Forrest, David Alexander James
The Muon Ionisation Cooling Experiment.
PhD thesis, University of Glasgow.
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Outstanding areas of ambiguity within our present understanding of the nature and behaviour of neutrinos warrant the construction of a dedicated future facility capable of investigating the likely parameter space for the theta 1,3 mixing angle, the Dirac CP violating phase and clarifying the neutrino mass hierarchy. A number of potential discovery venues have been proposed including the beta beam, superbeam and neutrino factory accelerator facilities. Of these, the neutrino factory significantly outperforms the others.
A neutrino factory will deliver intense beams of 10^21 neutrinos per year, produced from muons decaying in storage rings. This specification, coupled with the constraints of the short muon lifetime warrant the inclusion of a novel cooling channel to reduce the phase space volume of the beam to fall within the acceptance of the acceleration system.
Ionisation cooling is the only viable cooling technique with efficacy over the lifetime of the muon, however, it has yet to be demonstrated in practice. In a full cooling channel, a muon beam will traverse a periodic absorber and accelerator lattice consisting of low Z absorbers enclosed by focusing coils and accelerating radio-frequency cavities. Energy loss in the absorbers reduces both transverse and longitudinal momentum. The latter is restored by the accelerating cavities providing a net reduction in transverse momentum and consequently reducing the phase space volume of the muon beam.
The Muon Ionisation Cooling Experiment (MICE), under construction at the ISIS synchrotron at Rutherford Appleton Laboratory seeks to provide both a first measurement and systematic study of ionisation cooling, demonstrated within the context of a single cell prototype of a cooling channel. The experiment will evolve incrementally toward its final configuration, with construction and scientific data taking schedules proceeding in parallel. The stated goal of MICE is to measure a fractional change in emittance of order 10% to an error of 1%.
This thesis constitutes research into different aspects of MICE: design and implementation of the MICE configuration database, determination of the statistical errors and alignment tolerances associated with cooling measurements made using MICE, simulations and data analysis studying the performance of the luminosity monitor and a first analysis of MICE Step I data.
A sophisticated information management solution based on a bi-temporal relational database and web service suite has been designed, implemented and tested. This system will enable the experiment to record geometry, calibration and cabling information in addition to beamline settings (including but not limited to magnet and target settings) and alarm handler limits. This information is essential both to provide an experimental context to the analysis user studying data at a later time and to experimenters seeking to reinstate previous settings. The database also allows corrections to be stored, for example to the geometry, whereby a later survey may clarify an incomplete description. The old and new geometries are both stored with reference to the same period of validity, indexed by the time they are added to the configuration database. This allows MICE users to recall both the best-known geometry of the experiment at a given time by default, as well as the history of what was known about the geometry as required. Such functionality is two dimensional in time, hence the choice of a bi-temporal database paradigm, enabling the collaboration to run new analyses with the most up to date knowledge of the experimental configuration and also repeat previous analyses which were based upon incomplete information.
From Step III of MICE onwards, the phase space volume, or emittance, of the beam will be measured by two scintillating fibre trackers placed before and after the cooling cell. Since the two emittance measurements are made upon a similar sample of muons, the measurement errors are influenced by correlations. This thesis will show through an empirical approach that correlations act to reduce the statistical error by an order of magnitude.
In order to meet its goals MICE must also quantify its systematic errors. A misalignment study is presented which investigates the sensitivity of the scintillating fibre trackers to translational and rotational misalignment. Tolerance limits of 1 mm and 0.3 mrad respectively allow MICE to meet the requirement that systematic errors due to misalignment of the trackers contribute no more than 10% of the total error.
At present, MICE is in Step I of its development: building and commissioning a muon beamline which will be presented to a cooling channel in later stages of MICE. A luminosity monitor has been built and commissioned to provide a measurement of particle production from the target, normalise particle rate at all detectors and verify the physics models which will be used throughout the lifetime of MICE and onwards through to the development of a neutrino factory.
Particle identification detectors have already been installed and allow the species of particles to be distinguished according to their time of flight. This has enabled a study of particle identification, particle momenta and simulated and experimental beam profiles at each time of flight detector. The widths of the beam profiles are sensitive to multiple scattering and magnetic effects, providing an opportunity to quantify the success of the simulations in modelling these behaviours. Such a comparison was also used to detect offsets in the beam centre position which can be caused by misalignments of the detectors or relative misalignments in magnet positions causing asymmetrical skew in the magnetic axis. These effects were quantified in this analysis.
Particle identification combined with the earlier statistical analysis will be used to show that the number of muons required to meet the statistical requirements of MICE can be produced within a realistic time frame for each beam configuration considered.
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