Higgs production and decay channels
A Higgs boson is not a thing you can buy and store; it must be freshly made in a violent collision, and it falls apart almost the instant it appears. So studying the Higgs means understanding two separate questions: how do you make one (production), and what does it turn into when it decays (decay channels)? Both are governed by the same rule — the Higgs talks most strongly to the heaviest particles — which makes its behavior intricate but predictable, like a guest who only mingles with the biggest names in the room.
On the production side, the most common way to make a Higgs at the Large Hadron Collider is 'gluon fusion': two gluons from the colliding protons combine, by way of a loop of heavy top quarks, into a single Higgs. Other routes (such as two W or Z bosons fusing, or a Higgs produced alongside a W, Z, or pair of top quarks) are rarer but give cleaner experimental tags. On the decay side, a 125-GeV Higgs has many options. Most often it decays into a bottom-quark and antiquark pair, because the bottom is heavy and so couples strongly; but that channel is buried in background noise. The two-photon channel and the four-lepton channel are rare (the two-photon route happens only a couple of times per thousand Higgs bosons) yet so distinctive that they were the channels that nailed the 2012 discovery. The fraction of Higgs bosons going into each option is called that channel's branching ratio.
These channels are how the Higgs is tested in detail. By measuring how often the Higgs is produced and how often it decays each way, and comparing with the precise Standard Model predictions, physicists check whether this particle really is the textbook Higgs. So far the measured rates agree with predictions within experimental uncertainties, which is a striking success. The honest frontier: some rare channels (like the Higgs decaying to two muons, or its coupling to itself) are still being measured or remain out of reach, and any deviation could be the first crack pointing to new physics.
Roughly speaking, out of every 100 Higgs bosons made at the LHC, about 58 decay to bottom quarks, around 21 to W bosons, a handful to tau leptons, Z bosons, and gluons, and only about 0.2 decay to two photons. Yet that tiny diphoton slice, being so clean, was one of the two channels that revealed the Higgs in 2012.
The Higgs mostly decays to b quarks, but the rare, clean diphoton channel is what found it.
The most common decay is not the easiest to spot: the dominant b-quark channel drowns in background, while the rare two-photon channel is clean. 'Frequent' and 'detectable' are not the same thing.