On the west coast of Japan, the T2K far detector is located in a former mine under Mt. Ikenoyama. Super-Kamiokande reported its first results operating as a neutrino observatory in 1998, studying atmospheric and solar neutrinos. To shield it from cosmic ray muons Super-Kamiokande is 2,700 m.w.e (Meter Water Equivalent) or 1000 m of rock underground. Filled with 50 kt of water,
Figure 3.4: A diagram of the Super-Kamiokande Detector. Figure adapted from [66].
Super-Kamiokande is the largest neutrino detector in the world [65]. Super- Kamiokande I completed data taking in 2001, when the detector was upgraded. During a refill of the detector, a damaged photosensor imploded, the shock of which destroyed many of the remaining photosensors. Run II of the Super- Kamiokande experiment was carried out with 47% of the photosensors used in the full detector, and stopped in 2006 for new photosensors to be installed [43]. Returned to full photosensor coverage, Super-Kamiokande III took data until it was upgraded to Super-Kamiokande IV. Super-Kamiokande IV has been completed for T2K data taking, and consists of new electronics with updated reconstruction software.
The Super-Kamiokande detector is 42 m in height and 39 m in diameter, split into two regions - the inner detector (ID) and outer detector (OD). The inner detector consists of 11,146 photomultipliers of 50 cm diameter and is 36.2 m
high and 16.9 m in radius, and is used for observing signal events. The outer detector is used as a veto for cosmic ray muons that penetrate the rock, as well as other particle interactions in the material surrounding the detector. The outer detector uses 1,885 photomultipiers attached to a metal bulkhead separating the inner and outer detectors.
Events in Super-Kamiokande are classed as either FC (Fully Contained) or PC (Partially Contained). An event is FC if a signal is measured on the inner detector. If however, a signal is measured in the outer detector consistent with an outgoing particle, then the event is tagged PC. In a PC event it is only possible to put a lower limit on the energy of the neutrino, as at least some energy has escaped. Stopping muon events tagged by the outer detector as having come from the atmosphere are useful for calibration, and occur with a frequency of approximately 2 Hz.
3.4.1
Super-Kamiokande Reconstruction
A water ˇCerenkov detector, such as Super-Kamiokande, uses high gain Photo Multiplier Tubes (PMTs) to detect the ˇCerenkov light given off by a medium when a particle travelling faster than the phase velocity of light in that medium passes through. When a charged particle passes through a non-conductive material temporary polarisation occurs. If the particle is travelling faster than the phase velocity of light, then at a certain angle with respect to the direction of motion of the charged particle light will be emitted [67]. The angle is given by the expression,
cosθ = c
vn =
1
βn. (3.1)
the detector, centred on the direction of motion of the particle. Particle iden- tification of electrons and muons considers how well defined the ring of PMT hits are. A massive particle such as a muon will not radiate bremsstrahlung or scatter significantly in the water, creating a well defined ring as shown in Figure 3.5. Electrons however, lose energy radiatively and scatter off atoms; showering to create a number of lower momentum secondary electrons and positrons, which can also give off ˇCerenkov light. A ring due to an electron therefore has a poorly defined edge and is known as a ‘fuzzy ring. An example of such an event can be seen in Figure 3.5. In the case of a muon interaction, a second signal can be observed later in time, due to the Michel electron from the decay of the muon.
A large background in the electron neutrino analysis, due to π0 events, moti-
vates the P0D detector in ND280. The π0 background is caused by co-linear
photons, or highly asymmetric events where one photon takes most of the π0
energy and the second photon is not reconstructed. If both gammas are close to co-linear, then the pair of rings will lie on top of each other and the ring finding algorithm will see a ‘fuzzy’ ring and incorrectly identify the particle as an electron.
In Chapter 6 an electron neutrino analysis is designed using the ND280 de- tector. This analysis measures the electron neutrino flux at the near detector. Extrapolating this measurement to the far detector provides an estimate of what would be seen by Super-Kamiokande in the absence of νµ → νe oscilla-
tions. An excess of electron neutrinos at Super-Kamiokande would represent a non-zero value of θ13.
Figure 3.5: Event displays from Super-Kamiokande. An electron event with a ‘fuzzy’ ring is shown (top left) and a sharper muon ring (top right). A two ring
π0 event is shown (bottom left) as well as a more complicated event containing
three rings (bottom right). The relative timing of PMT hits is indicated by the colour of the square and the magnitude by its size [68].