Introduction
Modified Higgs couplings are studied in an effective theory approach, where higher-order modifications to the Lagrangian are modelled by coupling strength modifiers at tree-level. We study a model in which the couplings of the Higgs boson to vector bosons is modified by the constant κV, where custodial symmetry is assumed. The coupling to fermions is modified by a factor κF. In the Standard Model, κV = κF = 1. This benchmark model uses the explicit assumption that there are no additional loops in the production or decay of the Higgs boson and no undetectable contributions to the decay width are present. More details are given in the report by the Higgs boson working group.The main contributions from the modified Higgs couplings in this model to electroweak precision observables comes from the modified Higgs coupling to vector bosons. The corrections can be expressed in terms of the oblique parameters S and T. More information can be found in the latest publication.
Besides the parameters modifying the Higgs coupling strengths, the model incorporates a cut-off scale Λ, which represents the mass scale of new states. This cut-off scale is needed because the effective model is not renormalizable by itself. The scale Λ can be interpreted as the scale where new physics arises. It is connected to κV through the relation Λ = λ / sqrt( | 1 - κV2 | )
We derive constraints on this model using electroweak precision measurements, as described in the SM section. Additional information is obtained by using the latest Higgs coupling measurements from ATLAS and CMS.
Constraints on Higgs couplings
Constraints on the Higgs coupling parameters κV and κF are derived from a fit to electroweak precision data, as described in the SM section, together with data on Higgs coupling measurements, which are expressed as signal strength modifiers.
The electroweak fit results in the following values for κV:
κV = 1.037+0.029-0.026 for λ = 1 TeV
κV = 1.027+0.020-0.019 for λ = 3 TeV
κV = 1.021+0.015-0.014 for λ = 10 TeV
For κV = 1.03 and λ = 4 π v, the scale of new physics is Λ ≥ 13 TeV.
Contours of 68%, 95%, and 99% confidence level in the TS-plane. The reference point at which all oblique parameters vanish is defined by MH=125 GeV and mt=173 GeV. The red line shows the contribution to S and T from this model, when varying κV in the interval [0, 2] and the scale λ in the interval [1,10] TeV. |
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Contours of 68%, and 95% confidence level in the κF versus κV plane. The orange ellipses show the constraints obtained when using only Higgs coupling measurements, the blue ellipses show the constraints when combining Higgs coupling measurements with electroweak precision data. The black dot shows the Standard model prediction, while the open symbol shows the fit minimum. |
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Contours of 95% confidence level in the MW versus κV plane, where the measurements of MW and κV were not included in the fit (blue). The three ellipses are obtained with different values of the cut-off scale λ. The direct measureemnts at 68% and 95% confidence level are shown in green. |
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