Abstract: Dirac neutrino masses require two distinct neutral Weyl spinors per generation, with a special arrangement of masses and interactions with charged leptons. Once this arrangement is perturbed, lepton number is no longer conserved and neutrinos become Majorana particles. If these lepton number violating perturbations are small compared to the Dirac mass terms, neutrinos are quasi-Dirac particles. Alternatively, this scenario can be characterized by the existence of pairs of neutrinos with almost degenerate masses, and a lepton mixing matrix which has 12 angles and 12 phases. In this work we discuss the phenomenology of quasi-Dirac neutrino oscillations and derive limits on the relevant parameter space from various experiments. In one parameter perturbations of the Dirac limit, very stringent bounds can be derived on the mass splittings between the almost degenerate pairs of neutrinos. However, we also demonstrate that with suitable changes to the lepton mixing matrix, limits on such mass splittings are much weaker, or even completely absent. Finally, we consider the possibility that the mass splittings are too small to be measured and discuss bounds on the new, nonstandard lepton mixing angles from current experiments for this case.

Abstract: The main limitation for preinflationary breaking of Peccei-Quinn (PQ) symmetry is the upper bound on the Hubble rate during inflation from axion isocurvature fluctuations. This leads to a tension between high scale inflation and QCD axions with grand unified theory scale decay constants, which reduces the potential for a detection of tensor modes at next generation cosmic microwave background (CMB) experiments. We propose a mechanism that explicitly breaks PQ symmetry via nonminimal coupling to gravity, that lifts the axion mass above the Hubble scale during inflation and has negligible impact on today's axion potential. The initially heavy axion gets trapped at an intermediate minimum during inflation given by the phase of the nonminimal coupling, before it moves to its true CP-conserving minimum after inflation. During this stage, it undergoes coherent oscillations around an adiabatically decreasing minimum, which slightly dilutes the axion energy density, while still being able to explain the observed dark matter relic abundance. This scenario can be tested by the combination of next generation CMB surveys like CMB-S4 and LiteBIRD with haloscopes such as ABRACADABRA, DMRadio, or CASPEr-Electric.

Abstract: We propose a novel realization of the Nelson-Barr mechanism “seeded” by a dark sector containing scalars and vectorlike quarks. Charge parity (CP) and a Z8 symmetry are spontaneously broken by the complex vacuum expectation value of a singlet scalar, leaving a residual Z2 symmetry that stabilizes dark matter (DM). A complex Cabibbo-Kobayashi-Maskawa matrix arises via one-loop corrections to the quark mass matrix mediated by the dark sector. In contrast with other proposals where nonzero contributions to the strong CP phase arise at the one-loop level, in our case this occurs only at two loops, enhancing naturalness. Our scenario also provides a viable weakly interacting massive particle scalar DM candidate.

Abstract: We show how current LIGO data is able to probe interesting theories beyond the Standard Model, particularly dark sectors where a dark Higgs boson triggers symmetry breaking via a first-order phase transition. We use publicly available LIGO O2 data to illustrate how these sectors, even if disconnected from the Standard Model, can be probed by gravitational wave detectors. We link the LIGO measurements with the model content and mass scale of the dark sector, finding that current O2 data are testing a broad set of scenarios that can be mapped into many different types of dark-sector models where the breaking of SU(N) theories with Nf fermions is triggered by a dark Higgs boson at scales ? similar or equal to 108-109 GeV with reasonable parameters for the scalar potential.

Abstract: Recent studies have claimed that the strong CP problem does not occur in QCD, proposing a new order of limits in volume and topological sectors when studying observables on the lattice. In order to shed light on this issue, we study the effect of the topological theta-term on a simple quantum mechanical rotor that allows a lattice description. The topological susceptibility and the theta-dependence of the energy spectrum are both computed using local lattice correlation functions. The sign problem is overcome by considering Taylor expansions in theta, exploiting automatic differentiation methods for Monte Carlo processes. Our findings confirm the conventional wisdom on the strong CP problem.