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Description
Low-energy beams of positive muons, at tens of keV and below, allow for experiments in the field of fundamental particle physics, such as the search for the muon electric dipole moment [1], exotic atoms physics with muonium spectroscopy and gravitational experiments [2] and numerous material science applications thanks to muon-spin resonance (muSR) technique [3]. Current sources of muons suffer from large spatial and momentum spread, with transverse size on the ~cm scale, which limits the precision of previously mentioned experiments. The challenge of improving the beam quality stems from the relatively short lifetime of muons (~$2.2 \mu s$).
At Paul Scherrer Institute (PSI) we are developing a novel concept of phase-space cooling of positive muon beams (muCool), based on buffer gas cooling technique [4]. Muons of a few MeV are stopped in the cryogenic helium gas target with vertical density gradient, placed in the homogenous magnetic field combined with complex electric field. Engineering of the collisional frequency with helium atoms and guiding muons drift direction inside the gas target leads to transverse and longitudinal compression of the muon beam to the sub-mm size and cooling to a few eV energy [5]. Consequently muons are extracted from the helium gas target to the vacuum via windowless orifice, demonstrating the feasibility of generating low-energy and high-brightness beams of positive muons. Future steps will involve extraction out of the strong magnetic field and re-acceleration to keV energies to provide novel high quality muon beams to experiments. Tunable energy range at the acceleration stage, together with future High Intensity Muon Beam (HIMB) upgrades of the CHRISP facility at PSI [6], will pave a pathway to new generation precision measurements and development of new imaging technologies.
[1] A. Adelmann et al., Search for a muon EDM using the frozen-spin technique (2021), 2102.08838.
[2] A. Soter and A. Knecht, Development of a cold atomic muonium beam for next generation atomic physics and gravity experiments, SciPost Phys. Proc. p. 031 (2021), doi:10.21468/SciPostPhysProc.5.031.
[3] A. Yaouanc and P. de Réotier, Muon Spin Rotation, Relaxation, and Resonance: Applications to Condensed Matter, International Series of Monographs on Physics. OUP Oxford, ISBN 9780199596478 (2011).
[4] D. Taqqu, Compression and extraction of stopped muons, Phys. Rev. Lett. 97, 194801 (2006), doi:10.1103/PhysRevLett.97.194801.
[5] A. Antogniniet al., Phase space compression of a positive muon beam in two spatial dimensions, SciPost Phys. Core 8, 071 (2025), doi: 10.21468/SciPostPhysCore.8.4.071
[6] M. Aiba, A. Amato, A. Antognini, S. Ban, N. Berger, L. Caminada, R. Chislett, P. Crivelli, A. Crivellin, G. D. Maso, S. Davidson, M. Hoferichter et al., Science Case for the new High-Intensity Muon Beams HIMB at PSI (2021), 2111.05788.