Post #525769

#paperOfTheDay is "The axial vector current in beta decay" from 1960. This paper was written at a time where the #quantumFieldTheory of the weak force was not known, and the interactions of elementary particles were quite mysterious. There were all sorts of experimental observations, and theorists had to come up with models to describe these results. In practice, this means you propose that certain fields exist, and how they interact, and then you do a calculation to show that this gives the observed result. The present paper introduced what is now known the "nonlinear sigma model". The "linear sigma model" had been known before, it gave reasonable predictions, but postulated the existence of a particle that had not been observed. The key innovation in the present paper is to require another constraint, so that the un-observed field is not actually a particle, but it gets removed from the theory through the new constraint. This constraint amounts to demanding that the field sigma only changes its direction, but has constant magnitude. Today we know that the nonlinear sigma model does not describe neither strong nor weak force in nature. However, it has become one of the most widely studied "toy model" systems in theoretical #physics because it is relatively easy, and at the same time produces interesting non-trivial effects. #dailyPaperChallenge https://link.springer.com/article/10.1007/BF02859738