Higgs boson gets SMEFT treatment

The growing LHC dataset eight years after the discovery of the Higgs boson allows the experiments to study its properties more and more precisely, searching for hints of physics beyond the Standard Model (SM). New phenomena might occur at energy scales beyond the reach of the LHC, pointing to the existence of so-far undiscovered particles with masses too heavy to be directly produced in 13 TeV proton–proton collisions. Without knowing the exact nature of the new physics, LHC data can be analysed to systematically constrain new types of interactions in the framework of an effective field theory (EFT). One historical EFT example is Fermi’s effective interaction model for nuclear beta decay, which is valid as long as the probed energy scale is well below the mass of the W boson. The move to constrain EFTs rather than signal strengths for couplings marks a new, more comprehensive phase in SM tests at the LHC.

The move to constrain EFTs marks a new, more comprehensive phase in SM tests at the LHC

Almost all types of new physics would give rise to new interactions with SM particles, with different models leaving different EFT footprints. As the underlying dynamics is not known and effects can be subtle, it is important to combine as many measurements as possible across the full spectrum of the LHC research programme.

A new ATLAS analysis presented at the Higgs 2020 conference, held online from 26 to 30 October, takes a first step in this direction. The analysis combines measurements of production cross-sections and kinematic variables of Higgs-boson events in several decay channels (diphoton, four-lepton and di-b-quark decays) to constrain new phenomena within the so-called SMEFT framework. The combination of measurements allows multiple new interactions involving the Higgs boson to be constrained simultaneously. This approach requires fewer hypotheses on the other unconstrained interactions than studying the EFT terms one measurement at a time. The results are therefore more generic and easier to interpret in a broader context.

Predicted to vanish

Figure 1 shows the allowed ranges for the coupling coefficients of new EFT interactions to which the ATLAS combined Higgs analysis is sensitive. The coefficient c⁽³⁾_Hq, for example, describes the strength of an effective four-particle interaction between two quarks, a gauge boson and the Higgs boson. The SM predicts all these coefficients to vanish, as their corresponding interactions are not present. Significant positive or negative deviations would indicate new physics. For instance, a non-vanishing value of c⁽³⁾_Hq  would cause deviations from the SM in the ZH and WH cross-sections at high transverse momentum of the Higgs boson, which are not observed in the measured channels.

All measurements are compatible with the SM, indicating that if new physics is present it either has a mass scale larger than 1 TeV (the reference scale for which these results are reported) – or it manifests itself in interactions to which the available measurements are not yet sensitive. In the meantime, thanks to the design of the analysis, the results can be added to wider EFT interpretations that combine measurements from different physics processes (e.g. electroweak- boson or top-quark production) studied by ATLAS and other experiments, providing a consistent and increasingly detailed mapping of the allowed new physics extensions of the SM.