Unresolved Problems in Neutron Decay (video)

Chen-Yu Liu, Assistant Professor, CEEM/Physics Department, Indiana University

Neutron beta-decay is an archetype for all semileptonic charged-current weak nuclear processes. It transforms a d quark into a u quark and emits an electron and an (anti-)neutrino. As such, neutron beta-decay experiments played a pivotal role, in the 1950s, leading towards the construction of the universal V − A theory, which served as the foundation to unify the electromagnetic and the weak interactions. Since then, workers in the field have been developing tools, in precision polarimetry, spin manipulations, cold and ultracold neutron sources, and pushing for more accurate determination of the parameters that describe neutron decay. The neutron lifetime, along with neutron decay correlations, muon, nuclear and kaon decay data, can be used to test the unitarity of the CKM matrix (which quantifies the mixing between quarks) and probe physics beyond the standard model. Such tests are highly complementary to information that is anticipated from the LHC. In cosmology, the neutron lifetime determines weak interaction rates and therefore the helium yield of big bang nucleosynthesis (BBN).

Increased precision, however, brings out several data points of inconsistency. The value of quark mixing Vud extracted from the neutron lifetime and the beta asymmetry are off from the value measured using the 0+ → 0+ nuclear decays. The neutron lifetime measured by storing ultracold neutrons is 10 s shorter than the beta-decay lifetime measured by counting protons emerging from a beam of cold neutrons. The latter constitutes a 4 σ discrepancy, that has not been resolved with new measurements. In this talk, I will describe these modern experiments on neutron decay and discuss possible implications.