Time reversal violation beyond the Standard Model of elementary particles is one of the biggest problems in particle physics. The parity violation caused by weak interaction is largely enhanced in compound nuclei. There is a theoretical prediction that T-violating effects can also be enhanced by this mechanism in these nuclei implying that T-violation can be searched for with an enhanced sensitivity. We are planning an experiment to search for the T-violation beyond the Standard Model using compound nuclei in J-PARC. In order to estimate the enhancement of the T-violation, the angular distribution of the (n,γ) reaction was measured with 139La, which is a possible candidate nucleus for the T-violation search, by using a germanium γ-ray detector assembly at J-PARC. A clear angular dependence of the γ-rays from the p-wave resonance was found. The result indicates that the enhancement of the T-violation in the neutron absorption reaction of 139La is on the order of 106.
A new type of slow neutron detector which can realize a spatial resolution less than 100 nm was developed using fine-grained nuclear emulsion. The detector consists of a converter layer including a 50-nm-thick layer of 10B4C coated with 10-µm-thick layer of the emulsion. It was exposed to cold and ultracold neutrons at J-PARC. Its detection efficiency and the spatial resolution of incident neutrons were measured. The resolution was found to be in the range of 11–99 nm in the angle region of tanθ ≤ 1.9, where θ is the angle between a recorded track and the normal direction of the converter layer. The achieved spatial resolution corresponds to the improvement of one or two orders of magnitude compared with conventional technique. The detector will be a powerful tool to measure position distributions of neutrons for fundamental physics and neutron imaging.
Because of its unique characteristics, neutrons are used for many kinds of fundamental physics researches. In this article, I introduce experiments using neutrons for elementally particle, nuclear, and astrophysics with their physics backgrounds: neutron lifetime to know how the early universe was, neutron EDM and neutron-antineutron oscillation to solve mystery of matter and anti-matter, and unknown intermediate force to understand gravity.