Chiral anomaly

In theoretical physics, a chiral anomaly is the anomalous nonconservation of a chiral current. In everyday terms, it is equivalent to a sealed box that contained equal numbers of left and right-handed bolts, but when opened was found to have more left than right, or vice versa. In some theories of fermions with chiral symmetry, the quantization may lead to the breaking of this (global) chiral symmetry. In that case, the charge associated with the chiral symmetry is not conserved.

Quotes

 * Bell's most famous paper in this area — published with Roman Jackiw in Il Nuovo Cimento in 1969 and cited more than any other of his papers — was the discovery, clarified to some extent by Stephen Adler, of the Bell-Jackiw—Adler anomaly. At the time, theory predicted that the neutral pion could not decay into two photons, but this had been observed in experiments. Bell, Jackiw and Adler were able to explain the observed decays theoretically by adding an "anomalous" term resulting from the divergences of quantum field theory. A condition that the "anomaly" produced agreement with experiment was that the sum of the charges of the elementary fermions had to be zero. This provided support for the idea that quarks come in three colours, now part of the widely accepted Standard Model.
 * Andrew Whitaker, "John Bell and the most profound discovery of science", Physics World (December 1998)