Session S3: Invited Session: Electroweak Interactions, Neutrinos and Neutrons

Neutrino Driven Nucleosynthesis in the Early Galaxy

We discuss a neutrino driven neutron capture mechanism that occurs in the He shell of an early core-collapse supernova. We calculate the nucleosynthesis, both before and after the passage of shock, in recent pre-supernova models of $11-15 M_\odot$ stars with an initial metallicity of $[Z] < -3$. We find that for an inverted neutrino mass hierarchy with a hard neutrino spectra, neutrino interactions with He via $^4{\rm He}(\bar\nu_e,e^+n)^3{\rm H}$ produces free neutrons on a timescale of $\sim 5-6$ s, which are captured by Fe seeds present in the He shell to produce isotopes with $A > 200$. This process occurs on a timescale of $\sim100-300$ s at a temperature of $\sim 10^8 $ K which is much longer and colder compared to the conventional hot rapid neutron capture process. The process is uniquely sensitive to the neutron capture cross-sections since $(n,\gamma) \leftrightarrow (\gamma,n)$ equilibrium is not established. We find that variation of neutron capture rates can have a dramatic effect on both the timescale and the final abundance pattern. We also explore the sensitivity of the mechanism on the neutrino emission parameters and oscillations, as well as on explosion energy and progenitor metallicity. We discuss the implications of this process with respect to the observed abundance of neutron capture elements in metal-poor stars.   

Neutron Trapping using a Magneto-Gravitational Trap

Eighty years after Chadwick discovered the neutron, physicists today still cannot agree on how long the neutron lives. Measurements of the neutron lifetime have achieved the 0.1\% level of precision ($\sim$ 1 s). However, results from several recent experiments are up to 7 s lower than the (pre-2010) particle data group (PDG) value. Experiments using the trap technique yield lifetime results lower than those using the beam technique. The PDG urges the community to resolve this discrepancy, now 6.5 sigma. Measuring the absolute neutron lifetime is difficult because of several limitations: the low energy of the neutron decay products, the inability to track slow neutrons, and the fact that the neutron lifetime is long (880.1 $\pm$ 1.1 s). Slow neutrons are susceptible to many loss mechanisms other than beta-decay, such as upscattering and absorption on material surfaces. Often, these interactions act on time scales comparable to the neutron beta-decay, making the extraction of the beta-decay lifetime particularly challenging. We will revisit this measurement by trapping ultracold neutrons (UCN) in a hybrid magnetic-gravitational trap. The trap consists of a Halbach array of permanent magnets, which can levitate UCN up to 50 neV. These neutrons are also confined vertically up to 0.5 m by gravity. Such a trap minimizes the chance of neutron interactions with material walls. In addition, the open-top geometry allows room to implement novel schemes to detect neutrons and decay particles in-situ. The UCN$\tau$ experiment aims to reduce the uncertainty of the neutron lifetime measurement to below 1 second. In this talk, I will report results of our first attempt to trap UCN in 2013 and discuss plans to quantify systematic effects.   

The beta-decay of $^{6}$He: a sensitive window to search for physics beyond the standard model

The simplicity of the Gamow-Teller beta-decay of the $^{6}$He nucleus has attracted considerable experimental and theoretical attention in the past few years. Precision correlation measurements in this allowed transition have motivated the production of high intensity and high purity sources and beams at several facilities. The main purpose of such measurements is to search for new physics contributing to the weak interaction that would manifest itself through phenomenological tensor couplings. New measurement techniques, including the detection of recoiling ions from decays in ion and atom traps, have been developed in order to make the most efficient use of the available intensities and to reduce instrumental systematic effects. This talk will first present the result of a measurement of the beta-neutrino angular correlation in the decay of $^{6}$He which has been carried out by trapping ions in a Paul trap. The talk will then describe current efforts of second generation experiments aiming at new levels of sensitivity in the search for phenomenological exotic tensor couplings in $^{6}$He decay.