Testing the chiral anomaly

Abstract

Nuclear Physics Pi mesons, also known as pions, consist of a quark and an antiquark and are extremely unstable. Neutral pions have a lifetime of only ∼80 attoseconds, decaying into two photons. Quantum chromodynamics (QCD), the theory of quarks and gluons, predicts this decay and the associated lifetime using the mechanism of broken chiral symmetry—the so-called chiral anomaly. Measuring the lifetime to high precision then provides a benchmark for theories that aim to improve on this original prediction. Larin et al. measured this lifetime with an uncertainty that was half that of the previous most precise result (see the Perspective by Meyer). The measured value was consistent with the original QCD prediction and less consistent with other theoretical approaches. Science , this issue p. [506][1]; see also p. [469][2] [1]: /lookup/doi/10.1126/science.aay6641 [2]: /lookup/doi/10.1126/science.aba8063