Current status and prospects of muonium spectroscopy at PSI

Abstract

Recent and ongoing developments of low energy muon beamlines are heralding a new era of precision Muonium spectroscopy. While past spectroscopic measurements of Muonium were performed at pulsed muon facilities and were statistically limited, the advent of continuous low energy muon beams, such as at the LEM beamline at PSI, paired with the development of efficient muon-muonium converters and laser advancements, will overcome these limitations. Current experiments presently underway at the LEM facility and in the near future at the muCool beamline, which is under development at PSI, aim to improve the precision of both the 1S-2S transition determination and Lamb shift by several orders of magnitude. In this Chapter we give an overview of the current status and future prospects of these activities at PSI, highlighting how their projected significance fits into a broader context of other ongoing efforts worldwide.

Precision Measurement of the Lamb Shift in Muonium

“…With a longer lifetime of 2.2 μs limited by the muon decay, and a larger mass, M constitutes a promising system for spectroscopic measurements [18,19].…”

“…The realization of the high intensity muon beam (HiMB) at PSI [54] would open the way to push the accuracy of this experiment to its ultimate limit of few tens of kHz. Combined with other ongoing M spectroscopy experiments [19,55,56], one would probe the very interesting region of parameter space for a new scalar or vector boson with m ϕ > 300 keV where astrophysical bounds do not apply [57].…”

The proton structure in and out of muonic hydrogen

“…Note that a better benchmark test (δ ∼ 2×10 −9 ) of bound-state QED for a hyperfine transitions can be achieved for the muonium hfs, which the MuSEUM experiment (114) aims to measure with δ ∼ 2 × 10 −9 relative accuracy. To test the muonium hfs on this level, the MuMass experiment (115,116) has to determine the mµ me ratio to better than δ ∼ 1 × 10 −9 from the 1S-2S transition in muonium.…”

A theory vade mecum for PSI experiments

“…As the latter is a pointlike fermion, muonium is an excellent laboratory for QED tests, and for a precise determination of the muon mass. 7 As the muonium mass is dominated by antimatter, M is also an interesting option to study experimentally gravity of antimatter [75]. In addition, muonium-antimuonium oscillations M = (µ + e − ) ↔M = (µ − e + ) , (5.40) which are forbidden in the SM, are another channel to scrutinize BSM physics.…”