54)$ refers to the partial decay width of a meson, given by:
$\Gamma_{i} = \frac{p_{i}}{8\pi M^{2}}|M_{fi}|^{2}$
where $\Gamma_{i}$ is the partial decay width, $p_{i}$ is the momentum of the decay products, $M$ is the mass of the decaying meson, and
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