Validación de la propuesta

The overall prediction for the T2K beam flux, showing relative flavour fractions for neutrinos and antineutrinos at Super-K is shown in Figure2.7.

FIGURE 2.7: T2K beam flux prediction, showing relative flavour frac- tions for neutrinos and antineutrinos at Super-K. The left plot is for the neutrino beam mode made by focusing the positively charged particles, and the right is for the anti-neutrino mode made by focusing the negative

ones [72].

2.4

Super-K Detector

The Super-K detector is a neutrino observatory, located near the Kamioka section of the city of Hida in Gifu prefecture, Japan [73]. It is located 1000 m under Mount Ikeno and consists of a cyclindrical stainless steel tank filled with 50,000 tonnes of ultra pure water. Mounted around the detector are approximately 11,000 photomultiplier tubes (PMTs) which can detect tiny flashes of light in the large water volume. A schematic of the Super-K detector is shown in Figure2.8.

When a neutrino from a natural source or from the neutrino beam interacts with electrons or nuclei in the water, a charged particle can be produced. If this charged particle is travelling faster than the local speed of light in that medium (i.e. ultra pure water), a cone of light is produced. This is known as ‘Cherenkov Radiation’ [74]. The Cherenkov light cone projects a ring on to the wall of the detector. This is detected by the PMTs and a recorded ring is reconstructed by the software. If the event is caused

FIGURE2.8: A schematic view of the SuperK detector in Kamioka. Out- lined on the image are various important features of the detector used in

the T2K experiment [68].

by an electron-neutrino, in the charged-current case it will produce an electron in the interaction. Due to electromagnetic scattering in the water with atomic electrons, this results in a fuzzy ring projected on to the wall of the detector. If the event is caused by a muon neutrino, in the charged-current case then a muon will be created in the interaction. These are often highly relativistic and therefore travel in a straight line through the detector, creating a sharp edged ring in the detector. Example Super-K event displays of both of these typical cases are shown in Figure2.9.

FIGURE 2.9: Example Super-K event displays from a muon-neutrino interaction (left) and electron-neutrino interaction (right) showing the characteristic sharp and fuzzy Cherenkov radiation rings for these in-

teraction types. [75]

We know that neutrinos come in three weakly interacting flavours, and oscillate between these states. Therefore, we would expect someντ to interact in Super-K and

2.4. Super-K Detector 49

produce charged τ leptons. However, this does not happen commonly for three pri- mary reasons : the mass of theτ is large (1777 MeV) and thus a neutrino would require a significant amount of energy to produce aτ in a charged-current interaction. Most neutrinos produced by the T2K beam have a peak energy of 600-800 MeV, which is not sufficient to produce aτ in a charged-current interaction. Therefore they could only be produced by neutral current modes, which are more difficuly to detect due to the absence of a charged-lepton in the final state. Even if aτ is produced, due to its large mass, it is unlikely that aτ would reach super-local-luminal speeds in the detector, and given theτ lifetime is very short (6.99 x 10−13s) it would be unlikely to emit Cherenkov radiation in that time. Nonetheless, Super-K has performed an analysis of atmospheric

τ neutrinos, reconstructing∼180.1±44.3 (statistical)±17.8

15.2 (systematic) [76]τ leptons

produced in the 22.5 kton fiducial volume of the detector by tau neutrinos during the 2806 day running period. This was not achieved by direct observation of Cherenkov light fromτleptons, but instead by selecting a statistical sample of the hadronic decays ofτ leptons leptons using its multi-ring event sample.

FIGURE2.10: An electron ring (left) and aπ0_{fuzzy double ring (right) in}

the Super-K event display. The similarities of these stuctures is apparent, although theπ0_{does fully occur within a much shorter time period (as}

indicated by the z-axis colours) [78].

π0 particles can be reconstructed by Super-K, as the two photons produced by its decay (π0 → γγ 98.8% of the time [77]) create two separate electromagnetic showers,

which then produce electron-like Cherenkov radiation rings. These rings can be com- bined to reconstruct the originalπ0meson (by recombining its decay products). How- ever, in Super-K, if either of the decay photons is not energetic enough to produce an electromagnetic shower above the Cherenkov threshold, or the two electron-like Cherenkov rings indistinguishably overlap, then the event may be mis-identified as an electron-neutrino event. The Cherenkov threshold energies for Super-K are given for relevant particles here : Electron : 0.8 MeV, Muon : 160 MeV, Pion : 213 MeV, Proton : 1.4 GeV. Thereforeπ0from Neutral-Current interactions are potentially one of the most significant backgrounds to electron-neutrino appearance searches by the T2K experi- ment, and one of the motivations for this analysis.

In 2013, a new method for event reconstruction in large Cherenkov detectors was developed called FiTQun [79], which then supplemented the existing set of reconstruc- tion algorithms in Super-K. FitQun uses a likelihood fitting approach, utilising charge and time probability distribution functions and fitting using a number of kinematic variables (vertex position, track momentum, track direction) simultaneously. Before the implementation of FiTQun, 40% of the νe appearance background was from π0s [79], where the second photon was missed. The FitQun π0 fitter uses the result of a single track fit, yields an electron hypothesis, and then performs an additional fit, tak- ing into account the momenta and conversion lengths of each decay photon. Using a 2D likelihood ratio vsπ0mass cut, this new approach removes 75 % of the remaining

π0background, without any depreciation in electron signal efficiency. Nonetheless, de- spite these significant improvements, it is still beneficial to understandπ0_{production in}

charged-current events, not only for further background reduction, but for a multitude of physics reasons, explored in the motivation section of Chapter4.