Intense beam of metastable Muonium

G Janka,Z Salman,L Gerchow,R Nishi,P Crivelli,X Ni,N Kuroda,A Suter,T Prokscha,B Ohayon,M Tuzi,Z Burkley,C Vigo

doi:10.1140/epjc/s10052-020-8400-1

G Janka, Z Salman + Show 11 more

Open Access

https://doi.org/10.1140/epjc/s10052-020-8400-1

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Abstract

Precision spectroscopy of the Muonium Lamb shift and fine structure requires a robust source of 2S Muonium. To date, the beam-foil technique is the only demonstrated method for creating such a beam in vacuum. Previous experiments using this technique were statistics limited, and new measurements would benefit tremendously from the efficient 2S production at a low energy muon (<20 keV) facility. Such a source of abundant low energy {mu }^{+} has only become available in recent years, e.g. at the Low-Energy Muon beamline at the Paul Scherrer Institute. Using this source, we report on the successful creation of an intense, directed beam of metastable Muonium. We find that even though the theoretical Muonium fraction is maximal in the low energy range of 2–5 keV, scattering by the foil and transport characteristics of the beamline favor slightly higher {mu }^{+} energies of 7–10 keV. We estimate that an event detection rate of a few events per second for a future Lamb shift measurement is feasible, enabling an increase in precision by two orders of magnitude over previous determinations.

Highlights

Muonium (M) is the bound state of a positive muon (μ+) and an electron, two particles devoid of internal structure
Testing bound state quantum electrodynamics (QED) in the muonic sector is highly motivated by the inconsistencies which have arisen there, e.g. the deviation of the measured anomalous magnetic moment of the muon from its theoretical value [2], and the difference between the proton radius as measured by laser spectroscopy of muonic hydrogen [3] and several experiments in electronic hydrogen [4,5,6]
We have demonstrated the creation of an intense directed beam of Muonium in the long-lived 2S state by transmitting low energy muons from the LEM beamline through a thin carbon foil

Summary

Introduction

Muonium (M) is the bound state of a positive muon (μ+) and an electron, two particles devoid of internal structure. In the Born approximation, production of M with this beam-foil technique is expected to be comparable to hydrogen with protons at the same velocity [28], favouring energies of several keV [29,30] For this reason, the TRIUMF and LAMPF muon beams had to be significantly lowered in energy by degrader foils with the price of losing roughly half of the beam intensity. To enable the generation of precision measurements with metastable M, we set out to solve the main limitation affecting previous campaigns In this communication, we report on the efficient creation and detection of a nearly collinear M(2S) beam by the beam-foil technique. Combining our results with particle tracing simulations, we were able to quantify the M(2S) beam parameters These parameters enable a realistic estimation of the achievable event rate for a future Lamb shift measurement. We conclude that a significant improvement over the state-of-the-art is within reach

Experimental setup

Muonium fractions at different energies

Metastable beam intensity and Lamb shift outlook

Conclusions