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Computing Cross section

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Question
Consider a system of two real scalar fields \(\phi_1, \phi_2\)
described by the Lagrangian density
\begin{equation}
 \Lsc = \frac{1}{2} \partial_\mu \phi_i \partial^\mu \phi_i - \frac{1}{2} m^2
\phi_i\phi_i -\frac{1}{4} \lambda (\phi_i\phi_i)^2.
\end{equation}
Compute the scattering cross section to the lowest order in \(\lambda\). Find
the cross sections for the three processes

  1. \(\phi_1 + \phi_2 \longrightarrow \phi_1 +\phi_2\)
  2. \(\phi_1 + \phi_1 \longrightarrow \phi_1 +\phi_1\)
  3. \(\phi_1 + \phi_1 \longrightarrow \phi_2 +\phi_2\)

Write your answers as a constant times \(\sigma_0\equiv \frac{\lambda^2}{64\pi
s}\) where
\(s\) is the total  energy in the center of mass frame.


Answer :   a. \(4\sigma_0\)  b. \(36 \sigma_0\) c. \(4 \sigma_0\)

Hint : Find symmetry appropriate factor.
Remark : Good question


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