445 research outputs found
The nucleon and the nuclear force in the context of effective theory and path-integral methods
The nucleon structure and the nuclear force are investigated in the context
of the non-perturbative path-integral method of hadronization. Starting from a
microscopic quark-diquark model, the nucleon is generated as a relativistic
bound state and an effective chiral meson-nucleon Lagrangian is derived. Many
of the nucleon physical properties are studied using a theory of at most two
free parameters.Comment: 8 pages, 4 figures. To appear in the proceedings of Pan American
Advanced Studies Institute on New States of Matter in Hadronic Interactions
(PASI2002), Campos do Jordao, Brazil, 7-18 Jan 200
Quasifree processes from nuclei: Meson photoproduction and electron scattering
We have developed a relativistic formalism for studying quasi-free processes
from nuclei. The formalism can be applied with ease to a variety of processes
and renders transparent analytical expressions for all observables. We have
applied it to kaon photoproduction and to electron scattering. For the case of
the kaon, we compute the recoil polarization of the lambda-hyperon and the
photon asymmetry. Our results indicate that polarization observables are
insensitive to relativistic, nuclear target, and distortion effects. Yet, they
are sensitive to the reactive content, making them ideal tools for the study of
modifications to the elementary amplitude -- such as in the production,
propagation, and decay of nucleon resonances -- in the nuclear medium. For the
case of the electron, we have calculated the spectral function of He-4. An
observable is identified for the clean and model-independent extraction of the
spectral function. Our calculations provide baseline predictions for the
recently measured, but not yet fully analyzed, momentum distribution of He-4 by
the A1-collaboration from Mainz. Our approach predicts momentum distributions
for He-4 that rival some of the best non-relativistic calculations to date.Comment: To appear in the proceedings of International Symposium on
Electromagnetic Interactions in Nuclear and Hadron Physics (EMI 2001), Osaka,
Ibaraki, Japan, 4-7 Dec 200
Herpes simplex virus type 1 epidemiology in Latin America and the Caribbean: Systematic review and meta-analytics
Objectives To investigate the epidemiology of herpes simplex virus type 1 (HSV-1) in Latin America and the Caribbean. Methods Systematic review and meta-analytics guided by the Cochrane Collaboration Handbook and reported following the PRISMA guidelines. Results Thirty-three relevant reports were identified including 35 overall (and 95 stratified) seroprevalence measures, and five and nine proportions of virus isolation in genital ulcer disease (GUD) and in genital herpes, respectively. Pooled mean seroprevalence was 57.2% (95% CI: 49.7-64.6%) among children and 88.4% (95% CI: 85.2-91.2%) among adults. Pooled mean seroprevalence was lowest at 49.7% (95% CI: 42.8-56.6%) in those aged ?10, followed by 77.8% (95% CI: 67.9-84.8%) in those aged 10-20, 82.8% (95% CI: 73.1-90.8%) in those aged 20-30, 92.5% (95% CI: 89.4-95.1%) in those aged 30-40, and 94.2% (95% CI: 92.7-95.5%) in those aged ?40. Age was the strongest source of heterogeneity in seroprevalence, explaining 54% of variation. Evidence was found for seroprevalence decline over time. Pooled mean proportion of HSV-1 isolation was 0.9% (95% CI: 0.0-3.6%) in GUD and 10.9% (95% CI: 4.4-19.4%) in genital herpes. Conclusions HSV-1 is a widely prevalent infection in this region, but its epidemiology may be slowly transitioning, with still limited contribution for HSV-1 in genital herpes
Quasifree kaon-photoproduction from nuclei in a relativistic approach
We compute the recoil polarization of the lambda-hyperon and the photon
asymmetry for the quasifree photoproduction of kaons in a relativistic
impulse-approximation approach. Our motivation for studying polarization
observables is threefold. First, polarization observables are more effective
discriminators of subtle dynamics than the unpolarized cross section. Second,
earlier nonrelativistic calculations suggest an almost complete insensitivity
of polarization observables to distortions effects. Finally, this insensitivity
entails an enormous simplification in the theoretical treatment. Indeed, by
introducing the notion of a ``bound-nucleon propagator'' we exploit Feynman's
trace techniques to develop closed-form, analytic expressions for all
photoproduction observables. Moreover, our results indicate that polarization
observables are also insensitive to relativistic effects and to the nuclear
target. Yet, they are sensitive to the model parameters, making them ideal
tools for the study of modifications to the elementary amplitude --- such as in
the production, propagation, and decay of nucleon resonances --- in the nuclear
medium.Comment: 15 pages and 6 figures - submitted to PR
Pion-nucleus optical potential valid up to the DELTA-resonance region
We present in this article an optical potential for the -nucleus
interaction that can be used in various studies involving -nucleus
channels. Based on earlier treatments of the low energy -nucleus optical
potential, we have derived a potential expression applicable from threshold up
to the -resonance region. We extracted the impulse approximation form
for this potential from the scattering amplitude and then added to it
kinematical and physical corrections. The kinematic corrections arise from
transforming the impulse approximation expression from the center of
mass frame to the -nucleus center of mass frame, while the physical
corrections arise mostly from the many-body nature of the -nucleus
interaction. By taking advantage of the experimental progress in our knowledge
of the process, we have updated earlier treatments with parameters
calculated from state-of-the-art experimental measurements.Comment: 23 pages, 12 figures. Accepted for publication in Physical Review
Quasifree eta photoproduction from nuclei and medium modifications of resonances
We investigate the sensitivity of the differential cross section, recoil
nucleon polarization and the photon asymmetry to changes in the elementary
amplitude, medium modifications of the resonance masses, as
well as nuclear target effects. All calculations are performed within a
relativistic plane wave impulse approximation formalism resulting in analytical
expressions for all observables. The spin observables are shown to be unique
tools to study subtle effects that are not accessible by only looking at the
unpolarized differential cross section.Comment: 27 pages, 8 figures, Revtex, To be published in Phys. Rev.
