Measure cosmic ray pions and protons below surface of the Earth? In my particle physics lecture, the prof asserted that apparently cosmic ray muons can still be measured 1km below the surface of the earth (e.g. in a mine). This led me to ask the following two things:

*

*Why is that?

*Why can't one detect cosmic ray protons or cosmic ray pions in that depth?

A simplified answer (on conceptual level) is appreciated, as I'm not too fluent in this subject yet. ;)
 A: Protons and pions are hadrons: they participate in the strong nuclear force. Therefore, they collide with atomic nuclei in the overburden.
Various interaction parameters can be found here:
https://pdg.lbl.gov/2020/AtomicNuclearProperties/
The page for silicon looks like this:

The interaction lengths are on the order of 100 g/cm$^2$, so nothing makes it through 1 km of rock.
Muons are leptons: they interact via electromagnetism and the weak nuclear force. Weak interaction lengths are enormous (light years). Because muons are heavy (relative to electrons), interactions with the high-Z nuclear charge are suppressed, an the main for of energy loss is as a "minimum ionizing particle", which means it losses energy to sea of atomic electrons.
The table shows around 4 MeV/cm for silicon, so your average 1 GeV cosmic ray may only go 2.5m, the high energy tail is long. The muons are created from progenitor cosmic rays, with the spectrum shown in the figure:

With 400 GeV being the order-of-magnitude required to penetrate 1km of overburden, there is plenty of spectrum to do so.
While there is no radiation exposure to humans to worry about, physics experiments in deep mines are generally looking at rare events in huge targets.
