# Amplitude of Decay of mesons

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## Main Question or Discussion Point

M=sγ5d ee
is the amplitude of pure leptonic decay of pseudo scalar meson(say K→ee), while
M=sd ee
is the amplitude of the semileptonic decay of K→π ee. Kindly explain why does sd replace sγ5d ee in the amplitude?

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Hepth
Gold Member
It's about the parity conservation. The K has negative parity, as does the pion (pseudoscalars). On the right hand side, two electrons have positive parity, so the current should have negative parity, -1*-1=1.

Then for the PS to PS decay you have a final state with negative parity, so only currents with positive parity can contribute.

The gamma5 is negative parity.

I understand that the transition K→π conserves parity. While in pure leptonic decay, the quark current transition K→Vacuum does not conserve parity so it must include parity violating operator.