Neutrino properties at current and future facilities

The neutrino is the most elusive of the known elementary particles. They interact so rarely with ordinary matter that studying them requires gigantic detectors, like the 40m tall Super-Kamiokande (SK), which is built into a cavern beneath Mt. Ikeno In Japan, or the even bigger successor Hyper-Kamiokande (HK) which will be 70m tall and almost 10 times bigger overall.

Neutrinos are elusive but they are not rare: hundreds of billions pass through every square centimetre of Earth every second, without us noticing. Because they are so common it is possible for them to have significant effects in the formation and evolution of the universe. The most interesting question for modern experiments is whether neutrinos obey Charge-Parity (CP) symmetry or, more colloquially, if there is a difference between matter and antimatter. Because neutrinos are so common it is possible that a violation of their Charge-Parity symmetry could influence the rate of formation of other kinds of matter and antimatter after the Big Bang, leading to the universe as we observe it: containing matter and energy, but no antimatter. Violation of CP symmetry is a major focus of current in Particle Physics research, both in the UK and internationally.

The most interesting hint of this asymmetry to date came from the Tokai-to-Kamioka (T2K) experiment. A recent result published in Nature showed an indication (just short of what the field considers to be 'evidence') that neutrinos do indeed violate CP symmetry. Although not decisive, it is our current best lead on why the universe contains any "stuff" at all, rather than just light energy. The main goal of the next generation of neutrino experiments, including Hyper-Kamiokande, is to establish whether this indication of CP violation is really true.

The current analysis of T2K data uses artificial neutrinos, produced at an accelerator laboratory north of Tokyo, and directed 295 km through the earth's crust before being detected by Super - Kamiokande. A certain fraction of the neutrinos will change to a new type, in a now well-established process called neutrino oscillation. By looking at the difference in the fraction of neutrinos changing due to the oscillation and comparing this to the same measurement with antineutrinos it is possible to measure the amount of CP violation (or demonstrate that it must be small).

But the detector can also observe neutrinos from natural sources, including those produced by cosmic rays in the earth's atmosphere. As with the accelerator neutrinos, neutrinos produced at the far side of the earth can oscillate as they travel the 13000km through the Earth and be detected by Super-K. Combining the data from accelerator neutrinos and these natural 'atmospheric' neutrinos is more sensitive to the CP violation than either data set alone.

The first attempt at such a combined analysis is currently in development. The planned PhD project starts by working (as a member of the T2K and SK collaborations) to update this analysis and incorporate most recent developments in the handling of statistical and systematic uncertainties, using Markov Chain Monte Carlo or other techniques to estimate the likelihood functions. There is also potential to look at other ways to interpret and present the data to make it more useful in comparisons to external data sets.

With the analysis framework in place, it can also be adapted for next-generation HK experiment, which is currently under construction. The analysis could be used to estimate how sensitive HK will be in various scenarios and can be used in optimising the design and operation of the future experiment. Given how tantalisingly close the T2K results are to showing evidence of CP violation, it

is quite plausible that this work will lead to positive evidence of CP violation. Even if Nature is not so generous, the work will pave the way to a more definitive statement with Hyper-Kamiokande.