PI: Vincent Drach
Understanding the nature of the Standard Model Higgs boson is still an open problem. Extensions of the Standard Model in which the Higgs fields have a composite origin provide a compelling first-principles explanation of the origin of the Higgs boson and explain its mass in terms of spontaneous breaking of enlarged global symmetries of a novel strong interaction. Among minimal candidate realisations of Higgs compositeness is SU(2) gauge theory with two fundamental fermions. Given the non-perturbative nature of the novel force, first-principle calculations allow to determine quantitative predictions that are used to test the experimental viability of these theories.
Our long-term goal is to unravel the properties of the flavour singlet scalar resonance, referred to as 𝜎, in SU(2) gauge theory with two fundamental fermions. In general, in Composite Higgs scenarios, the physical Higgs boson is a mixture of Goldstone bosons and of the lightest scalar state. The presence of the scalar state alters the properties of the physical Higgs boson and gives rise to an additional scalar resonance with a larger mass, expected to be produced by the LHC in similar fashion to the SM Higgs. The model is not excluded by the current LHC data.
Dealing with the σ resonance on the lattice is a challenge in its own right, because of disconnected contributions and of a poor signal to noise ratio. Furthermore, chiral perturbation theory fails to unitarise the cross-section Goldstone bosons scattering amplitude in a regime where the σ resonance is unstable. In addition, one first must reach the regime in which the σ resonance is unstable, which constrained us to resort to an improved action and overcome the challenges to run simulations in an unexplored regime.
Reliable predictions require a careful assessment of the systematics. We are now in a position to examine lattice discretisation errors at a fixed reference mass using four lattice spacings and currently drafting a publication summarising our results. The results are particularly interesting to the lattice community because we are using a new discretisation refer to as non-perturbatively improved exponential clover Wilson fermions/ In parallel we are investigating the mass of the scalar meson using a two-point correlation function on our most chiral ensemble and were able to reach a regime that have never been explored before. It will allow us to constrain the properties of scalar resonance for the first time. Both results were presented at the latest international symposium in lattice field theory. Our work contributes to the understanding of the role of resonances in the phenomenology of the class of composite models characterised by the strong sector we are considering, irrespective of its embedding.
Fig. 1: Left: The effective mass of the 𝜎 against lattice time, the decay threshold is shown in orange. Right: Continuum extrapolation of 𝑤0 𝑓PS