The Small Neutral One

MORE than twenty years ago W. Pauli hypothesized the need for an undiscovered, uncharged particle of extremely small mass as an explanation for the puzzling disappearance of energy in the radioactive process of beta decay—thus preserving the respectability of the law which demands that energy be conserved. Pauli's small neutron (which Fermi reduced to the diminutive “neutrino” to avoid confusion with Chadwick's big neutron) has received powerful theoretical support bolstered by a certain amount of indirect but persuasive experimental evidence for its existence, even though it has been shown that at best the free neutrino can be expected to interact only slightly with matter.