  The scattering cross section is the effective area of collision when two
particles collide. Quantum mechanically, it is a measure of the probability for
a specific process to take place. Employing wave packets to describe the
scattering process, we compute the entanglement entropy in 2-to-2 scattering of
particles in a general setting using the $S$-matrix formalism. Applying the
optical theorem, we show that the linear entropy $\mathcal{E}_2$ is given by
the elastic cross section $\sigma_{\text{el}}$ in unit of the transverse size
$L^2$ of the wave packet, $\mathcal{E}_2 \sim \sigma_{\text{el}}/L^2$, when the
initial states are not entangled. The result allows for dual interpretations of
the entanglement entropy as an area and as a probability. Since
$\sigma_{\text{el}}$ is generally believed, and observed experimentally, to
grow with the collision energy $\sqrt{s}$ in the high energy regime, the result
suggests a "second law" of entanglement entropy for high energy collisions.
Furthermore, the Froissart bound places an upper limit on the entropy growth.
