Pions: the original Nambu–Goldstone bosons
An introduction and precision pion physics
Centre for High Energy Physics, Indian Institute of Science, 560012, Bangalore, India
Accepted: 11 January 2022
Published online: 27 January 2022
In this special issue being brought in the centenary year of the birth of Yoichiro Nambu, we exemplify on his discovery of spontaneous symmetry breaking in elementary particle physics, and review precision pion physics in the present era. The notion of spontaneous symmetry breaking in elementary particle physics was introduced by Nambu, and found a realization in the strong interaction sector. It allows one to view the pions as the approximate Nambu–Goldstone bosons of spontaneously broken axial-vector symmetries associated with the (near) masslessness of quarks. Inspired by the phenomenon of superconductivity of condensed matter physics, Nambu found this application in a remarkable tour de force. Nambu’s work in collaboration with G. Jona-Lasinio gave a dynamical model where such pions may arise. Pions today play the role of being sensitive probes of the ground state of quantum chromodynamics, the Lagrangian field theory of the strong interactions with (confined) quark and gluon degrees of freedom, and whose ground state spontaneously breaks the approximate chiral symmetry. The presence of non-zero quark masses renders the symmetries approximate, and yet the properties of the low-energy sector can both be described and measured at high precision both in experiment and on the lattice. Notable physical quantities include the neutral pion lifetime and pion scattering lengths. An important role of pions is their contribution to the hadronic radiative corrections to the anomalous magnetic moment of the muon, which is being measured at high precision at Fermilab. We review some of the important aspects of the state of the art. We also say some words about the outstanding contributions of the recently departed Murray Gell-Mann who was a pioneer in the field initiated by Nambu.
© The Author(s), under exclusive licence to EDP Sciences, Springer-Verlag GmbH Germany, part of Springer Nature 2022