Two is the magic number

摘要

In quantum theory, the magnetic moment of the muon should be twice the value calculated classically, although in fact their ratio is not exactly two. Theory and experiment disagree on quite how far from two it is. There are essentially three ways of exploring the strange phenomena that might occur at very high energies or, equivalently, at very high temperatures: build a powerful accelerator to produce new particles (as was done for the discoveries of the W and Z particles and the top quark); search in the present-day Universe for any 'fingerprints' of exotic particles that might have existed fleetingly in the first moments after the Big Bang; or make precise measurements of those particle properties that might be influenced by high-energy quantum processes. In this last category, one such measurement is that of the anomalous magnetic moment of the muon. Its latest value, reported by G. W. Bennett and colleagues, does not match the theoretical prediction as well as might be expected -and might be a hint of something as yet unknown.