We report an improved measurement of the free neutron lifetime τ_{n} using the UCNτ apparatus at the Los Alamos Neutron Science Center. We count a total of approximately 38×10^{6} surviving ultracold neutrons (UCNs) after storing in UCNτ's magnetogravitational trap over two data acquisition campaigns in 2017 and 2018. We extract τ_{n} from three blinded, independent analyses by both pairing long and short storage time runs to find a set of replicate τ_{n} measurements and by performing a global likelihood fit to all data while self-consistently incorporating the β-decay lifetime.
View Article and Find Full Text PDFThe precise value of the mean neutron lifetime, τ, plays an important role in nuclear and particle physics and cosmology. It is used to predict the ratio of protons to helium atoms in the primordial universe and to search for physics beyond the Standard Model of particle physics. We eliminated loss mechanisms present in previous trap experiments by levitating polarized ultracold neutrons above the surface of an asymmetric storage trap using a repulsive magnetic field gradient so that the stored neutrons do not interact with material trap walls.
View Article and Find Full Text PDFIn this paper, we describe a new method for measuring surviving neutrons in neutron lifetime measurements using bottled ultracold neutrons (UCN), which provides better characterization of systematic uncertainties and enables higher precision than previous measurement techniques. An active detector that can be lowered into the trap has been used to measure the neutron distribution as a function of height and measure the influence of marginally trapped UCN on the neutron lifetime measurement. In addition, measurements have demonstrated phase-space evolution and its effect on the lifetime measurement.
View Article and Find Full Text PDFObjectives: To examine the efficacy of sexual risk reduction interventions among South African youth.
Methods: Electronic databases were searched to identify studies published between 2007 and early 2013. Studies were eligible if they (1) targeted youth age 9-26, (2) evaluated sexual risk reduction interventions and (3) reported at least one behavioral outcome.
The purpose of this meta-analytic study is to summarize the current science concerning the (a) prevalence of alcohol use, (b) prevalence of sexual risk behaviors, and (c) association between alcohol use and sexual risk behaviors among people livingwithHIV/AIDS (PLWHA). Studies were included if they sampled PLWHA, evaluated both alcohol use and sexual risk behavior (e.g.
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