Muon polarisation in the neutrino factory

Muon polarisation leads to interesting effects and could be useful in enhancing one flavour of neutrinos over the other. Preserving it leads to some design constraints on the accelerators, on the beam energy, and on the storage ring. The natural polarization of 27% or more can be preserved in the capture cooling and acceleration process without too much difficulty. Preserving this level against the depolarising effect of energy spread in the storage ring requires either (i) a bow-tie geometry (no spin precession) or (ii) for a standard ring with spin precession, an RF bunching system that induces synchrotron oscillations with a synchrotron tune Qs≃σ(E)/E. In the case of a standard ring, spin precession will normally reduce the effective longitudinal polarisation from 27% to 18%, unless the beam energy is chosen to be exactly (i.e. within 2×10−4) E=45.311 GeV, or multiple thereof. Odd multiples of this number provide the additional advantage of automatic polarisation reversal turn after turn. These constraints do not apply for bow-ties, which however require spin manipulation devices prior to injection in the muon decay ring. The polarisation figure of merit being the product of the neutrino flux by the square of the polarisation, improvements on the natural muon polarisation at the source have to be weighted accordingly against the loss of flux.