PI: Davide Vadacchino
A central open question in particle physics is whether the Higgs boson is an elementary scalar or a composite state emerging from a previously unprobed strongly interacting sector Beyond the Standard Model. Addressing this requires first-principles, non-perturbative calculations of candidate gauge theories. In 2025, we carried out large-scale lattice simulations of an Sp(4) gauge theory that realises a minimal UV completion of a composite Higgs framework with partial top compositeness. Our results determine the spectrum of mesons and chimera baryons — bound states that encode how electroweak symmetry breaking and fermion mass generation could arise dynamically. The first figure summarises these mass determinations across multiple ensembles, demonstrating a stable and well-resolved hierarchy of states, including excited resonances. By combining high statistics with spectral-density reconstruction techniques, we obtain quantitatively robust predictions that provide essential input for effective field theory descriptions and phenomenological studies of composite Higgs models.
Beyond zero-temperature spectroscopy, understanding the thermal behaviour of new strong sectors is essential for assessing their cosmological implications. In particular, a first-order phase transition in the early universe would provide the out-of-equilibrium conditions required for baryogenesis and could also generate a potentially observable gravitational-wave signal. Using the Logarithmic Linear Relaxation (LLR) density-of-states algorithm, we reconstructed the entropy of the theory with high precision across a finite-temperature transition. The second figure shows the entropy as a function of microcanonical temperature for the Sp(4) lattice gauge theory: its characteristic structure clearly signals phase coexistence across a range of spatial lattice volumes and allows us to determine the critical temperature and latent heat directly on the lattice. Because LLR gives access to thermodynamic quantities without relying on conventional importance sampling, it overcomes the severe slowing down that affects standard Monte Carlo simulations near first-order transitions and enables controlled studies toward the continuum limit.
Together, these results establish both the particle spectrum and the phase structure of a key composite Higgs candidate theory, demonstrating the power of advanced algorithms combined with leadership-class DiRAC computing resources.
