Abstract
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.
Highlights
A free neutron decays into a proton, electron, and anti-neutrino with a mean lifetime τn ∼ 15 min denoted as, n → p + e− + ν e . (1)Figure 1 shows the measured neutron lifetime in these twenty years
The predicted Y p is the cross point of the band of τn and baryon to photon ratio η, which is determined by the Planck satellite from the observation of cosmic microwave background (CMB) [1]
The neutron lifetime was obtained from the counting ratio of these two detectors. This experiment published the result of τn = 887.7 ± 1.2 ± 1.9 s. In another beam method using Time Projection Chamber (TPC) [17], neutron lifetime is obtained from the simultaneous measurement of an electron from β decay and 3 He(n,p)3 H reaction
Summary
A free neutron decays into a proton, electron, and anti-neutrino with a mean lifetime τn ∼ 15 min denoted as, n → p + e− + ν e. There are two types of methods, one is called “storage method” and the other is “beam method”. The discrepancy between these two methods of 8.6 s or 4.1σ is called “neutron lifetime anomaly”. Before explaining the measurement methods in detail, the physical significance of τn will be introduced . The predicted Y p is the cross point of the band of τn and baryon to photon ratio η, which is determined by the Planck satellite from the observation of cosmic microwave background (CMB) [1]. There are two bands of τn by the measurement methods. Since the observed accuracy of Y p and η is improving year by year, the ambiguity of τn should be resolved
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