At a typical energy of about 100 GeV, a muon has a Lorentz factor of about 1000, an electron about 200.000. The flight time to the detector should be around 30 ns (assuming d= 10m from the collision point). While e- would practically have the speed c, the muon's speed is about 0.9999995 c, causing a delay of about 15 fs, barely measurable I think. Is there a way to distiguihs e- from mu, unless we assume prior knowledge (that they have passed the electric calorimeter, about the penetrating power etc). Or, to put it in another way: what is the energy threshold below which they can be distinguished directly?

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    $\begingroup$ I their behavior in the calorimeter is how they are distinguished because (as you noted) all ultra-relativistic particles have the same time or flight. At high enough energy it may simply be harder to distinguish them. $\endgroup$ Aug 20, 2013 at 17:55

1 Answer 1


The electron and the muon have the same charge, but the muon is 200 times heavier than the electron. The bremsstrahlung emitted by all charge particles in interactions with electric or magnetic fields depends on the mass of the particle:

In limiting cases examined the total radiation goes as at least m^- 4

which accounts for why electrons lose energy to bremsstrahlung radiation much more rapidly than heavier charged particles (e.g., muons, protons, alpha particles)

This is utilized by constructing electromagnetic calorimeters which absorb all the energy of the electrons while the same momentum muons pass through with minimum ionization losses, and also pass through the hadronic calorimeters without interacting, since they only have electromagnetic and weak interactions.

In this CMS detector slice one sees how the calorimeters perform.

cms slice

One can see the muon going through all that mass of detectors, while the electrons shower in the electromagnetic calorimeter and are contained.


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