Coupling constant

"That the pion-nucleon (πN) coupling constant is fundamental in our understanding of the Cosmos has been adequately emphasised in numerous works. In meson-exchange models of the , a significantly weaker coupling between the pions and the nucleons would have prevented the s from combining fast with s in the ; they would have decayed before they had any chance to be enmeshed first in s, then in other light nuclei. According to the , within half an hour of the Big Bang, all existing matter had assumed the form of free electrons, protons, and helium nuclei (as well as traces of other nuclei up to 7). On the contrary, a significantly stronger coupling would have resulted in the rapid creation of bound diprotons and would have led to a helium-dominated Universe. It is hard to imagine how life could emerge in such a Universe: typical stars burn hydrogen to helium for about 90 % of their lives."

"... Roughly speaking, in renormalizable theories no coupling constants can have the dimensions of negative powers of . But every time we add a field or a space-time derivative to an interaction, we reduce the dimensionality of the associated coupling constant. So only a few simple types of interaction can be renormalizable. ..."