An urgent question in Particle Physics is related to the possible existence of new interactions beyond the Standard Model. The existence of those interactions is strongly suggested by currently open problems such as the nature of Dark Matter and the naturalness of the measured value of the Higgs mass.  A framework that could provide a solution to a significant number of open problems is near-conformal dynamics beyond the Standard Model. This framework is based on the existence of a novel strong force whose dynamics, rather than being confining, is conformal in the infrared (i.e., at asymptotically large distances).

Figure 1: The ratio of the mass of the vector meson over the mass of the scalar meson as a function of the constituent fermion mass for the SU(2) gauge theory with one adjoint fermion flavour. Different colours indicate different values of the gauge coupling, and larger couplings correspond to smaller lattice spacings. As the constituent mass goes to zero, this ratio is expected to diverge if the theory is confining and to be approximately constant if the theory is infrared conformal.

Currently, no example of a model displaying near-conformality in the infrared is known in nature. Therefore, as a part of the investigation effort, a set of signatures for near-conformality needs to be identified. The problem being a typically strong coupling one, Monte Carlo simulations of the model discretised on a spacetime lattice provide the best quantitative first principles approach. Previous  simulations pioneered by our collaboration and conducted on earlier DiRAC hardware have shown that a robust signal of near-conformality is provided by the behaviour of ratios of spectral masses: in the near-conformal case, these ratios are very mild functions of the mass of the constituent fermions, while in the better understood confining case there is a state (the Goldstone boson associated with the breaking of a global internal symmetry) whose mass goes to zero as the constituent mass is decreased towards zero, with all other masses staying instead finite in the same zero constituent mass limit. Taking the continuum limit of spectral ratios in nearly conformal theories is a remarkable computational challenge that can be addressed only with the latest hardware technologies.

Using the capabilities of the latest DiRAC3 Extreme Scaling facility, we have obtained preliminary results that, if confirmed by more extended simulations, will provide the first evidence of the persistence of near-conformal behaviour in the SU(2) gauge theory with one Dirac fermion flavour in the adjoint representation as the lattice spacing is reduced (see Fig. 1). This finding enables us to take the studied model as a robust template of near-conformal behaviour that will be used in successive investigations to inform phenomenologically relevant studies. 

Categories: 2021 Highlights