Personal Memories of 50 Years of Quarkonia

TL;DR

This paper recounts 50 years of quarkonium discoveries, from the 1974 J/psi particle to recent LHC findings of potential toponium states, highlighting key experimental milestones and theoretical predictions.

Contribution

It provides a personal historical perspective on the developments and discoveries in the field of quarkonium physics over five decades.

Findings

Observation of J/psi confirmed charm quark existence

Discovery of bottomonium expanded quarkonium family

Recent LHC results suggest possible toponium state detection

Abstract

The world of particle physics was revolutionised in November 1974 by the discovery of the J/psi particle, the first particle to be identified as a quarkonium state composed of charm quarks and antiquarks. The charmonium interpretation of the J/psi was cemented by the subsequent observations of a spectrum of related c \bar c states, and finally by the discovery of charmed particles in 1976. The discovery of charmonium was followed in 1977 by the discovery of bottomonium mesons and particles containing bottom quarks. Toponium bound states of top quark and antiquarks were predicted to exist in principle but, following the discovery of the top quark in 1995, most physicists thought that its observation would have to wait for next-generation e^+ e^- collider. However, in the second half of 2024 the CMS Collaboration reported an excess of events near the threshold for \bar t t production at the LHC that is most plausibly interpreted as the lowest-lying toponium state. These are the personal recollections of an eyewitness who followed closely these 50 years of quarkonium discoveries.