The muonium atom as a probe of physics beyond the standard model

Abstract

Abstract. The observed interactions between particles are not fully explained in the successful theoretical description of the standard model to date. Due to the close confinement of the bound state muonium (M = µ + e − ) can be used as an ideal probe of quantum electrodynamics and weak interaction and also for a search for additional interactions between leptons. Of special interest is the lepton number violating process of sponteanous conversion of muonium to antimuonium.

Muonium-Antimuonium Conversion

“…M-M conversion is of great interest and new experiments with improved apparatus exploiting the time dependence of the conversion process could reach substantially more stringent bounds [15]. In the recent years the upper limit established in the MACS experiment has been exploited to disfavor single flavor-violating axion-like particle (ALP) based explanations for anomalies observed in electron and muon g-2 measurements [6].…”

“…Since the MACS experiment reached its possible sensitivity limit, an improved concept and a refined setup are required to establish tighter bounds. At a pulsed muon source one can benefit from exploiting the time evolution of the conversion process [15]. All muon decay related background decreases on a time scale given by the µ + lifetime.…”

“…In particular he ratio of M to µ + -decays increases further with time. Therefore an enhanced signal to background ratio could be expected from experiments in which the time from M formation and the subsequent M-or M-decay can recorded [15]. This would favor future experiments at intense future pulsed muon sources [17].…”

Lepton flavour violation experiments—some recent developments

“…The setup at PSI (Fig. 5) [44] is designed to employ the signature developed in a predecessor experiment at LAMPF, which requires the coincident identification of both particles forming the antiatom in its decay [45,46]. Muonium atoms in vacuum with thermal velocities, which are produced from a SiO 2 powder target, are observed for antimuonium decays.…”

Muonium-antimuonium conversion in models with heavy neutrinos

“…This phenomenon was first suggested almost 50 years ago [1] and since then several studies [2,3] and experimental searches [4] have been done. Many extensions of the SM could cause muonium to antimuonium conversion [5] (left-right symmetric gauge models, extra neutrinos, extra higgses, SUSY, bileptonic gauge bosons, etc).…”