Speaker
Description
A neutron decays into a proton, an electron, and antineutrino in a lifetime of about 880 s. The neutron lifetime is one of the important parameters for particle physics and astrophysics. For instance, it dominates the uncertainty on 4He abundance in the Big Bang Nucleosynthesis and it also determines Vud term in the Cabibbo-Kobayashi-Maskawa quark mixing matrix. Although the neutron lifetime is very important in modern physics, there is a 4-sigma (8.5 s) discrepancy between the results of two typical methods: the beam method and the storage method. The beam method measures the neutron flux and decay protons by different detectors, and the storage method counts survival neutrons after some storage times. The discrepancy is called the neutron lifetime puzzle and is not yet settled. The possibility that unknown systematic errors and new physics such as dark decays are the cause has been discussed.
We have been carrying out a neutron lifetime measurement with a new method at J-PARC to solve the puzzle. In our method, we measure the neutron flux and decay electrons simultaneously by a Time Projection Chamber filled by working gas and 3He. To reduce the background event rate, the neutron beam is shaped to bunches shorter than the length of the sensitive region by the Spin Flip Chopper (SFC) and injected into the TPC. Since the neutron flux and decay rate are counted by the same detector in our method, the systematic uncertainty is different from the typical beam method. Additionally, we can measure dark decays with electrons if it exists. We are aiming for 1 s (0.1%) precision determination of the neutron lifetime to achieve a definitive result.
We have been constructed the experimental and analysis procedure to determine the neutron lifetime by our new method by the last year and the result is 898 +/- 10 (stat.) +15 -18 (sys.) s. Towards 1 s accuracy, we are now installing a new SFC of 3 times flux and it will enable us to analyze more classified events to reduce systematic uncertainties.
This presentation will report the first physics result of our experiment with acquired data during 2014 - 2016, and a detailed status and prospect of upgrades both in the apparatus and the analysis towards 1 s precision.
