The Discoveries of Rare Pion Decays at the CERN Synchrocyclotron

Abstract

In 1957 the CERN 600 MeV Synchrocyclotron started to operate and could detect for the first time already in 1958, and later in 1962, the two still missing β-decays of the charged pion, providing crucial verifications of the universal V-A coupling.

The 600 MeV Synchrocyclotron: Laying the Foundations

“…They failed to observe it, posing an apparent serious obstacle to the understanding of the weak interaction and to the V-A theory. One SC Team took up the challenge and succeeded, because it developed imaginative ways to suppress the background of electrons produced in the dominant decay of the pion and quickly discovered the rare decay mode at the predicted level [Highlight 2.3] [6]: a world-class experiment in the first year of SC operation! These early years saw other related fundamental studies, such as the first observation ever of the extremely rare decaymode π + → π 0 + e + + ν, a further crucial test of the theory of weak interactions.…”

“…Feynman (Nobel Prize 1965) presented an invited paper, "Theory of Beta Decay", dealing with the apparent absence of the π → eν. Not convinced by the arguments advanced by Feynman and despite the prevailing view that the π → eν decay might be exceedingly rare (or non-existent), an initiative was launched for a new experiment at CERN [6]. In this experiment (Fig.…”