Physical Review Letters ( IF 8.385 ) Pub Date : 2020-10-15 , DOI: 10.1103/physrevlett.125.164802
A. Antognini; N. J. Ayres; I. Belosevic; V. Bondar; A. Eggenberger; M. Hildebrandt; R. Iwai; D. M. Kaplan; K. S. Khaw; K. Kirch; A. Knecht; A. Papa; C. Petitjean; T. J. Phillips; F. M. Piegsa; N. Ritjoho; A. Stoykov; D. Taqqu; G. Wichmann (muCool Collaboration)

We demonstrate efficient transverse compression of a $12.5\text{\hspace{0.17em}}\text{\hspace{0.17em}}\mathrm{MeV}/\mathrm{c}$ 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 (rms) within $3.5\text{\hspace{0.17em}}\text{\hspace{0.17em}}\mu \mathrm{s}$. The simulation including cross sections for low-energy ${\mu }^{+}$-He elastic and charge exchange (${\mu }^{+}↔$ 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.

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