PI: Biagio Lucini

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 allowed 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. We have obtained the first determination of the mesonic spectrum in this model that makes use of the HLT spectral density method, a novel technique that has a lower systematic error than more traditional methods. Another advantage of the HLT method over non-spectral techniques is a more robust determination of excited state masses. 

The figure shows the lowest-lying flavour triplet meson spectrum for representative ensembles of configurations at different lattice parameters, for both fundamental and antisymmetric representation fermions, found through spectral densities fitting analysis. Capital (lower-case) letters indicate states in the fundamental (antisymmetric) representation. For each channel, a tower of masses corresponding to ground, first and (where available) second excited states is shown in units of the physical scale of the theory. The vertical midpoint of each colour block is the best estimate of the corresponding mass, whereas the heights are comprehensive of statistical and HLT systematic errors, summed in quadrature. The figure shows five ensembles determined at different values of lattice parameters, which enables us to gauge systematic errors coming from other sources. Horizontal offsets are used to visually separate the five ensembles. M2 is the reference ensemble, with lattice parameters chosen in such a way that finite-size, discretisation and finite-mass corrections are negligible. This is explicitly verified studying additional ensembles. The ensembles M1 and M3 differ from M2 for the value of the lattice temporal extent, and hence provide information on the systematics related to the numerical extraction of masses due to contamination of further excited states in lattice correlation functions.  The ensembles M4 and M5 differ from M2 for the value of the bare fermion mass, and hence enable us to estimate the systematics related to the explicit breaking of the chiral global symmetry in flavour space.