Hello! My name is Jo Dudek.
I'm a theoretical nuclear physicist here at Jefferson Lab.
In particle and nuclear physics, 
a common experiment 
Is to accelerate a beam of particles 
And fire them at a target.  
And when you do that
You can produce new particles.
And many different new particles 
can be produced.
Let’s focus on one possibility.
Suppose we fire the beam in and produce 
let’s say three particles.
We’d like to be able to describe how often this happens.
Now, of course, the more particles we send in
The more often this is going to happen.  
That’s fairly obvious.
So we need to discount the fact 
of how many particles do we send in. 
So what a cross-section does 
Is it counts up the number of times 
This particular final state of three particles happens.
And divides by how many particles did we send in.
That describes how likely is it 
For this particular final state to be produced.
