What does the symbol sigma represent in electromagnetic calorimetry equations?

[GoBluePhysics]

Quick question regarding E-Cals. I am giving a presentation on them soon and I understand just about everything I am supposed to, but I keep encountering one symbol and I simply don't understand what it is supposed to mean.
$\frac{d\sigma}{dx}=\frac{A}{X_0N_A}\bigg[1-\frac{4}{3}x(1-x)\bigg]$
$\sigma_{pair}\approx \frac{7A}{9N_AX_0}$
These equations are supposed to describe pair production. However, I simply don't understand what the sigma is meant to be. I think it is something along the lines of a distribution of resulting particles from the pair production given an intensity of incident particle, but the same $\sigma$ is used to describe things like photoelectric effect and compton scattering. Any assistance would be greatly appreciated! Thanks!

 

[Quantum Defect]

Quick question regarding E-Cals. I am giving a presentation on them soon and I understand just about everything I am supposed to, but I keep encountering one symbol and I simply don't understand what it is supposed to mean.
$\frac{d\sigma}{dx}=\frac{A}{X_0N_A}\bigg[1-\frac{4}{3}x(1-x)\bigg]$
$\sigma_{pair}\approx \frac{7A}{9N_AX_0}$
These equations are supposed to describe pair production. However, I simply don't understand what the sigma is meant to be. I think it is something along the lines of a distribution of resulting particles from the pair production given an intensity of incident particle, but the same $\sigma$ is used to describe things like photoelectric effect and compton scattering. Any assistance would be greatly appreciated! Thanks!

Usually sigma is a cross section. It is an effective area. Sometimes measured (cheekily) in "barns." It can be thought of as being akin to the size of target attached to a particular particle, the "hitting" of which will result in a particular process. Large cross section --> likely process. Small cross section --> unlikely process.