where A is assumed to be a constant and Qq is the charge of the annihilating quarkpair. For the φ which is a pure ss state, the matrix element
M(φ → e
+
e

) ∼ he
+
e

|Qˆ
q|ssi = AQs = −
1
3
A .
For the ρ
0 with wavefunction


ρ
0
, =
E
1

2
(uu − dd), the phases of the two components are important and the total amplitude depends on the coherent sum of the
contributions from the decays of the uu and dd.
9.6 Answer: The meson mass formulae works well for all of the mesons in
21 Symmetries and the Quark Model
the question, with the exception of the η
0
, where the prediction of approximately
350 MeV is very different from the measured mass of 958 MeV. However it should
be noted that the η
0
is a flavour singlet state and in principle it could mix with
flavourless purely gluonic bound states and given the special nature of the η
0
, it is
not surprising that the simple mass formula does not work.