

\subsection{\bf Density Matrix Element Example---I}

\begin{slide}
\begin{itemize}
\item An example of importance of off-diagonal error matrix elements
can be found from high energy physics in the decay of a spin 1
particle.

\item One can define the angular distribution by density matrix
elements
\begin{eqnarray*}
x_1 & = & \rho_{00} \\
x_2 & = & \rho_{11} \\
x_3 & = & \rho_{10}
\end{eqnarray*}
in some kinematic coordinate system (Lorentz frame)!  

\item Some years ago, these were always given in one
frame---the so-called $t$ channel frame.  

\item ``Recently'' (1975---these really are old notes!), it was found
that the values in another frame (the ``$s$ channel'') were more
meaningful physically.

\item Unfortunately, latter values, $y$, were related by precisely such
a linear transformation $y=Tx$ and one can no longer estimate errors
in $y$ from recorded experimental data.
\end{itemize}
\end{slide}

