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The Higgs boson, discovered at the Large Hadron Collider (LHC) in 2012, has a singular role in the standard model of particle physics. The most remarkable is the affinity of the Higgs boson to the mass, which can be likened to the electric charge of an electric field: the larger the mass of a fundamental particle is, the greater is the strength of its interaction with the Higgs boson. The deviations from these predictions could be a feature of the new physics in this still unexplored part of the standard model.
Higgs boson couplings are manifested by the speed of production of the Higgs boson at the LHC, and by its decay branching ratios in various final states. These rates were precisely measured by the ATLAS experiment at CERN, using up to 80 fb -1 of data collected at a proton-proton collision energy of 13 TeV from 2015 to 2017. Measurements were made in all the main decay pathways of the Higgs boson: photon pairs, W and Z bosons, bottom quarks, taus and muons. The overall production rate of the Higgs boson was measured as consistent with the predictions of the standard model, with an uncertainty of 8%. Uncertainty is reduced by 11% in the previous combined measurements released last year.
The measurements are decomposed into production modes (assuming decay branching rates of the standard model), as shown in Figure 1. The four main modes of production observed at ATLAS with a significance of more than 5 deviations types: the long-established gluon-gluon fusion pattern, recently associated production observed with the top-quark pair and the latest low-boson fusion mode, presented today by ATLAS. These results give a complete picture of the production and disintegration of the Higgs boson
Physicists can use these new results to study the couplings of the Higgs boson with other fundamental particles. These couplings are in excellent agreement with the prediction of the standard model over a range covering 3 orders of magnitude, from the top quark (the heaviest particle of the standard model and therefore the strongest interaction with the Higgs boson) to much lighter muons. (for which only an upper limit of coupling with the Higgs boson has been obtained to date.)
The measurements also probe the coupling of the Higgs boson to the gluons in the gluon-gluon fusion production process , which goes through a diagram loop and is therefore particularly sensitive to new physics. In the standard model, the loop is mainly mediated by the upper quarks. Therefore, possible new physical contributions can be tested by comparing gluon coupling with the direct measurement of higher quark coupling in Higgs boson production in association with higher quarks, as shown in Figure 2.
The excellent agreement with the standard model that is observed throughout, can be used to set strict limits on new physics models. These are based on possible modifications of the Higgs couplings and complement the direct searches carried out at the LHC.
Learn more:
Boson of Higgs observed in decomposition in b quarks
More information:
Combined measurements of the production and decay of the Higgs boson using up to 80 fb-1 of 13 TeV proton-proton collision data collected with the ATLAS experiment: atlas.web.cern.ch / Atlas / GROUPS … ATLAS-CONF-2018-031 /
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