First muon antineutrino disappearance measurement at T2K performed by Oxford graduate student

22 May 2015

The first measurement of muon antineutrino disappearance on the T2K Experiment, performed by Oxford graduate student Kirsty Duffy, was released on 18 May 2015.

T2K (Tokai to Kamioka) is a long-baseline neutrino oscillation experiment in Japan, studying neutrino oscillations and interactions. Researchers from 59 institutes in 11 countries search for oscillations from muon neutrinos to electron neutrinos and tau neutrinos, and study neutrino interactions at the two near detectors (ND280 and INGRID) and the far detector (Super-Kamiokande), 295km away from the initial beam.

Since May 2014, T2K has been running with a beam composed mostly of muon antineutrinos, and 59.8 events would have been detected in T2K's far detector Super-Kamiokande if there were no antineutrino oscillations. However, only 17 events were observed, which matches the expected "disappearance" due to muon antineutrinos oscillating into tau antineutrinos.

In the first analysis of its kind on T2K, Kirsty worked with collaborators from Imperial College London and Tokyo University's Kavli IPMU to measure the antineutrino oscillation parameters \sin^2\bar{\theta}_{23} and \Delta \bar{m}^2_{32}. The first of these parameters is a measure of the fraction of antineutrinos that oscillate, while the second is a measure of the difference between the squares of the masses of two types of antineutrino. T2K's measurement of the antineutrino oscillation probability parameter \sin^2\bar{\theta}_{23} is the world's best.

The graph below shows the best-fit values and 68% and 90% credible intervals in the antineutrino oscillation parameters \sin^2\bar{\theta}_{23} and \Delta\bar{m}^2_{32}. These results have also been compared with T2K's earlier measurement of muon-neutrino disappearance, and no difference found between neutrinos and antineutrinos in these oscillations.

Kirsty and T2K are now preparing an analysis of electron antineutrino appearance (where electron antineutrinos appear in the mostly muon antineutrino beam), and will update the muon antineutrino disappearance measurement with more data over the summer. These measurements could give a first insight into CP-violation in the neutrino sector, which would help to understand the differences between matter and antimatter.

Best-fit values and 68% and 90% credible intervals from muon antineutrino disappearance fit

More information about this analysis is available here.