Thursday, April 28, 2022

Seventh Higgs Boson Decay Channel Confirmed

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).

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 fractions B(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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