Experiments performed with cold and ultracold neutrons constrain our understanding of
particle physics at the TeV scale and beyond. In particular, we focus on beta-decay
measurements with ultracold neutrons, including the UCNA and UCNτ experiments,
where our ongoing experimental program promises a nuclear structure-independent
determination of the fundamental parameters of the charged weak current of the nucleon.
Ultracold neutrons provide a unique handle on some of the key systematic uncertainties
in these measurements and continue to present opportunities to improve the accuracy of
our understanding of the decay of the neutron. We review the status of motivation and
status of these experiments, and the path forward to probe neutron beta-decay in
next-generation experiments with precision at the 0.1% level and below.
We also present ideas for measurements with greatly improved sensitivity for
neutron-antineutron oscillations. This process would be produced by a beyond-standard-model
interaction which violates baryon number by 2 units (ΔB=2). Neutron anti-neutron oscillations
is expected in a number of models in which oscillations can be produced at energy scales
down to the current limits and for which the classic, ΔB=1 processes such as proton decay
are not observable. The detection of such a process would have enormous implications for
our understanding of the baryon-antibaryon asymmetry, and touch on other important topics
such as our understanding of the origin of neutrino mass. Our concept would utilize
state-of-the-art neutron optics (developed in part by the group of H. Shimizu) coupled to
a dedicated spallation source, to produce an intense beam of cold neutrons directed at
a graphite target several hundred meters from the source. Antineutrons produced via
oscillations are detected by observing annihilation events in the graphite target.
This geometry can improve the sensitivity to detect oscillations by a factor of 1000
over existing experiments, probing effective energy scales in excess of 100 TeV and
potentially setting the most stringent limits for years to come.