A CW-pion focusing horn for low-energy muon neutrino beams

Low-energy muon neutrino beams can be produced from pion decays in-flight at high-intensity accelerators, such as the Los Alamos Meson Physics Facility (LAMPF), providing a new tool to study the role of the weak interaction in nuclear and particle physics. Employing a pion focusing device can enhance the neutrino flux by large factors, and reduce backgrounds by sign-selection of the parent pions. However, LAMPF's long beam pulse and high repetition rate makes it impractical to use pulsed horns like those found at high-energy accelerators. In this paper we discuss a CW-pion focusing device that uses coils wound inside vanes mounted radially around the beam axis to provide an azimuthal field. From our studies with a prototype magnet, we have found the optimum field configuration needed to focus pions at LAMPF energies can be obtained by adjusting the radial density of turns in the coils. This optimum yields an eight-fold increase in neutrino flux above the muon threshold over the bare-target case. Our calculations also indicate a correlation between the arrival time of the neutrinos in the detector and their energy.