PI: David Schaich
DiRAC Project dp162/APP13862, “Stealth Dark Matter Confinement Transition and Gravitational Waves”, is exploring the possibility that the dark matter of the universe may consist of composite particles arising from a new strongly interacting dark sector beyond the standard model of particle physics. This possibility offers the exciting prospect of being able to observe effects of dark matter through a stochastic background of gravitational waves generated from a first-order confinement phase transition in the early universe. Pulsar timing array experiments recently reported evidence of such a stochastic gravitational-wave background in the nanohertz frequency range, and future space-based observatories will extend this search to higher frequencies more directly relevant to dark-sector confinement phase transitions. Quantitative analyses of these first-order transitions and the resulting spectrum of gravitational waves demand non-perturbative numerical analyses using the computational framework of lattice field theory.
The specific focus of this project is on the Stealth Dark Matter model, a concrete realization suitable for lattice studies, which still provides general insight into the broad landscape of composite dark matter. Stealth Dark Matter is an SU(N) gauge theory with four massive fermions, and in our work we consider the minimal case N=4, which is both the most tightly constrained and also the easiest to analyse through lattice field theory calculations. Any even value of N produces bosonic baryons composed of N valence fermions, and the lightest scalar baryon is the massive, stable dark matter candidate. Our previous work identified a first-order confinement transition for a particular range of fermion masses at a fixed lattice spacing, and our ongoing computations employing DiRAC resources at Cambridge will provide new non-perturbative predictions at a smaller lattice spacing, enabling extrapolations to the physical continuum limit.
In the framework of lattice field theory, first-order phase transitions are identified by critical scaling in the susceptibility of the order parameter, in this case the Polyakov loop. The accompanying figure presents preliminary results for this susceptibility, determined from calculations using L^3×12 lattice volumes up to L=48. The large temporal extent Nt=12 provides a smaller lattice spacing compared to previous work using Nt=6 and Nt=8. The clear peak in the susceptibility at the critical lattice coupling beta_F~14.9 confirms the presence of a phase transition. The critical scaling mentioned above corresponds to the clear growth in the height of the peak as L increases — note the logarithmic scale on the vertical axis!
