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\title{\boldmath{ $\pi^0$}\ \ \ \ \ \ \ $*****$}
\maketitle
The $\pi^0$ is the lightest meson. It decays to two photons, with the possiblity of the photon branching to give an electron-positron pair. The properties are taken from \cite{Yao:2006px} with minor adjustments to the branching ratios to ensure they sum to one. The adjustments are within the errors on the branching ratios. All the modes with branching ratios above $1\times10^{-6}$ are included including the mode $e^+e^-e^+e^-$ which has a low branching ratio. The mode including the production of positronium or a single $e^+e^-$ pair which have branching ratios of order $10^{-8}$ are not included,
The $\pi^0$ has mass $134.9776$~MeV and is unstable. The $\pi^0$ has spin $0$, charge $0$ and is colour neutral.
 The $\pi^0$ is a meson and is from the $1^1S_0$ multiplet.The properties of the particle and its antiparticle are taken to be charge-conjugate to each other.
 The decay length is $2.51\times10^{-8}$~m. The PDG code is 111.
\setlongtables
\begin{longtable}[t]{|c|c|c|p{1in}|p{2.5in}|c|}
\hline
Branching & Rating & On/ & Outgoing  & Description & Decayer  \\ 
Ratio     &                                 & Off & Particles &             &         \\ 
\hline
\endhead
\caption{The decay modes of the $\pi^0$ (continues).}
\endfoot
\caption{The decay modes of the $\pi^0$.}
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\label{tab:111decaymodes}
0.987989 & $*****$ & on & $ \gamma, \gamma $  & The decay of the $\pi^0$ to two photons dominates the $\pi^0$ decays. This decay could be handled by the decayer for a pseudoscalar to two vectors but given we are not including width effects and the outgoing particles are photons we use the phase-space decayer to improve efficiency.   & Herwig::Hw64::DecayME0 \\ 
\hline
0.011980 & $*****$ & on & $ \gamma, e^-, e^+ $  & The Dalitz decay of the $\pi^0$ to a photon and an electron-positron pair.   & Herwig::PScalarVectorFermions::PVff \\ 
\hline
0.000031 & $****$ & on & $ e^-, e^+, e^-, e^+ $  & Decay of the pion to two electron-positron pairs with branching ratio from \cite{Yao:2006px}.   & Herwig::PScalar4Fermions::PScalar4f \\ 
\hline
\end{longtable}The $\pi^0$ decay modes are given in Table~\ref{tab:111decaymodes} and the total branching ratio is $1$.


 There is no mass generator for the $\pi^0$.
 There is no width generator for the $\pi^0$.

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 The particle was checked by Peter Richardson at 12:19:8 on the 18th of October 2006. The most recently changed decay mode was modified at 22:13:16 on the 16th of July 2005. The particle data was last modified at 16:24:39 on the 10th of October 2006.

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