Lifetime of the free neutron

The average lifetime of the free neutron, 878.4 ± 0.5 s [S. Navas et al.], is a fundamental constant of nature which impacts various areas of fundamental research. Most importantly, the neutron decays via the weak interaction  n → p + e^- +  ν̃_e  and hence can serve as a tool to contribute to a precision determination of the Cabibbo-Kobayashi-Maskawa quark-mixing matrix element V_ud [M. Gorchtein and C.-Y. Seng]. The exact value of the neutron lifetime is also an important parameter of the model describing Big-Bang nucleosynthesis. As soon as the temperature of the Universe has fallen below the value corresponding to the neutron-proton mass difference, the free neutron lifetime determines the time available to form light elements [S. Navas et al.]. Their abundance can be determined by astronomy. 

Since about 20 years, the lifetime of the free neutron is heavily disputed amongst particle physicists, as the exact values derived from significantly distinct measurement methods, namely from neutron storage experiments [A. T. Yue et al.] and from neutron beam experiments [R. Musedinovic et al.], differ by about five standard deviations. All the past experiments have been thoroughly scrutinized for systematic errors, but no obvious mistakes were discovered. In addition, some new and exotic theoretical ideas were communicated, however, these were disproved. The big enigma persists nowadays [G.L. Greene and P. Geltenbort, F. E. Wietfeldt]. 

This is where the neutron lifetime experiment τSPECT wants to contribute. The goal of the τSPECT experiment is the precise and accurate determination of the free neutron lifetime with very small and different systematics than previous experiments. In the experiment, ultracold neutrons are stored in a fully magnetic trap for varying storage times up to 2000 seconds, after which the number of surviving UCNs gets counted. τSPECT employs a unique method called "double spin-flip loading" to fill the trapping volume [J. Auler, M. Engler et al.]. Purely magnetic trapping ensures no influence from wall collisions. Surviving UCNs are detected using a movable in-situ detector inside the storage volume [J. Auler, U. Bajpai et al.]. The τSPECT apparatus has been developed and built at Johannes Gutenberg University Mainz in recent years by the groups of W. Heil, D. Ries and M. Fertl [url], repurposing the magnet and cryostat of the aSPECT experiment. After initial commissioning at the UCN source at the research reactor TRIGA Mainz, τSPECT has been installed at the PSI UCN source. In 2025, physics data taking started.

The τSPECT home page at Johannes Gutenberg University Mainz you find here.