Meanwhile there have lately been intriguing hints of a possible violation of the conservation of lepton number. In the standard model this conservation law is responsible for keeping the neutrinos massless, and with this conservation law violated we would expect neutrinos to have small masses, about a hundredth to a thousandth of a volt (or in other words, about one billionth the mass of an electron). This mass is much too small to have been noticed in any laboratory experiment done so far, but it could have a subtle effect, of allowing neutrinos that start out as electron-type neutrinos
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