Neutrino Decay of Positronium in the Standard Model and Beyond

Whether in the Standard Model or beyond it, neutrinos contribute to the invisible decay mode of orthopositronium but practically not at all to that of parapositronium. Although this remark does not resolve the orthopositronium decay puzzle, it allows for upper bounds to be set on neutrino magnetic moments. UCL-IPN-96-P01 February 1996 E-mail address: [email protected] 1. Positronium decay[1, 2] has provided for a number of years a somewhat uncomfortable crack in this otherwise most beautiful edifice which is QED. Indeed, the two most precise experimental rates for the C-odd triplet orthopositronium (o-PS) decay[3, 4], λ T = 7.0514 ± 0.0014 μs , λ T = 7.0482 ± 0.0016 μs , (1) are in flagrant conflict with the QED prediction[5, 6, 2], λ T (PS → 3γ) = 7.03831 ± 0.00005 μs . (2) Note that the experimental results correspond, respectively, to a 9.4 σ and a 6.2 σ deviation from the theoretical expectation, and in relative terms, to the differences, λ T − λ T λ T = ( 1.86 or 1.41 ) · 10 . (3) In contradistinction, the C-even singlet parapositronium (p-PS) decay rate, of which the most precise measurement gives[7], λ S = 7990.0 ± 1.7 μs , (4) compares better with the theoretical expectation[8, 6], λ S (PS → 2γ) = 1 2 αmec 2 h̄ [ 1− α π ( 5− π 2 4 )