Speaker
Description
An unbound neutron decays via the weak interaction into a proton, electron, and antineutrino with a lifetime $\tau_n$ of approximately 15 minutes. Within the Standard Model of particle physics, $\tau_n$ is precisely related to two other parameters, the nucleon axial form factor $g_A$ and the CKM matrix element $V_{ud}$. Thus, measurements of two of these parameters determines the third, or precise measurements of all three serves as a test for beyond Standard Model physics. Also, $\tau_n$ is an input to models of big bang nucleosynthesis where it impacts primordial abundance of light elements. Most of the recent measurements of $\tau_n$ have employed ultracold neutrons (UCN), which are neutrons with extremely small kinetic energy which undergo total external reflection from material surfaces. In these experiments, UCN are confined to a bottle and the number of survivors are counted after prescribed holding periods to determine a characteristic storage time $\tau_s$. The free neutron lifetime $\tau_n$ is then found by applying corrections for the finite loss probability when a UCN bounces from the material wall. The difference between $\tau_s$ and $\tau_n$ has been typically much larger than the final quoted uncertainty in $\tau_n$, demanding very good understanding of the extrapolation to effectively infinite storage volume. To overcome systematic uncertainties of material bottle experiments, the "UCNtau" experiment uses an array of permanent magnets to repel UCN from the bottle surface, avoiding material interactions altogether. The recent result from the UCNtau experiment, with overall uncertainty matching that of the previous most-precise experiment, will be discussed in this talk, along with prospects for further improvement to the $\tau_n$ measurement.
| Early Consideration | No |
|---|---|
| Graduate Student | No |
