De Leener- Rosier, N., Naviliat- Cuncic, O., Amsler, C., Schaad, M., Truoel, P., Joseph, C., Perroud, J. P., Tran, M. T., Deutsch, J., Lebrun, M., Prieels, R., & van Elmbt, L. (1991). Search for massive neutrinos in pi ---> e neutrino decay. Physical Review D, Particles, Fields, Gravitation and Cosmology, 43, 3611-3618. https://doi.org/10.1103/PhysRevD.43.3611 (Original work published 1991)