New monochromatic muon beam channel using two body decay K⁺ → μ⁺v at rest in a production target

A new type of monochromatic μ⁺ beam channel of high resolution was designed and constructed at the KEK 12 GeV proton synchrotron (KEK-PS). The principle of the channel is to transport 236 MeV/c μ⁺ emitted in the two body at rest decay of positive kaons, K⁺ → μ⁺v, in the production target as a beam. In order to eliminate background μ⁺ from in-flight π⁺ decay, the channel has the following features. Firstly, the μ⁺ beam is extracted at a backward angle of 147° from the direction of the 12 GeV primary proton beam, where the π⁺ yield is relatively small. Secondly, the distance from the production target to the first bending magnet is designed to be as short as possible. Thirdly, the high momentum resolution of the beam channel also enables us to observe a very sharp peak in the μ⁺ momentum spectrum, so that the background is efficiently subtracted. The typical intensity of the 236 MeV/c monochromatic μ⁺ is 3500 per 10¹² incident protons on a thin Pt target. The signal-to-background ratio finally achieved at the peak momentum was 60. This pure Kμ₂ muon beam was successfully utilized for an experiment designed to search for V + A currents in the weak interaction from a precise determination of the μ⁺ longitudinal polarization.