A silicon detector for neutrino physics

In order to demonstrate the feasibility of conducting future muon neutrino - tau neutrino oscillation searches using a high-resolution, large-area silicon microstrip detector, the Silicon TARget (STAR) detector was built. STAR was installed in the NOMAD short baseline neutrino oscillation experiment at the CERN SPS neutrino beam, where it recorded approximately 10000 neutrino interactions during the operation of the detector in the period 1997-98. It consists of five layers of silicon detectors interleaved with four layers of passive boron carbide as the target. The target mass is 45 kg, while the total silicon surface area is 1.14 square-meters and contains 32000 readout channels. The individual modules have a length of 72 cm, the longest built to date. The detection of tau particles, produced in tau neutrino charged-current interactions, would require a tracking detector with a precision of a few tens of microns in order to measure the position of the neutrino interaction vertex as well as the impact parameter of the tau decay products. The performance of STAR was studied by reconstructing the decays of K-short mesons produced in muon neutrino charged-current interactions. For both the primary and secondary vertices, the resolution in the y direction was found to be approximately 20 microns, while that in the z direction was found to be approximately 100 microns. The double vertex resolution, a measure of how accurately the distance between the vertices can be measured, was found to be approximately 20 microns and 300 microns in the z direction. The impact parameter resolution of the muons resulting from muon neutrino charged-current interactions, with respect to the primary vertex, was found to be 25 microns. The vertex resolution and impact parameter results show that a microstrip silicon detector would be well-suited to measuring muon neutrino - tau neutrino oscillations. The high precision of silicon detectors has several other applications within neutrino physics, such as using a silicon detector as the near-detector in a future neutrino factory facility.