observed (expected) limit at 95% confidence level on the non-resonant Higgs boson pair production cross-section is 6.9 (10) times the predicted Standard Model cross-section. Limits are also set on the ratio (kappa(lambda)) of the Higgs boson self-coupling to its Standard Model value. This ratio is constrained at 95% confidence level in observation (expectation) to -5.0 < kappa(lambda) < 12.0 (-5.8 < kappa(lambda) < 12.0). In addition, limits are set on the production of narrow scalar resonances and spin-2 Kaluza-Klein Randall-Sundrum gravitons. Exclusion regions are also provided in the parameter space of the habemus Minimal Supersymmetric Standard Model and the Electroweak Singlet Model. For complete list of authors see http://dx.doi.org/10.1016/j.physletb.2019.135103</p
Searches for lepton-flavour-violating decays of the Higgs boson in TeV collisions with the ATLAS detector
This Letter presents direct searches for lepton flavour violation in Higgs boson decays, H → eτ and
H → μτ , performed with the ATLAS detector at the LHC. The searches are based on a data sample
of proton–proton collisions at a centre-of-mass energy √s = 13 TeV, corresponding to an integrated
luminosity of 36.1 fb−1. No significant excess is observed above the expected background from Standard
Model processes. The observed (median expected) 95% confidence-level upper limits on the leptonflavour-violating branching ratios are 0.47% (0.34+0.13−0.10%) and 0.28% (0.37+0.14−0.10%) for H → eτ and H → μτ , respectively.publishedVersio
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