Measuring the Running Coupling Constant of the Strong, the Electromagnetic and Weak Forces

Abstract

Lord Kelvin once wrote that every basic principle worth mentioning had already been discovered and that all that was needed were more precise measurements. Shortly after that, in 1897, J.J. Thomason discovered the first lepton (the electron) and opened the first chapter of elementary particle physics. The discovery of the J particle in 1974, interpreted as the bound state of a quark-antiquark pair with a new flavor, charm (much in the same fashion as the hydrogen atom is the bound state of the proton and electron), put the finishing touch to the hypothesis that quarks are the fundamental building blocks of all hadrons. Now we know our universe is likely to be made of 6 leptons and 6 quarks. They interact with each other by means of 4 interactions: nuclear (or strong), electro-magnetic, weak forces and gravity; each is distinctively different in nature and in magnitude at the energy scale commonly available in nature. Each of these forces is characterized by a coupling constant which roughly determines the size of the interaction. Most of our daily phenomena, from the maximum height of a mountain and living beings to temperatures of the sun and the earth can be explained in terms of these constants. Since Einstein’s unsuccessful efforts to unify gravity with electromagnetic forces at low energies, physicists have attempted, over many years, to unify these interactions at much higher energies than presently available in the laboratories. According to these speculations, the coupling constants of all forces will change with energy and at sufficiently high energy they will become comparable in size (Fig. 1). This property of changing with energy resulted in the name of “running coupling constants”. In these lectures we will take a few examples, primary from e+e− annihilation to illustrate the effort in measuring these constants at different energies.