Particles decay into other particles over time. Decay channels are the possible transformations a particle can undergo as it decays. For example, the Higgs could decay into several different channels, such as two photons or two W bosons or two Z bosons.
Physicists can calculate how long a particle should last and the ways it should decay. Knowing a particle’s decay channels can help physicists spot new particles created in ATLAS collisions, even if the particle decays before a detector can capture it. If experimentalists can’t see the particle itself, they can see the products of its decay.