There are 62 institutes across 12 different countries contributing to the T2K experiment. Within this broad collaboration Warwick has made some important and significant contributions towards construction, quality assurance, calibration hardware and software analysis.

P0D ECal Construction

The T2K near-detector (ND280) is made up of mulitple sub-detectors, each of which is modular in construction. Around the P0D (Pi Zero detector) is placed an electromagnetic calorimeter (ECal) for measuring the energy of photons from decaying Pi0s in the P0D.

The detector consists of two side modules and four smaller modules on the top/bottom and all six are being assembled in the Warwick EPP lab. Each contains six layers of scintillator bars sandwiched between lead sheets, those belonging to the side modules are also manufactured in Warwick while those for the top/bottom modules are constructed by our collaborators at the University of Sheffield.

During construction each layer is craned into the module in-turn and scanned using a radioactive source, testing the integrity of the scintillator bars and the optical fibres within them. Once all the layers have been put in we procede to instrument the module with photosnsors and read-out electronics, cooling pipes temperature sensors etc.

Construction of the last of the six modules was completed at the start of August 2010 well within schedule for installation in September/October. They will play an important role in catching particles "escaping" from the central P0D detector, which will help T2K increase precision on its target to measure ν_μ → ν_e oscillations.

Event Reconstruction

The Warwick group is heavily involved in the reconstruction software. The involvement is based on reconstruction of particles in the electromagnetic calorimeters, and this work ranges from the simulation of the module triggering process to the particle identification algorithms.

Work is also underway on the analysis of data. The group is working on tagging electrons via emitted Bremsstrahlung photons in order to measure the electron neutrino contamination in the neutrino beam. Studies have also begun on looking into finding charged pion events in an attempt to measure the cross sections for the process - an essential measurment for predicting the neutrino signal at Super-K.

Photosensors

All of the scintillator-based subdetectors in ND280, including the calorimeter modules, are read out using a newly developed photodetector, the Multi-Pixel Photon Counter (MPPC). These devices are capable of similar performance to photomultiplier tubes, while being cheaper and much more compact.

Top: Exploded view of a photosnesor, its protective shrouding and PCB connector

Bottom: Comparison of MPPC data with the prediction from the simulation

Because T2K is pioneering the use of MPPCs, it is essential that we test the devices before they go into the detector, to ensure that they meet our requirements. The test stands used to perform these checks were designed and built at Warwick; Warwick has also played a leading role in the development of the analysis software for these tests, and was responsible for the physical testing of half of the devices destined for the downstream ECal.

Warwick has also made large contributions to the development of a Monte-Carlo simulation of the MPPCs. This allows us to accurately simulate the response of an MPPC to light signals of different durations and intensities. This code forms part of a larger simulation of ND280, which accurately models the behaviour of the whole detector.

Calibration

Variation between the different photosensors and their respective readout electronics leads to a varying detector response, depending on where particles pass through. Environmental conditions, such as the ambient temperature also affect the detector performance.

All of these effects must be accounted for by calibrating the detector to have a uniform response to known signals. Part of the work carried out by the group is to develop a scheme for calibrating each component, and working out how to obtain the calibration data. The group is also writing the software framework that allows many different types of calibration to be developed independently and chained together.