Anomalous Higgs couplings as a window to new physics

Abstract

The initial data on the production and decay of the Higgs boson reported significant deviations from the Standard Model (SM) expectations, prompting much speculation about its couplings to the other particles. Although the latest data has veered towards conformity with the SM, there is yet room for a sizable deviation from the SM values of the coupling of the Higgs boson with tt, and to a smaller extent, of that with W + W − and ZZ. Keeping the fluid nature of the data in mind, this opens up an interesting avenue to explore regarding unitarity of gauge boson scattering and the stability of the electroweak vacuum in the presence of anomalous couplings. We show that, for some typical benchmark points, unitarity in gauge boson scattering breaks down between 1 and 10 TeV. We also show that if there are no new light degrees of freedom, the Higgs quartic coupling becomes negative at around the same point, making the electroweak vacuum unstable. Thus, some new ultraviolet completing new physics is demanded at that scale to cancel both these anomalous behaviours if such deviations from the SM couplings are indeed established.

Laboratory-frame observables for probing the top-Higgs boson interaction

“…Here we only show the contours for positive values of a f and κ V . An excess seen initially in the h → γγ channel (excess which is now reduced in ATLAS data and absent in CMS data) pointed to negative values of a f , which would have had serious consequences on unitarity [68,69]. In the figure, the black dot at (0.97, 1.06) indicates the best-fit value, while the yellow, green and blue regions correspond to the 68%, 95% and 99.7% confidence level regions, respectively.…”

Scattering unitarity with effective dimension-6 operators

“…A study somewhat similar in approach, assuming that the SM vertices may allow some deviations from their predicted values, through possibly higher-order interactions, was undertaken in Refs. [30,31].…”

Higgs, Supersymmetry and Dark Matter After Run I of the LHC

“…In short, significant deviations from the SM values are preferred by the currently available data and should certainly be considered viable. Since having C U ∼ −1 (in the C V > 0 convention) is not easy to achieve in a realistic model context, and leads to unitarity violation in W W → tt scattering at scales that can be as low as 5 TeV [227,228], it would seem that new physics contributions to the effective couplings of the Higgs to gluons and photons are the preferred option. (The possibility of a second, degenerate Higgs boson contributing to the observed signal remains another interesting option, not considered here.)…”