We’ve seen how to draw simple diagrams for particle collisions, which we’ve referred to as collision diagrams.

Now, there are two very similar interactions that we need to take a look at more closely. 

First, we’ve seen electron capture before 

Where a proton captures an electron… and emits a neutron and an electron neutrino

Now, there’s a very similar interaction that can occur in a particle accelerator. 

Where a proton collides with an electron… 

And that interaction emits a neutron and an electron neutrino

If we were to just look at these equations then we’d say that…

If we were to just look at these equations then we’d see that the two interactions were exactly the same. 

But now, let’s draw the particle interaction diagrams

Where, this shows electron capture

And this shows an electron-proton collision

Again… so far so similar. 

The only difference is with these

Where the exchange particle involved in electron capture is the W plus boson 

And the exchange particle involved in an electron-proton collision is the W minus boson. 

These interactions have the same incoming particles, the same outgoing particles and are represented by the same particle interaction equation. 

The only difference between the two is the exchange particle! 

Now, we can use an easy trick to remember which way around these exchange particles go. 

For instance, electron capture has a T here – which sort of looks like a plus sign for the W plus boson. 

And an electron-proton collision has an I and an N here…

Which helps us remember that it’s the W minus boson. 

So now, which of these show the correctly drawn particle interaction diagrams for these interactions?

This shows the particle interaction diagram for electron capture

Whilst this shows the particle interaction diagram for an electron-proton collision. 

Finally, in this course, we’ll always have these exchange particles this way around. 

But at university, things get slightly more complex and the role of probability needs to get taken into account.

But  for the time being… we don’t have to worry about that. 

So to sum up for now…  the exchange particle involved in electron capture is the…

The exchange particle involved in electron capture is the W plus boson. 

Whereas, the exchange particle involved in an electron-proton collision is the W minus boson.