More on His Book and How It Was Received

Abstract

Källén’s book mirrors his general approach to physics. It starts with “down to earth” issues that every particle physicist should master, such as how does one determine the masses, spins, etc. of various particles. How to deal with relativistic kinematics is another such basic topic. Then there are treatments of strong, electromagnetic and weak interactions of elementary particles, calculations of cross sections, etc. The book treats both formal theory and comparison with experiments.

The book was published at a time when books on elementary particle physics were rare. Källén introduced new elegant concepts, from mathematics into physics, that were quite useful, such as what young people started calling the “Källén λ-function”, λ(x, y, z) = x ² + y ² + z ² − 2x y − 2y z − 2z x, that is used to determine the area of a triangle with sides x, y, z and thus enters into two-body kinematics.

From the beginning, Källén had realized that the field of elementary particle physics was progressing very rapidly and his book would need future revisions. There can be no doubt that he had intended to do so (see below). However, methods and techniques would often be useful, even if the field changed direction.