Precise Neutron Lifetime Measurement Using Pulsed Neutron Beams at J-PARC*

The neutron decay lifetime (~ 880 sec) is an important parameter in the weak interaction. For example, it can be used to determine the Vud parameter of the CKM quark mixing matrix. However, experimental measurements of the neutron lifetime today are significantly different (4.0 sigma or 8.4 sec) depending on the method used. We are therefore performing an experiment using a different method with a goal precision of 1 sec on the neutron lifetime.

The experiment is carried out at the polarized beam branch of BL05, MLF, J-PARC. A Time Projection Chamber (TPC) operated with He and CO2 gas is used as a beta-decay detector. Neutrons are formed into bunches, the length of which is half of the TPC length, by Spin Flip Chopper. The neutron flux can be evaluated by the number of 3He(n, p)3H events in the TPC and its cross section.

The data acquisition started at J-PARC in 2014, and by 2018 we already took the data giving a statistical error of a few seconds on the neutron lifetime. We will present the analysis result of the neutron lifetime. We also present some future upgrade plans to achieve our goal precision of 1 sec.