into any odd number of photons) is forbidden by the C-symmetry of the electromagnetic interaction: The intrinsic C-parity of the is +1, while the C-parity of a system of photons is (−1). The second largest decay mode () is the Dalitz decay (named after Richard Dalitz), which is a two-photon decay with an internal photon conversion resulting a photon and an electron-positron pair in the final state: {| | || → || || + || || + || . |} The third largest established decay mode () is the double-Dalitz decay, with both photons undergoing internal conversion which leads to further suppression of the rate: {| | || → || || + || || + || || + || . |} The fourth largest established decay mode is the loop-induced and therefore suppressed (and additionally helicity-suppressed) leptonic decay mode (): {| | || → || || + || . |} The neutral pion has also been observed to decay into positronium with a branching fraction on the order of . No other decay modes have been established experimentally. The branching fractions above are the PDG central values, and their uncertainties are omitted, but available in the cited publication. [a] Make-up inexact due to non-zero quark masses. See also Pionium Quark model Static forces and virtual-particle exchange Sanford-Wang parameterisation References Further reading Gerald Edward Brown and A. D. Jackson, The Nucleon-Nucleon Interaction (1976), North-Holland Publishing, Amsterdam External links Mesons at the Particle Data Group Mesons