The dynamical origin of the Higgs sector of the standard model is still an open problem in elementary particle physics. A theoretically appealing possibility is the existence of a novel strongly interacting beyond the standard model gauge theory undergoing spontaneous breaking of a global symmetry. The Higgs sector of the standard model would then emerge from the Goldstone sector of that theory, as an effective theory at the scale of the TeV. In this framework, the Higgs boson is a composite particle of the new interaction.
Observed standard model interactions strongly constrain models of Higgs compositeness. Among phenomenologically possible realisations, a particularly attractive one is the model based on an Sp(4) gauge theory with two Dirac fermions transforming in the fundamental representation and three transforming in the antisymmetric representation of the gauge group. The major appeal of this theory derives from the possibility of having bound states of mixed representation fermions—chimera baryons—that can couple to the QCD top quark, providing an explanation for its comparatively large mass. This mechanism is known as top partial compositeness. Our project has provided the first simulation of the Sp(4) model underlying top partial compositeness, obtaining the first determination of chimera baryon masses, reported – together with other states of the theory – in Fig. 1 (left).
Like Quantum Chromodynamics, novel strong interactions would undergo a change of regime from deconfined to confined as the early universe cools down. If this change of regime is a first-order phase transition, the corresponding signal could be detected at gravitational wave experiments. The expected intensity of the signal is determined by the latent heat of the transition and by the dynamics of nucleations. Studying the latent heat in a first-order phase transition is a numerically challenging problem, since the presence of metastable states in the physical system reflects in an autocorrelation time that diverges exponentially with the volume. Using the logarithmic linear relaxation method, an algorithm developed by some of us to deal with strong metastabilities, we have provided the first determination of the free energy at the phase transition of SU(3) Yang-Mills theories that unambiguously show the multi-valuedness of the function, paving the way to an accurate determination of the parameters needed to determine the strength of the gravitational wave signal resulting from this and similar potential phase transitions.
Fig. 1: (Left) Quenched spectrum of the Sp(4) gauge theory in the continuum and massless-hyperquark limit. The glueball states are labelled using the JP notation, wih J the angular momentum and P the parity of the state, while PS (ps), V (v), T(t), AV (av), AT (at) and S (s) denote the pseudoscalar, vector, tensor, axial-vector, axial-tensor and scalar mesons composed of fundamental (antisymmetric) hyperquarks. (Right) The free energy per unit of volume, f, of SU(3) Yang-Mills on a 4×243 lattice around the critical region. The butterfly wing behaviour is typical of a first-order phase transition point.
PI: Biagio Lucini