Welcome to the digital edition of the November/December 2024 issue of CERN Courier.

This edition marks the 50th anniversary of the November Revolution in particle physics. In 1974, a year after the discovery of neutral currents, experiments at Brookhaven and Stanford discovered an unexpected resonance at 3.1 GeV. Its remarkable stability suggested a new quantum number. Was the state long-lived because it bore the charm quantum number predicted by Glashow, Iliopoulos and Maiani? Shortly after, another narrow resonance appeared at 3.7 GeV. As the Courier reported at the time, the new particles were completely unexpected (p41).

It soon became clear that the new resonances did contain charm – hidden charm, to be precise. A rich “charmonium” spectrum followed until, two decades ago, experiments at electron–positron colliders discovered charm–anticharm states with exotic features such as long lifetimes, net electric charges and strangeness. These were the experimental harbingers of a bestiary of tetraquarks and pentaquarks, with a further 23 now discovered at the LHC (p26). They pose a fascinating theoretical puzzle in QCD. Nature employs two very different mechanisms to create them, and for many states it is not yet clear which of the two is at play (p33).

Also in this edition: QCD calculations are key to probing hints of new physics in the charm sector (p37); new results throw a spotlight on anomalous measurements of the mass of the W boson (p7) and of the magnetic moment of the muon (p8); the latest developments in string theory (p21) and machine learning for statistics (p19); and much more.

j CERN Courier November/December 2024