The branching ratio or branching fraction is the fraction of a specific particle's total decays that result in a particular final state or decay mode. It measures the relative probability of that specific decay pathway occurring compared to all other possible ways the particle can decay.

For example, the Higgs boson has a high branching ratio of 58% for decaying into two b-quarks, but a tiny 0.2% ratio for decaying into two photons. These values can be measured directly using the ATLAS experiment.