Neutron Lifetime Measurement Initiated at J-PARC/MLF/BL05

The neutron lifetime $\tau_{n}$ is one of the important parameters for the Big Bang Nucleosynthesis (BBN) which predicts an abundance of the light elements in the early universe. However, the He/(H+He) ratio recently measured by Izotov et al. has been deviated from that of the BBN prediction. Thus a precise $\tau_{n}$ measurement is desired. Historically, there are two methods for $\tau_{n}$ measurement, and there exists 3.8$\sigma$ deviation between their results. We are therefore conducting the $\tau_{n}$ measurement at the J-PARC/MLF/BL05 by using a third method. In this experiment we count the number of decay electrons by using Time Projection Chamber (TPC). We expect 1\% accuracy with the collected data in JFY 2014. In this presentation, we will report some analysis results and future plan.