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.

A status report is given on an experiment started in 1987 at PSI to search for the lepton flavour violating muon to electron conversion process. The motivation for this experiment, the design and performance of the detector (SINDRUM II), and results of a first run are presented. No candidate of the process mu -Ti to e-Ti was found, and, using two independent determinations of the muon stop rate, an upper limit for the branching ratio relative to muon capture Bmu e<4.4.10-12 (90% C.L.) was obtained. This result confirms the current best value for the upper limit found at TRIUMF (4.6.10-12).

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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...

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