I know that a particle must decay to decay products that have less than or equal mass to the original particle. However, I am confused how this works for composite particles. Is it any different? Eg. in the decay of a meson, is the meson treated as a single particle or can it be thought of as two quarks colliding?

In the following interaction, the mass of two $\tau$ particles is greater than the original $J/\psi$ mass. My intuition is that it is therefore impossible but I want to make sure there are no complications due to $J/\psi$ being a composite particle.

\begin{equation} J/\psi \rightarrow \tau^{+}+\tau^{-} \end{equation}


1 Answer 1


As long as one works with the four vectors describing the particles, the answers will be consistent, whether composite or not.

Here is the pion decay :

pion decay

The pion four vector is the (four vector) sum of the up and antidown four vectors.

In the rest mass system of the decaying particle the energy given by its mass has to accommodate the sum of the masses of the particles it can decay into.

See this table for the measured decays of the J/psi . Note how the branching ration diminishes the larger the masses of the decay products. The phase space becomes small.

The pion four vector can be considered the addition (fourvector) of the up antidon which compose it.


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