A report from the CMS experiment.
Following the discovery of the Higgs boson in 2012, the CMS collaboration has been exploring its properties with ever-increasing precision. Data recorded during LHC Run 2 have been used to measure differential production cross-sections of the Higgs boson in different decay channels – a pair of photons, two Z bosons, two W bosons and two tau leptons – and as functions of different observables. These results have now been combined to provide measurements of spectra at the ultimate achievable precision.
Differential cross-section measurements provide the most model-independent way to study Higgs-boson production at the LHC, for which theoretical predictions exist up to next-to-next-to-next-to-leading order in perturbative QCD. One of the most important observables is the transverse momentum (figure 1). This distribution is particularly sensitive both to modelling issues in Standard Model (SM) predictions and possible contributions from physics-beyond-the-SM (BSM).
In the new CMS result, two frameworks are used to test for hints of BSM: the κ-formalism and effective field theories.
The κ-formalism assumes that new physics effects would only affect the couplings between the Higgs boson and other particles. These new physics effects are then parameterised in terms of coefficients, κ. Using this approach, two-dimensional constraints are set on κc (the coupling coefficient of the Higgs boson to the charm quark), κb (Higgs to bottom) and κt (Higgs to top). None show significant deviations from the SM at present.
Effective field theories parametrise deviations from the SM by supplementing the Lagrangian with higher-dimensional operators and their associated Wilson coefficients (WCs). The effect of the operators is suppressed by powers of the putative new-physics energy scale, Λ. Measurements of WCs that differ from zero may hint at BSM physics.
The CMS differential cross-section measurements are parametrised, and constraints are derived on the WCs from a simultaneous fit. In the most challenging case, a set of 31 WCs is used as input to a principal-component analysis procedure in which the most sensitive directions in the data are identified. These directions (expressed as linear combinations of the WCs) are then constrained in a simultaneous fit (figure 2). In the upper panel, the limits on the WCs are converted to lower limits on the new physics scale. The results agree with SM predictions, with a moderate 2σ tension present in one of the directions (EV5). Here the major contribution is provided by the cHq3 coefficient, which mostly affects vector-boson fusion, VH production at high Higgs-boson transverse momenta (V = W, Z) and W-boson decays.
The combined results not only provide highly precise measurements of Higgs-boson production, but also place stringent constraints on possible deviations from the SM, deepening our understanding while leaving open the possibility of new physics at higher precision or energy scales.
