HEP Seminar: Recent results from T2K

Dr Nick Grant, University of Lancaster

Friday 08 November 2013, 1300-1400
Frankland Colloquium Room

T2K means "Tokai to Kamioka" and is a neutrino physics experiment in Japan. Neutrinos are elementary particles that come in 3 types or "flavours": electron, muon and tau. They have the unusual property that, as they travel, they change from one flavour to another. T2K produces a beam of muon neutrinos in Tokai on the east coast of Japan, and sends it 295 km to Kamioka in west Japan. The properties of this beam are measured twice: first in a near detector that is 280 metres from where the neutrinos are created, and the second time in the far detector at Kamioka. Using the near detector measurement, it is predicted that 205 $pm$ 17 (syst) muon neutrinos would be seen in the far detector if no flavour changes occur. However only 58 muon neutrinos have been seen in the far detector, and this is known as "muon-neutrino disappearance". Furthermore, 28 electron neutrinos have been seen in the far detector, whereas only 4.6 $pm$ 0.5 (syst) would be expected without flavour changes; this is known as "electron-neutrino appearance". T2K has made measurements of the mixing angles and mass-squared differences that parametrise these flavour changes. Muon-neutrino disappearance is parametrised by the mixing angle $sin^{2}theta_{23}$ and the mass-squared difference $Delta m^{2}_{32}$, and the best-fit T2K measurements of these parameters are $sin^{2}theta_{23}$ = 0.514 and $Delta m^{2}_{32}$ = 2.44 $times$ 10$^{-3}$eV$^{2}$/c$^{4}$. Electron-neutrino appearance is parameterised by the mixing angle $sin^{2} 2 theta_{13}$, and the best-fit T2K measurement of this parameter is $sin^{2} 2 theta_{13}$ = 0.154$^{+0.041}_{-0.034}$ for the normal mass hierarchy and $sin^{2} 2 theta_{13}$ = 0.184$^{+0.046}_{-0.041}$ for the inverted mass hierarchy. The p-value for seeing 28 electron neutrinos when 4.6 $pm$ 0.5 are expected is 10$^{-13}$, and this represents the first observation of the appearance of a neutrino of one flavour in a beam of another flavour. The non-zero value of $sin^{2} 2 theta_{13}$ means that it will be possible in the future to search for charge-parity (CP) violation in neutrinos.