<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>π</mml:mi><mml:mo>−</mml:mo><mml:mi>π</mml:mi></mml:mrow></mml:math> scattering, QED, and finite-volume quantization
Physical review. D/Physical review. D.