Demonstration of Muon-Beam Transverse Phase-Space Compression

We demonstrate efficient transverse compression of a 12.5 MeV/c muon beam stopped in a helium gas target featuring a vertical density gradient and crossed electric and magnetic fields. The muon stop distribution extending vertically over 14 mm was reduced to a 0.25 mm size (nns) within 3.5 mu s. The simulation including cross sections for low-energy mu(+)-He elastic and charge exchange (mu(+) <-> 44 muonium) collisions describes the measurements well. By combining the transverse compression stage with a previously demonstrated longitudinal compression stage, we can improve the phase space density of a mu(+) beam by a factor of 10(10) with 10(-3) efficiency.