Muons are subatomic particles that are similar to electrons but are more massive. They are created in the upper atmosphere when high-energy cosmic rays, typically originating from outer space, collide with the atoms in the Earth's atmosphere. These collisions produce a shower of particles, including muons.
Muons are unstable and have a relatively short lifetime, decaying into other particles within a fraction of a second. However, due to their high energy, they can travel significant distances before decaying. This is in contrast to other unstable particles that are typically created in the upper atmosphere, which decay much more quickly.
Since muons have a longer lifetime compared to other particles produced in the atmosphere, they have a better chance of reaching the Earth's surface before decaying. This is because the higher in the atmosphere they are produced, the more time they have to travel downwards before decaying. The muons that do reach the Earth's surface are often moving at velocities close to the speed of light.
Furthermore, muons are not significantly affected by the Earth's magnetic field, so they can travel in straight lines without being significantly deflected. This property allows them to penetrate through the atmosphere and reach the surface.
In summary, muons reach the Earth's surface because they are created at high altitudes in the atmosphere by cosmic rays, they have a longer lifetime compared to other particles produced in the same processes, and they are not greatly affected by the Earth's magnetic field, allowing them to travel straight and relatively unimpeded to the surface.