Numerically, the decays of a 125 GeV Higgs boson in the Standard Model (which is a little below the experimentally measured value) are approximately as follows (updated per a PDG review paper with details of the calculations, for example, here and here):

b-quark pairs, 57.7% (observed)

W boson pairs, 21.5% (observed)

gluon pairs, 8.57%

tau-lepton pairs, 6.27% (observed)

c-quark pairs, 2.89%

Z boson pairs, 2.62% (observed)

photon pairs, 0.227% (observed)

Z boson and a photon, 0.153% (observed)

muon pairs, 0.021 8% (observed)

electron-positron pairs, 0.000 000 5%

The total adds 99.9518005% rather than to 100% due to rounding errors, and due to omitted low probability decays including strange quark pairs (a bit less likely than muon pairs), down quark pairs (slightly more likely than electron-positron pairs), up quark pairs (slightly more likely than electron positron pairs), and asymmetric boson pairs other than Z-photon decays (also more rare than muon pairs).

Six decay channels of the Higgs boson to ZZ, two photons, WW, bb, two tau leptons, and two muons have been observed previously and are tolerably close to the expected values in the chart above.

A new paper from the CMS experiment at the Large Hadron Collider (LHC) detects a seventh Higgs boson decay channel predicted in the Standard Model, a decay to a Z boson and a photon, although not yet at the five sigma "discovery" level of significance.

The signal strength μ, defined as the product of the cross section and the branching fraction [σ(pp→H)B(H→Zγ)] relative to the standard model prediction, is extracted from a simultaneous fit to the ℓ+ℓ−γ invariant mass distributions in all categories and is found to be μ=2.4±0.9 for a Higgs boson mass of 125.38 GeV.The statistical significance of the observed excess of events is 2.7 standard deviations.This measurement corresponds to σ(pp→H)B(H→Zγ) = 0.21±0.08 pb.

The observed (expected) upper limit at 95% confidence level on μ is 4.1 (1.8). The ratio of branching fractionsB(H→Zγ)/B(H→γγ) is measured to be 1.5+0.7−0.6, which agrees with the standard model prediction of 0.69 ± 0.04 at the 1.5 standard deviation level.

In absolute terms, the predicted branching fraction is a little less than 0.2%, and the measured branching fraction is about 0.3% but subject to large uncertainties.

As of March 2022, one of the most important decay channel of the Higgs boson not yet definitively observed is the decay of a Higg boson into a charm quark-charm antiquark pair. ATLAS and CMS have so far only been able to place upper bounds on the branching fraction of these decays (in part, because backgrounds from Z boson decays to charm quark/charm antiquark pairs make direct decays of a Higgs boson to charm quark/charm antiquark pairs difficult to distinguish).

Gluon pairs decays are likewise difficult to distinguish from other background sources of the same hadrons.

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