Elementary particles
THE urge and the interest to find those ultimate elements in terms of which everything else is made of are almost as old as the human civilization. However, as our knowledge increases what were thought to be elementary may turn out to be composites. Consequently, the class of these supposedly fundamental elements changes with time. Such was, for example, the periodic table of atoms in the last century. Today we know that all different molecules, atoms, and nuclei are complexes resulting from the existence and the interactions of some thirty particles which are called "elementary particles".
AS early as it as of was ft physicists decay, already suggested by several different groups of physicists that the different weak processes such as ß decay, μ decay, and μ capture may be characterized by a single universal form of interaction. However, at that time because of the lack of detailed and accurate knowledge of these reactions it was difficult to subject this attractive idea to quantitative tests.
Since the establishment of nonconservation of parity, the discovery that in a decay process the neutrino carries away not only energy and momentum but also a definite (longitudinal) angular momentum gives a new possibility of investigating the dynamics of weak interactions by measuring angular momenta. These new measurements on angular momenta together with other already existing experiments lead now to a much simpler phenomenological description of the weak interactions. 41 Indeed, it was found 42 quantitatively that a certain coupling constant in the beta decay appears to be exactly the same as that which occurs in the ju, decay in spite of the difference that nucleons have strong interactions but juT and e ± have only electromagnetic and weak interactions. Such identity and other universal characters of these interactions may lead to a deeper and unifying principle underlying all different weak reactions.
,It was realized in the 192O's that by analyzing the energy spectrum of the electron in beta decay there was an apparent nonconservation of energy. Pauli 10 resolved this difficulty by postulating the existence of a neutral particle with spin = \ h and zero (or very small) mass. Subsequently, Fermi l l developed the theory of beta decay. This neutral particle was called the neutrino v and its antiparticle the antineutrino.
,Further experimental confirmations of this particle came later from the measurement of the recoil of the final nucleus, from the capture experiment of the antineutrinos and from the over-all verifications of Fermi's beta-decay theory.