Postulated in 1930 by Pauli and discovered more than 50 years ago by Reines and Cowan, the neutrino is still the most enigmatic of the fundamental building blocks of matter. Understanding the behaviour of neutrinos could provide the key to why the universe consists of matter and not antimatter, while detecting neutrinos from astrophysical sources could provide unique insights into the origins of cosmic rays and the inner workings of violent astrophysical objects such as quasars and gamma-ray bursts.
The phenomenon of neutrino oscillations or neutrino mixing was first observed in the late 1990s and involves a neutrino of one flavour (such as the electron neutrino) changing into a different flavour neutrino (such as the muon neutrino) in free flight. This can be understood by invoking quantum mechanics whereby the weak (or flavour) eigenstates through which the neutrino interacts are not aligned with the mass eigenstates via which the neutrino propagates. The phenomenon of neutrino oscillations is theoretically described via the so-called PMNS mixing matrix, analogous to the CKM matrix in the quark sector.
The PMNS matrix is very different in structure from the CKM matrix: many of the off-diagonal entries are large, in stark contrast to the nearly diagonal nature of the CKM matrix. Compared to the quark sector, the elements of the neutrino mixing matrix are poorly constrained, with large error bars. The Sheffield PPPA group is an active member of T2K which aims to measure parameters such as sin22ϑ23 and sin22ϑ13, and is involved with other members of the UK T2K community in its proposed successor, Hyper-Kamiokande. The group is also involved in R&D in Liquid Argon aimed at future neutrino experiments, especially LBNF at Fermilab.
A neutrino factory, generating an intense beam of neutrinos from muon decay, is potentially the most valuable tool in terrestrial neutrino physics, but is a difficult technical challenge. The Sheffield neutrino physics group is working on the Muon Ionisation Cooling Experiment, MICE, which is tackling the problem of producing a well collimated monoenergetic beam of muons within the short timescale imposed by the muon lifetime. The Sheffield MICE group built the target which produces the MICE muon beam, and is also engaged in design studies for targets for a future neutrino factory (UKNF), which would have to withstand a proton flux an order of magnitude higher than those used in present-day neutrino beamlines.
The Sheffield PPPA group is currently working on the following neutrino physics projects: