<Tr> <Td> π </Td> <Td> → </Td> <Td> π </Td> <Td> + </Td> <Td> </Td> <Td> + </Td> <Td> ν </Td> </Tr> <P> The rate at which pions decay is a prominent quantity in many sub-fields of particle physics, such as chiral perturbation theory . This rate is parametrized by the pion decay constant (ƒ), related to the wave function overlap of the quark and antiquark, which is about 6989208282943310000 ♠ 130 MeV . </P> <P> The π meson has a mass of 7002135000000000000 ♠ 135.0 MeV / c and a mean lifetime of 6983840000000000000 ♠ 8.4 × 10 s . It decays via the electromagnetic force, which explains why its mean lifetime is much smaller than that of the charged pion (which can only decay via the weak force). The main π decay mode, with a branching ratio of BR = 0.98823, is into two photons: </P> <Dl> <Dd> <Table> <Tr> <Td> π </Td> <Td> → </Td> <Td> γ . </Td> </Tr> </Table> </Dd> </Dl>

What is the electric charge on a pion having quark composition ud