Neutron is a unique object, since it does have a rest mass but does not have an electric charge, and shows a wave-like property at low energies. We are promoting studies of fundamental physics using such features of neutrons. The following experimental programs are ongoing at the J-PARC Material and Life-science experimental Facility (MLF).

(1) High-precision measurement of the neutron lifetime
The neutron lifetime is an important parameter related to the Big-bang nucleosynthesis and the unitarity test of the Cabibbo-Kobayashi-Maskawa (CKM) matrix. There is a systematic difference as large as 1% or 3.8 sigma between currently available data obtained with the different experimental methods. In order to solve this problem, we are making an experiment with an independent method based on the pulsed neutron beam and an active target. One of the advantages of our method is that the systematic error is much suppressed by measuring the decay rate of the neutrons relative to the event rate of the 3He(n,p)3H reaction whose corss section is known with the accuracy of 0.3%.

(2) Test of the inverse-square law of the gravity and search for exotic interaction
Recent unified theories of the fundamental interactions including gravity introduce the spatial dimensions greater than three. Since we do not experience such additional dimensions in daily life, they should be compactified in a very tiny scale. In such a small scale, the distance dependences of the fundamental interactions, especially the gravity, may become different from those in larger scales. The inverse-square law (IQL) of the gravity has been verified with good accuracies down to the distance of around one micron, but has not in still shorter distance. We are testing IQL of the gravity and searching for new gravity-like interactions by means of the neutron small angle scattering of the targets made of nano-particles. The advantage of our method is the very low background of the intermolecular forces thanks to the extremely small electric polarizability of the neutron.

(3) Test of time-reversal invariance in compund nuclei (NOPTREX collaboration)
Our universe is known to be dominated by not antimatter but matter. The origin of the matter dominance, however, is still an open question. We wish to find the source of the matter-antimatter asymmetry by searching for the equivalent time-reversal asymmetry. It has been known that the symmetry violation in the simillar parity reversal is enhanced in the neutron-induced nuclear reaction via compound states by a factor of upto about one million. It is expected that the same enhancement process will work also in the time-reversal violation. We are promoting a new test of the time-reversal invariance in compound nuclei formed by the interaction between neutron and heavy nucleus.