| Abstract |
Neutrons are stable inside atomic nuclei. Outside the confines of the nucleus, they
decay into a proton, electron, and antineutrino, with a lifetime of about 880 s. The
rate of decay can be precisely calculated, using the theory of electroweak
interactions, with an uncertainty on the order of 1e-4. Recent measurements using
bottled neutrons have achieved uncertainties below 1 s (0.1 %), but other
measurements observing neutron decay in flight disagree by 10 s. Attempts to resolve
this discrepancy have spawned much experimental effort as well as exotic theoretical
conjectures, thus far without a clear conclusion. In this colloquium, I will discuss
the challenges of precision measurement of the neutron lifetime, illustrating the
UCNtau experiment. It eliminates the dominant loss mechanisms present in previous
bottle experiments by levitating polarized ultracold neutrons above the surface of a
large magnetic trap. Using this approach, a new result, 877.75 +/- 0.28 (stat)
+0.22/-0.16 (sys) s [PRL 127, 162501 (2021)], is the most precise measurement of the
lifetime. This result, together with improved measurements of the axial coupling
constant, will provide a determination of the CKM matrix element Vud, independent of
nuclear decays, and address the recent tension in the test of CKM unitarity.
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