Properties of Muons #
Muons (μ particles) are elementary particles belonging to the same family as electrons (leptons).
| Property | Value |
|---|---|
| Mass | ~207 times the electron mass (105.7 MeV/c²) |
| Charge | −1 (antimuon: +1) |
| Spin | 1/2 |
| Mean lifetime | ~2.2 microseconds (rest frame) |
Muons are far heavier than electrons and have high penetrating power, passing through most matter with ease.
Why Do They Reach the Ground? #
Muons are created in the upper atmosphere (about 15 km altitude). With a rest-frame lifetime of ~2.2 μs, they should only travel about 660 m at light speed. Yet they reach the surface thanks to special relativity.
Relativistic Effects #
For muons moving at near-light speed, time passes more slowly (time dilation). From the ground’s perspective, the muon’s lifetime appears extended by tens of times. This allows them to travel over 15 km and reach the surface.
Interaction with Earth’s Magnetic Field #
East-West Effect #
Earth’s magnetic field deflects the trajectories of muon parent particles (mainly protons). The number of particles arriving from the east and west becomes asymmetric — this is the east-west effect. It is also evidence that cosmic rays are predominantly positively charged.
Latitude Effect #
Earth’s magnetic field is stronger near the equator, deflecting low-energy cosmic rays. As a result, muon intensity is weaker near the equator and stronger near the poles.