Knowledge Bank

The discovery of a rapidly growing number of short-lived, excited systems among high-energy nuclear particles suggests the possibility of an underlying molecular type of structure for these particles to which the concepts of molecular spectroscopy appear to be applicable. It is found that the observed spin and masses of the heavy mesons and resonance'' particles can be accounted for in terms of the rotational excited states of pionic-molecules'' consisting of 2, 3, 4, and $5\pi$-mesons. The bond-length and binding energy per bond can be obtained from a relativistic electron-positron model of the $\pi$-mesons which is also able to account for the observed masses, spins, and life-times of the $\pi$-mesons themselves. Such a molecular structure of nuclear particles permits one to obtain a physical interpretation of the isotopic spin'' and strangeness'' quantum numbers introduced for empirical reasons in terms of the internal structure and angular momenta of these systems. Furthermore, it provides an explanation for the usefulness of group-theoretical symmetry arguments which have recently been widely applied to the classification of the newly discovered particles. The desirability of applying the powerful techniques developed for the analysis of molecular spectra to the problem of nuclear particle structure is suggested.