6,244 research outputs found
Cosmological constant in a quantum fluid model
Possible analogies between vacuum state and quantum fluid provide a model to
study vacuum energy density induced by thermal corrections, space-time
curvature, boundary conditions and quantum back-reaction. We find that vacuum
energy density in this quantum fluid model is not naturally of the order of the
matter energy density. We show how higher-order corrections in quantum
back-reaction can also contribute to vacuum energy density, and how the
cosmological expansion is a manifestation of an universe out of mechanical
equilibrium. This last fact implies that simple thermodynamic arguments are not
enough to explain the cosmological constant problem due to the calculation of
the associated vacuum energy density requires first the knowing of the
underlying microscopic physics of vacuum.Comment: Latex file, 12 pages, some references added, typos correcte
Vulnerability Factors and Pathways Leading to Underage Entry into Sex Work in two Mexican-US Border Cities
The current wave of interest in human trafficking and the commercial sexual exploitation of children has exposed a lack of knowledge about the vulnerabilities leading to underage entry into sex work. This knowledge is necessary for the development of effective prevention programs to identify girls who are most at-risk, especially in Latin America, a region that is believed to be a large source of persons moved across international borders for the purposes of sexual and labor exploitation. The objective of this study was to explore and increase understanding of the vulnerability factors and pathways leading to underage entry into sex work experienced by women currently engaging in sex work in two cities on the northern border of Mexico. From August 2013 to October 2014, 20 female sex workers (FSWs) with a history of entry into sex work prior to age 18 were recruited for in-depth interviews from a larger time-location sample of female sex workers (FSWs) participating in a quantitative survey in Tijuana and Ciudad Juarez. The median age of entry into sex work was 14 (range 10-17); 12/21 participants reported being forced into sex work and, of these, 7 were transported to another city where they began engaging in sex work. Family dysfunction (e.g., domestic violence between parents, parent drug use, neglect, etc.), sexual and physical abuse, and teen pregnancy were among the key themes that emerged as vulnerabilities to underage entry into sex work. Womenâs narratives clearly illustrated that the vulnerabilities and pathways leading to underage entry are manifold, complex, and often intersect with each other. Our findings begin to lay the groundwork for understanding the potential vulnerabilities and pathways leading to underage entry into sex, and may have relevance to Latin America in general. This study also provides a foundation for further research to explore what may mitigate these vulnerabilities as well as creating evidence-based interventions to prevent commercial sexual exploitation of minors in the region
Respuesta de gramas nativas a la fertilizaciĂłn de nitrĂłgeno, fĂłsforo y potasio en el trĂłpico hĂșmedo
Propuesta pedagĂłgica en torno a tĂłpicos introductorios de fĂsica nuclear
Teaching physics research in school environment has shown a great tendency in different countries to propose new objectives inside it, among which are also, the recognition and description of introductory concepts about modern physics in high school, in our particular interest, teaching of nuclear physics. On the other hand, the methods used in teaching of some of those topics do not suppose an efficient methodology, raising the need of a new focus to approach this type of topics to optimize their learning in school. We propose different activities from cooperative-colaborative learning methodology, with the objective to approach the teaching of introductory topics of nuclear physics (atomic structure and nuclear fusion); where these are teach from different experiences in the classroom that simulate the phenomena that are related to the different themes. Adjunct to the activities we included animations that support the material used in class.Investigaciones en el ĂĄrea de la enseñanza de la fĂsica en el ĂĄmbito escolar han demostrado una gran tendencia en diferentes paĂses a proponer nuevos objetivos dentro de Ă©sta, entre los cuales ademĂĄs se encuentra el reconocimiento y descripciĂłn de conceptos introductorios de fĂsica moderna en la secundaria, para nuestro caso de interĂ©s en particular la enseñanza de la fĂsica nuclear. Por otra parte, la forma como son abordadas muchas de las temĂĄticas involucradas no suponen una metodologĂa eficiente, lo que plantea la necesidad de un nuevo enfoque a la hora de abordar Ă©ste tipo de tĂłpicos en la escuela, para asĂ optimizar la enseñanza de estos. Se proponen diferentes actividades desde la metodologĂa del aprendizaje cooperativo-colaborativo con el fin deabordar la enseñanza de temas introductorios de fĂsica nuclear (estructura atĂłmica y fusiĂłn nuclear); en donde estos son afrontados a partir de experiencias en el aula que simulen los fenĂłmenos que estĂĄn ligados a las diferentes temĂĄticas. Anexo a las actividades se incluyeron animaciones para respaldar el material empleado en el aula
