Cambridge MINOS Group

MINOS Far Detector

Andy Blake, John Chapman, Andy Culling, Caius Howcroft, John Marshall, Jessica Mitchell, Mark Thomson, Ruth Toner, David Ward, Pat Ward

Introduction

The MINOS ("Main Injector Neutrino Oscillation Search") experiment was a neutrino oscillation experiment located at Fermilab in the United States. The experiment started taking data in 2005 and published results on muon neutrino disappearance and other topics related to neutrino oscillation. The Cambridge MINOS group was involved in several of these research efforts.

Neutrino Oscillations

Neutrino physics has become an increasingly exciting field in particle physics over the past two decades. Numerous experiments have suggested that neutrinos, one of the fundamental particles in the Standard Model of particle physics, can undergo oscillation from one lepton flavour to another. There are three known flavours of neutrino: electon, muon, and tau. The oscillation between these flavours can be described by a theoretical mixing matrix (the "PMNS Matrix"). The PMNS matrix is a set of "mixing angle" parameters (θ23, θ13,etc) which describe the probability of one flavour oscillating to another. Neutrino oscillation also relates to another problem, that of neutrino mass. While neutrinos were previously thought to be massless, the presence of oscillation suggests that they have some small unknown mass. The differences between the three mass states, such as |Δm32 2|=|m32 - m22|, also affect the oscillation probability. The aim of MINOS was to observe oscillations and explore these neutrino parameters with greater accuracy than previous experiment.

The MINOS experiment

The MINOS experiment consisted of two separate detectors: a "Near" detector at Fermilab (Illinois, USA), and a "Far" detector in Soudan (Minnesota, USA). A beam of muon neutrinos (the NuMI beamline) was created at Fermilab from Main Injector protons and aimed at the two detectors. Both detectors consisted of alternating layers of steel calorimeter and plastic scintillator, immersed in a toroidal magnetic field. This design permitted both calorimetric and topological event reconstruction. The combined Near and Far detectors were used to investigate neutrino oscillation over the distance (735 km) separating the two detectors. This could appear as a deficit of muon neutrinos in the Far detector, for instance, or as a surplus of electron neutrinos.

The Cambridge group was involved in many aspects of the experiment, from detector operations through to data analysis. Our main activities are summarized below:

  • MINOS Physics Analyses: We collaborated with physicists from across the experiment in using the MINOS data to perform a precise measurement of the neutrino oscillation parameters. One focus of our research was the development of techniques to improve the precision of this measurement by exploiting the intrinsic sensitivity of individual neutrino interactions. We also lead the effort to analyse atmospheric neutrinos in the MINOS Far detector. This was the first massive underground detector to be magnetized, enabling atmospheric neutrinos and anti-neutrinos to be studied separately for the first time. This allowed MINOS to perform direct tests of CPT conservation in the neutrino sector.

  • Data Reconstruction and Calibration: We developed algorithms for reconstructing the trajectories and kinematics of particle tracks in the MINOS data, enabling the identification and reconstruction of neutrino interactions. We were also responsible for calibrating the timing system at the Far detector. This enabled the identification of muons travelling upwards through the detector, a characteristic signature of atmospheric neutrino interactions.

  • Data Acquisition and Monitoring: We were responsible for the control software used to operate both the MINOS Near and Far detectors. We also shared the responsibility for monitoring the data collected by each detector.