 The neutrino was a <unk> particle first proposed theoretically by Wolfgang <unk> on December 4 , 1930 , to explain undetected energy that escaped during beta decay when neutron decayed into a proton and an electron so that the law of conservation of energy was not violated . Enrico Fermi renamed it the neutrino , Italian for " little neutral one " , and in 1934 , proposed his theory of beta decay which explained that the electrons emitted from the nucleus were created by the decay of a neutron into a proton , an electron , and a neutrino : 
