lifetime

Most of the particles physicists make do not last. Created in a collision, they exist for a flicker and then fall apart into lighter particles. This impermanence is so common that one of the first questions asked about any new particle is simply: how long does it live? The answer is its lifetime, and it ranges across a staggering span — from a near-eternity for the proton down to less than a billionth of a billionth of a second for the heaviest particles.

A particle's lifetime is its average time of survival before it decays. Decay is a random, quantum process: you can never say exactly when a single particle will break apart, only the odds. So the lifetime is a statistical average over many identical particles, much like the half-life used for radioactivity (the two are directly related). A free neutron, for example, lives about fifteen minutes on average before decaying; a muon lives about two-millionths of a second; many particles made at colliders survive less than a trillionth of a trillionth of a second. Some particles, like the electron and (as far as we can tell) the proton, are stable and effectively never decay.

Lifetime matters because it shapes how a particle can be detected at all. A long-lived particle leaves a visible track in a detector; a short-lived one decays almost instantly and must be reconstructed from its decay products. Lifetime is also tightly bound, through the uncertainty principle, to a particle's decay width: short-lived particles have a broad spread in measured mass, long-lived ones a sharp one. A common subtlety worth flagging: a fast-moving unstable particle appears to live longer in the lab because of time dilation, which is why short-lived particles created at high speed can still travel measurable distances before decaying.