New precision measurements of the muonic X-ray transition in 24Mg and 28Si: Vacuum polarisation test and search for muon-hadron interactions beyond QED

Abstract Motivated by the importance of scalar particles in the current work on gauge theories of fundamental interactions, we have performed an improved muonic-atom experiment to search for long-range muon-hadron interactions. We remeasured the wavelengths of the 3 d 5 2 −2 p 3 2 X-ray transitions in 24 Mg and 28 Si with the bent-crystal spectrometer at the SIN muon channel. The relative difference between the X-ray wavelength λ exp and the theoretical value as obtained from QED calculations, averaged over the two elements, is λ exp -λ QUED /λ QUED = (-0.2 +-3.1) × 10 6 Assuming the validity of the QED calculations, we can put limits on an additional muon-nucleon interaction: If such an interaction were mediated, for example, by a scalar or vector (isoscalar) boson with a mass smaller than about 1 MeV, the corresponding coupling constant is g μ g N ⩽ 0.8 × 10 −6 × e 2 . Alternatively, if additional muon-hadron interactions are negligible, our result corresponds to a 950 ppm test of the vacuum polarisation effect in QED. The result can also be interpreted as a 3 ppm measurement of the negative muon mass.