Muons are very small particles (also called elementary particles, of the family of Leptons), that are created up above in the atmosphere, as a consequence of cosmic rays decay (they are created when pions disintegrate). Their speed is very close to the speed of light (0.9c), and they have the same charge as an electron. However, their mass is considerably larger than the electron’s, about 200 times larger, hence they have the ability to penetrate walls quite deeply. Muon’s lifetime is extremely short (around 2 microseconds, which is a hundred thousand times faster than the time it takes to blink).
However, when muons are created at about 15km from the atmosphere, they don’t seem to decay as fast, as we are able to detect them at sea level. At first glance, this seems to be impossible; their lifetime is too short for them to reach the ground. Then, why are we able to detect muons on the Earth’s surface?
As Einstein’s Relativity Theory explains, particles that move so fast, perceive time very slowly. It sounds weird, but you can think of it the following way: imagine you go on a spaceship, and your watch shows the same time as a clock on Earth. With that spaceship, you go very fast to a near Galaxy, and you come back to Earth, at velocities close to the speed of light. When you arrive, you compare the time your watch says, with the clock that has remained on Earth. They will not be showing the same time: yours will seem to have been slowed down. Relativity explains this phenomenon: when you move very fast, time slows down. The faster you move, the slower time will go by. This way, you could have measured that your journey on the spaceship has taken about a week, whereas people on Earth may have measured your journey has lasted for a month. The same will happen to you if you get on a plane, or on a train, or if you go running: the faster you move, the slower time will go for you, compared with those who are at rest, not moving. However, this time dilation effect is utterly unnoticeable unless you go to incredibly high speeds, close to the speed of light (this is why in everyday life, when we get on a plane, train or go running, we notice nothing at all).
Nonetheless, this is the case for muons. They move so fast that they experience the time dilation effect. For them, time goes very slow. This is basically why we see that they take a longer time to decay, from Earth.
And this is where our experiment comes into play. We want to measure the decay of muons at each height. With this data, we will obtain the muons’ lifetime, and we will compare it with the prediction obtained from Relativity Theory. Our goal is to observe how theory matches our experimental data.