Does infection with Chlamydia trachomatis induce long-lasting partial immunity? Insights from mathematical modelling.
OBJECTIVES: To explore whether existence of long-lasting partial immunity against reinfection with Chlamydia trachomatis is necessary to explain C. trachomatis prevalence patterns by age and sexual risk, and to provide a plausible estimate for the effect size, defined here as a reduction in susceptibility to reinfection. METHODS: A population-based mathematical model was constructed to describe C. trachomatis natural history and transmission dynamics by age and sexual risk. The model was parameterised using natural history, and epidemiological and sexual behaviour data, and applied for UK and US data. Sensitivity analyses were conducted to assess the robustness of predictions to variations in model structure and to examine the impact of alternative assumptions for the mechanism underlying partial immunity. RESULTS: Partial immunity against reinfection was found necessary to explain observed C. trachomatis prevalence patterns by age and sexual risk. The reduction in susceptibility to reinfection was estimated at 93% using UK data (95% uncertainty interval (UI)=88%-97%) and at 67% using US data (95% UI=24%-88%). The model-structure sensitivity analyses affirmed model predictions. The immunity-mechanism sensitivity analyses suggested a mechanism of susceptibility reduction against reinfection or a mechanism of infectious-period duration reduction upon reinfection. CONCLUSIONS: A strong long-lasting partial immunity against C. trachomatis reinfection should be present to explain observed prevalence patterns. The mechanism of immunity could be either a reduction in susceptibility to reinfection or a reduction in duration of infection on reinfection. C. trachomatis infection appears to naturally elicit a strong long-lasting immune response, supporting the concept of vaccine development
Trends and Predictors of Syphilis Prevalence in the General Population: Global Pooled Analyses of 1103 Prevalence Measures Including 136 Million Syphilis Tests.
Background: This study assessed levels, trends, and associations of observed syphilis prevalence in the general adult population using global pooled analyses. Methods: A standardized database of syphilis prevalence was compiled by pooling systematically gathered data. Random-effects meta-analyses and meta-regressions were conducted using data from the period 1990-2016 to estimate pooled measures and assess predictors and trends. Countries were classified by World Health Organization region. Sensitivity analyses were conducted. Results: The database included 1103 prevalence measures from 136 million syphilis tests across 154 countries (85% from women in antenatal care). Global pooled mean prevalence (weighted by region population size) was 1.11% (95% confidence interval [CI], .99-1.22). Prevalence predictors were region, diagnostic assay, sample size, and calendar year interacting with region. Compared to the African Region, the adjusted odds ratio (AOR) was 0.42 (95% CI, .33-.54) for the Region of the Americas, 0.13 (95% CI, .09-.19) for the Eastern Mediterranean Region, 0.05 (95% CI, .03-.07) for the European Region, 0.21 (95% CI, .16-.28) for the South-East Asia Region, and 0.41 (95% CI, .32-.53) for the Western Pacific Region. Treponema pallidum hemagglutination assay (TPHA) only or rapid plasma reagin (RPR) only, compared with dual RPR/TPHA diagnosis, produced higher prevalence (AOR >1.26), as did smaller sample-size studies (2.16). Prevalence declined in all regions; the annual AORs ranged from 0.84 (95% CI, .79-.90) in the Eastern Mediterranean to 0.97 (95% CI, .97-1.01) in the Western Pacific. The pooled mean male-to-female prevalence ratio was 1.00 (95% CI, .89-1.13). Sensitivity analyses confirmed robustness of results. Conclusions: Syphilis prevalence has declined globally over the past 3 decades. Large differences in prevalence persist among regions, with the African Region consistently the most affected
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