branching ratio

When a road forks several ways, you can ask what fraction of cars take each branch — most go straight, a few turn left, almost none take the rough side track. An unstable particle is like that fork: it can usually decay in more than one way, into different sets of products, and each path gets its own share of the decays. That share, the probability of one particular decay path, is called the branching ratio.

A branching ratio is the fraction of decays of a particle that go through a specific channel, written as a percentage or a decimal. By definition, all the branching ratios of a particle add up to one, since the particle must decay through some channel. A famous example is the Higgs boson: about 58 percent of the time it decays into a bottom quark and its antiquark, about two-tenths of a percent of the time into two photons, and so on through a list of channels. Some decays are common, some rare; a branching ratio of one in a billion describes a path the particle almost never takes, which can be exactly the path a physicist is hunting for.

Branching ratios matter because they are how theory predictions get tested in detail and how rare new physics gets searched for. The Standard Model predicts a precise branching ratio for each decay channel; measuring those fractions and comparing is a stringent test, and a channel that should be vanishingly rare becomes a sensitive place to look for unexpected effects. One thing to keep clear: a branching ratio is about which way a particle decays, not how fast. How fast is the lifetime or width; into what, and how often each, is the branching ratio. The two together fully describe an unstable particle's decay.