Colours in muon detector
The FAQ includes the following information about the colours of markings in the muon detector.
There are sometimes also red, blue, yellow and green rectangular markings on the outside of the ring, starting at the muon detector. The red ones are always at the end of muon lines, so they are obviously the energy measured from our muons. Blue ones often appear opposite a muon and indicate a jet that has likely come from the decay of a bottom quark. Yellow and green ones mean photons and electrons, respectively (?).
However in this diagram http://hypatia.phys.uoa.gr/Simplified_Basics/img/atlaslayers.jpg photons and electrons are shown as being absorbed by the EM calorimeter and don't reach the muon detector.
Sometimes I also see yellow or green markings in the muon detector which don't seem to be directly associated with signals in the EM calorimeter. For example
So are these marks really photons and electrons or could they be something else?
by DZM admin
A good question. Perhaps the yellow rectangle in the muon detector is some type of artifact, but it's a good question why there's yellow showing up there when it should be entirely absorbed by the EM calorimeter according to that diagram.
Will be interested in a scientist's response!
by peterwatkins scientist
The yellow rectangle does indeed represent a photon at that angle (10 o'clock)
This symbol is always drawn at the outside of the detector. However it is not a signal in the muon detector because as you say the photon is absorbed in the electromagnetic calorimeter. The green rectangles that represent electrons are also drawn at the outside of the detector.
The red symbols for muons are drawn there too but in this case it is also where the muon chamber hits are recorded.
Ah, so in other words: the fact that the symbol appears in the muon detector doesn't mean that it was detected in the muon detector.
Thank you for clearing that up!