What is the significance of using pseudorapidity in HEP experiments?

[HAMJOOP]

I read some of the articles related to particle physics experiment and don't know the meaning of it.

1. minimum bias event

2. pile up

Also, η (pseudo-rapidity) is used instead of θ to describes the angular distribution, but why ?

Can someone explains to me ?

 

[mfb]

(1) experiments cannot keep all events, just a tiny bit passes the trigger steps. But those are not representative for all collisions any more (because you specifically look for "interesting" things - that's the idea of the trigger), so they are biased. To study the general particle distributions, a low rate of events is written to disk with just minimal trigger requirements (to make sure there was a collision at all, basically).

(2) multiple collisions happening in the same bunch crossing. Up to ~40 for ATLAS and CMS.

Also, η (pseudo-rapidity) is used instead of θ to describes the angular distribution, but why ?

It is a more convenient scale. If you plot "interesting things" over the angle, you get a large peak for small θ and there is a huge difference between 1° and 3°, for example. This does not happen with pseudo-rapidity.

 

[HAMJOOP]

η (pseudo-rapidity) is something related to special relativity.
Is there any reasons related to special relativity for using η ?

 

[mfb]

Differences in rapidity are invariant under Lorentz transformations (along the beam axis). It's not exactly the same as the pseudorapidity η, but for large energy of the particles they are very similar.