where m12 is the invariant mass of particles 1 and 2, $cos\phi_{3}$ is the
cosine of the angle 3 makes with 2 in the rest frame of 12 (cosine of the
angle 3 makes with 1 in the rest frame of 12 is, obviously, $-cos\phi_{3}$).
Note that if two of the three daughters (for example 2 and 3) are identical,
one has to take into account the contributions from two possible combinations
12 and 13, with corresponding signs for the cosines (for spin 1 and higher), and a normalization factor of 1/2.
Another member function, relBrWig( int ), returns the relativistic Breit-Wigner amplitude for the $K^{*} \pi$ (in which case the integer argument should be equal to 1) or $K \rho$ (in which case the argument should be $\neq$ 1) resonances. More precisely, for a P-wave decay of a scalar meson (which I'll denote $S$), the amplitudes are given by:
\begin{equation}
BW(m_{ij}^2) = \frac{\sqrt{\Gamma_{0}}\,M}{(m_{R}^2 - m_{ij}^2) - i \Gamma m_{R}}
\end{equation}
where the matrix element M is:
\begin{equation}
M = (P_{S} - P_{k})^{\mu}(g^{\mu\nu} - P_{ij}^{\mu}P_{ij}^{\nu}/m_{R}^2)(P_{i}^2 - P_{j}^2)
Here, $m_{R}$ and $\Gamma_{0}$ are the mass and width of the $m_{ij}^2$ resonance; $m_{S}$, $m_{i}$, $m_{j}$, $m_{k}$ are the masses of the parent and of the $i^{th}$, $j^{th}$, $k^{th}$ particles, respectively; $p_{j}$ is the magnitude of the 3-momentum of the $j^{th}$ particle in the $i-j$ rest frame, and $p_{jR}$ is the same when $m_{ij}^2= m_{R}^2$. The value of $R$ for the ``centrifugal barrier penetration factor'' is taken to be 2 fm for the $K^{*}$ and 5 fm for the $\rho$.
\subsection{EvtSecondary}
\index{EvtSecondary}
\index{classes!EvtSecondary}
Allows EvtGen not to write secondary particles to StdHep. This
class will most likely be removed.
\subsection{EvtSpinDensity}
\index{EvtSpinDensity}
\index{classes!EvtSpinDensity}
\index{spin-density}
This class represents spin-density matrices of
arbitrary dimensions. (Well, this is not quite true, at the
moment it is limited to dimension 5 which is the number of
degrees of freedom of a spin 2 particle.)
Functions are provided to manipulate the components of the
spin density matrix as well as to
calculate probabilites.
\subsection{EvtSpinType}
\index{EvtSpinType}
\index{classes!EvtSpinType}
Defines the folowing enum for the different particle types