  A neutron decays into a proton, an electron, and an anti-neutrino through the
beta-decay process. The decay lifetime ($\sim$880 s) is an important parameter
in the weak interaction. For example, the neutron lifetime is a parameter used
to determine the |$V_{\rm ud}$| parameter of the CKM quark mixing matrix. The
lifetime is also one of the input parameters for the Big Bang Nucleosynthesis,
which predicts light element synthesis in the early universe. However,
experimental measurements of the neutron lifetime today are significantly
different (8.4 s or 4.0$\sigma$) depending on the methods. One is a bottle
method measuring surviving neutron in the neutron storage bottle. The other is
a beam method measuring neutron beam flux and neutron decay rate in the
detector. There is a discussion that the discrepancy comes from unconsidered
systematic error or undetectable decay mode, such as dark decay. A new type of
beam experiment is performed at the BL05 MLF J-PARC. This experiment measured
neutron flux and decay rate simultaneously with a time projection chamber using
a pulsed neutron beam. We will present the world situation of neutron lifetime
and the latest results at J-PARC.
