A new upper limit for the probability of spontaneous muonium to antimuonium conversion was established at P MM ≤ 8.2 · 10 −11 (90%C.L.) in 0.1 T magnetic field, which implies consequences for speculative extensions to the standard model. Coupling parameters in R-parity violating supersymmetry and the mass of a flavour diagonal bileptonic gauge boson can be significantly restricted. A Z 8 model with radiative mass generation through heavy lepton seed and the minimal version of 331-GUT models are ruled out.

Article (Published Version) http://sro.sussex.ac.uk Meyer, V, Boshier, M G and et al, (2000) Measurement of the 1s-2s energy interval in muonium.

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A new experiment has been set up at the Paul Scherrer Institut to search for muonium to antimuonium conversion. No event was found to fulfil the requested signature which consists of the coincident detection of both constituents of the antiatom in its decay. Assuming an effective ͑V 2 A͒ 3 ͑V 2 A͒ type interaction an improved upper limit is established for the conversion probability of P MM # 8 3 10 29 (90% C.L.), which is almost 2 orders of magnitude lower compared to previous results and provides a sensitive test for theoretical extensions of the standard model. [S0031-9007(96) The hydrogenlike muonium atom (M m 1 e 2 ) consists of two leptons from different generations. Because of the close confinement of the bound state it offers excellent opportunities to study precisely the fundamental electron-muon interaction as described in standard theory and to search sensitively for additional so far unknown interactions between these two particles. A spontaneous conversion of muonium into antimuonium (M m 2 e 1 ͒ would violate additive lepton family number conservation by two units. In the standard model, which is a very successful description of experimental particle physics, this process is not provided like other decays which are searched for, e.g., the muon decay modes m 1 ! e 1 n m n e [1], m ! eg [2], m ! eee [3], and m ! e conversion [4]. However, in the framework of many speculative theories, which try to extend the standard model in order to explain further some of the features like parity violation in weak interaction or the particle mass spectra, lepton number violation appears to be natural and muonium to antimuonium conversion is an essential part in several of those models (Table I) [5-10]. The coupling constant G MM in an effective four fermion interaction [11] could be as large as the present experimental limit of G MM # 0.16G F (90% C.L.) established at LAMPF in Los Alamos [12], or the bound of G MM # 0.14G F (90% C.L.) [13] very recently proposed from an experiment at the Phasotron in Dubna, Russia, where G F is the Fermi coupling constant of the weak interaction. In particular, in the framework of minimal left-right symmetric theory a lower bound has been predicted with the assumption of a muon neutrino mass m m n larger than 35 keV͞c 2 .At the Paul Scherrer Institut (PSI) in Villigen, Switzerland, a new experiment has been set up (Fig. 1), which utilizes the powerful signature for a conversion developed in the recent LAMPF experiment [12]. It requires the coincident identification of both constituents of the antiatom, TABLE I. Muonium to antimuonium conversion is allowed in some speculative extensions to the standard model. The lower limit given for minimal left-right symmetry corresponds to a muon neutrino mass limit of m nm # 160 keV͞c 2 [21]. Model Limit Ref. Minimal left-right symmetry with extended Higgs sector; conversion G MM $ 2 3 10 24 G F [5] through exchange of doubly charged Higgs boson D 11 Z 8 model with fourth generation of heavy leptons and radiative mass G MM # 10 22 G F [7] gene...

A new experimental search for muonium-antimuonium conversion was conducted at the Paul Scherrer Institute, Villigen, Switzerland. The preliminary analysis yielded one event fulfilling all required criteria at an expected background of 1.7(2) events due to accidental coincidences. An upper limit for the conversion probability in 0.1 T magnetic field is extracted as 8 · 10 −11 (90% CL).

In the standard model lepton number violating processes are forbidden, but are allowed in many extensions to it. Muonium to antimuonium conversion has been explicitly allowed in the framework of left-right-symmetric models or in supersymmetric models. A new experiment at PSI has demonstrated its ability to improve the sensitivity to such a rare process significantly.

A new measurement of the 1S-2S energy splitting of muonium by Doppler-free two-photon spectroscopy has been performed at the Rutherford Appleton Laboratory in Chilton, Didcot, UK. Increased accuracy is expected compared to a previous experiment [1]. Spectroscopy of this transistion promises an improvement of the muon mass value.One-electron atoms, being the most fundamental atomic systems, provide excellent tests for bound state quantum electrodynamics (QED) and render the possibility of highly precise measurements of fundamental constants. As the energy levels of the natural hydrogen isotopes (hydrogen, deuterium and tritium) and hydrogen-like exotic systems with hadronic nuclei (e.g. muonic helium, pionium and many others) are inuenced by the nite size and internal structure of the hadrons, the interpretation of highly precise measurements in such systems is limited by t o d a ys insucient knowledge of the nuclear size eects. The hydrogen-like m uonium atom ( + e ) consists of two leptons from two dierent generations [2]. No internal structure is known for leptons down to dimensions of order 10 18 m; therefore muonium is free from nuclear structure eects. The level energies can be calculated to very high accuracy exclusively by the theory of bound state Quantum Electrodynamics (QED). The potential for high precision studies has been demonstrated in a long series of microwave measurements and theoretical calculations of the ground state hyperne structure splitting [2], from which accurate values for fundamental constants (muon mass m and ne structure constant ) w ere obtained [2]. The optical 1S-2S transition oers a higher resolution than the ground state hyperne structure splitting, because of the much higher transition frequencies (and QED contributions) and the same 144 kHz narrow natural linewidth, which is due to the muon lifetime 2.2sec. This experiment was performed at the worlds brightest pulsed surface muon source at the Rutherford Appleton Laboratory (RAL) in Chilton, UK.The 1 2 S 1=2 (F=1) ! 2 2 S 1=2 (F=1) transition was induced by Doppler-free two-photon laser spectroscopy using two counter-propagating laser beams of wavelength = 244 nm [1]. The atoms were formed by electron capture after stopping positive muons close to the surface of a SiO 2 powder target. A fraction of these diused to the surface and left 1