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Do I have a misunderstanding here at all? The question states:

One of the following equations represents a possible decay of the K$^+$ kaon. \begin{align}\mathrm K^+&\to\pi^++\pi^0\\\mathrm K^+&\to\mu^+ +\overline{\nu_\mu}\end{align} State, with a reason, which one of these decays is not possible.

I then see that on the first one that the strangeness isn't conserved, on the right, the strangeness is $+1$ and on the left the strangeness is $0$, then on the second one the lepton number on the left is $0$ then on the right it is $0$ too, the answer however states that the second one is incorrect as the lepton number isn't conserved, despite me working out that it is.

I could be very wrong here, any ideas?

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closed as off-topic by dmckee Apr 13 '16 at 19:00

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  • 2
    $\begingroup$ The $\mu^+$ and $\bar{\nu}_\mu$ are both antileptons so their total lepton number is -2. Strangeness is not a conserved quantity in decays via the weak interaction. $\endgroup$ – John Rennie Apr 13 '16 at 17:35
  • $\begingroup$ What about the first one, surely the strangeness works as its under the strong force giving $1$ before and $0$ after? Or does this one interaction ignore the rules? $\endgroup$ – Jan Apr 13 '16 at 17:43
  • $\begingroup$ The $\bar{s}$ decays to $\bar{u}$ and a $W^+$ in a weak process. This does not conserve strangeness. The $W^+$ decays to $\bar{d}$ and $u$. The original $u$ and the new $\bar{d}$ form a $\pi^+$ and the $u$ and $\bar{u}$ form a $\pi^0$. $\endgroup$ – John Rennie Apr 13 '16 at 17:53
  • $\begingroup$ Uuuuh... the first one doesn't conserve electric charge... $\endgroup$ – Gremlin Apr 14 '16 at 15:48
  • $\begingroup$ @Eoin: that was an error introduced when the question was edited. The original version did have the first decay as $K^+ \rightarrow \pi^+ + \pi^0$. $\endgroup$ – John Rennie Apr 14 '16 at 16:18