Relationship between propagule pressure and colonization pressure in invasion ecology: a test with ships' ballast
Increasing empirical evidence indicates the number of released individuals (i.e. propagule pressure) and number of released species (i.e. colonization pressure) are key determinants of the number of species that successfully invade new habitats. In view of these relationships, and the possibility that ships transport whole communities of organisms, we collected 333 ballast water and sediment samples to investigate the relationship between propagule and colonization pressure for a variety of diverse taxonomic groups (diatoms, dinoflagellates and invertebrates). We also reviewed the scientific literature to compare the number of species transported by ships to those reported in nature. Here, we show that even though ships transport nearly entire local communities, a strong relationship between propagule and colonization pressure exists only for dinoflagellates. Our study provides evidence that colonization pressure of invertebrates and diatoms may fluctuate widely irrespective of propagule pressure. We suggest that the lack of correspondence is explained by reduced uptake of invertebrates into the transport vector and the sensitivity of invertebrates and diatoms to selective pressures during transportation. Selection during transportation is initially evident through decreases in propagule pressure, followed by decreased colonization pressure in the most sensitive taxa
Long-term prognostic value of quantitative myocardial perfusion in patients with chest pain and normal coronary arteries
BACKGROUND: Patients with chest pain and no obstructive coronary artery disease have shown a high incidence of major adverse cardiovascular events (MACE). We evaluated the role of absolute myocardial perfusion quantification in predicting all-cause mortality and MACE during long-term follow-up in this group of patients. METHODS: We studied 79 patients who underwent Nitrogen-13 ammonia PET for quantification of global myocardial blood flow (MBF) and myocardial flow reserve (MFR) due to suspected impaired myocardial perfusion. Patients with coronary artery disease (i.e.,â>â30% stenosis in one or more coronary arteries) were excluded. We assessed all-cause mortality and MACE. MACE was defined as the composite incidence of death, myocardial infarction (MI), or hospitalization due to heart failure. RESULTS: Median follow-up was 8 (IQR: 3-14) years. Univariate Cox regression showed that only MFR (Pâ=â0.01) was a predictor of all-cause mortality. Univariate Cox regression analysis showed that both MFR and Stress MBF were predictors of the composite endpoint of MACE (Pâ<â0.001 and Pâ=â0.01, respectively). CONCLUSION: Quantitative assessment of myocardial perfusion may predict all-cause mortality and MACE in patients with chest pain and normal coronary arteries in the long-term follow-up
Observation of an Excited Bc+ State
Using pp collision data corresponding to an integrated luminosity of 8.5 fb-1 recorded by the LHCb experiment at center-of-mass energies of s=7, 8, and 13 TeV, the observation of an excited Bc+ state in the Bc+Ï+Ï- invariant-mass spectrum is reported. The observed peak has a mass of 6841.2±0.6(stat)±0.1(syst)±0.8(Bc+) MeV/c2, where the last uncertainty is due to the limited knowledge of the Bc+ mass. It is consistent with expectations of the Bcâ(2S31)+ state reconstructed without the low-energy photon from the Bcâ(1S31)+âBc+Îł decay following Bcâ(2S31)+âBcâ(1S31)+Ï+Ï-. A second state is seen with a global (local) statistical significance of 2.2Ï (3.2Ï) and a mass of 6872.1±1.3(stat)±0.1(syst)±0.8(Bc+) MeV/c2, and is consistent with the Bc(2S10)+ state. These mass measurements are the most precise to date
Search for the rare decays and
A search for the rare decay of a or meson into the final
state is performed, using data collected by the LHCb experiment
in collisions at and TeV, corresponding to an integrated
luminosity of 3 fb. The observed number of signal candidates is
consistent with a background-only hypothesis. Branching fraction values larger
than for the decay mode are
excluded at 90% confidence level. For the decay
mode, branching fraction values larger than are excluded at
90% confidence level, this is the first branching fraction limit for this
decay.Comment: All figures and tables, along with any supplementary material and
additional information, are available at
https://lhcbproject.web.cern.ch/lhcbproject/Publications/LHCbProjectPublic/LHCb-PAPER-2015-044.htm
Measurement of the lifetime
Using a data set corresponding to an integrated luminosity of ,
collected by the LHCb experiment in collisions at centre-of-mass energies
of 7 and 8 TeV, the effective lifetime in the
decay mode, , is measured to be ps. Assuming
conservation, corresponds to the lifetime of the light
mass eigenstate. This is the first measurement of the effective
lifetime in this decay mode.Comment: All figures and tables, along with any supplementary material and
additional information, are available at
https://lhcbproject.web.cern.ch/lhcbproject/Publications/LHCbProjectPublic/LHCb-PAPER-2016-017.htm
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