Celso C. Nishi

We investigate Nelson-Barr (NB) solutions to the strong C P problem in which spontaneous C P violation is transmitted to the Standard Model (SM) through mixing with a vectorlike partner of the SM quark doublet. We show that these constructions constitute compelling and phenomenologically viable alternatives to the more widely studied singlet-based NB models. A key result of our analysis is that an accidental symmetry of the renormalizable theory delays the leading contributions to θ ¯ until three loops, naturally suppressing hadronic C P violation. We outline the main phenomenological constraints, including future electric dipole moment experiments, as well as the main differences between these scenarios and generic models with doublet vectorlike quarks.

The recent result of the Fermilab Muon g-2 experiment is still in conflict with the standard model (SM) prediction if dispersive methods are used as input for the hadronic vacuum polarization. We seek models where the same set of mediators around the weak scale or above contribute to neutrino masses and to the muon g − 2 through a chiral enhanced contribution. As family lepton number violation is inherent to the former, non-negligible charged lepton flavor violation will be induced. We arrive at the following field content additional to the SM: one Higgs doublet, one singly charged scalar, and one right-handed neutrino. Neutrino masses will be generated by a combination of mechanisms: the tree-level seesaw, Zee mechanism, and Grimus-Neufeld mechanism, while lepton flavor violation will be minimized by considering Yukawa-aligned Higgs doublets. The setting is flexible enough to account for ( g − 2 ) μ , the Cabibbo angle anomaly, and the W mass deviation, while avoiding dangerous charged lepton flavor violation.

We study the flavor invariants of the SM augmented by one singlet vector-like quark. Aided by the Hilbert series, we construct all the basic invariants with which any flavor invariant can be written as a polynomial. In special, this theory contains one CP odd invariant of degree six which has degree much lower than the usual Jarlskog invariant of the SM. We find the nonlinear polynomial relations (syzygies) of lowest degrees involving these basic invariants, including the expression of the square of the CP odd invariant of lowest degree in terms of CP even invariants. The SU(3) identity underlying this syzygy is uncovered in terms of invariant tensors, which can be applied to rewrite any square of a CP odd invariant of the same form, involving three hermitean matrices of size three. We demonstrate by an example that there is CP violation that is not detected by the CP odd invariants proposed in the literature so far but it can be detected with the full list of CP odd invariants found here.

We review the theory and phenomenology of isosinglet vector-like quarks (VLQs). In recent years, interest in VLQs has been increasing, due to their contributions to new physics effects that can be tested in experiments at LHC and High-Luminosity LHC. The similarities of models with isosinglet VLQs and the seesaw framework in the leptonic sector are pointed out. The existence of VLQs leads to flavour-changing neutral currents at tree level and deviations from unitarity of the CKM matrix, introducing rich phenomenological implications. These new effects are naturally suppressed by the masses of the new quarks, that are constrained to be above the electroweak scale. In addition, striking new effects can be achieved with the inclusion of an extra complex scalar singlet. Such a minimal extension of the SM can give rise to new sources of CP violation with profound theoretical implications, allowing for a solution to the strong CP problem and a possible explanation for the baryon asymmetry of the Universe. We list and explain strong motivations to consider this class of models. We also briefly review how models with VLQs can be matched to the SM effective field theory (SMEFT). A detailed analysis of flavour observables that can be affected by the presence of VLQs is presented. Current bounds from collider searches of VLQs are summarized. We point out that the discovery of VLQs can be within the reach of present or future colliders being planned.

The seesaw extension of the SM explains the tiny neutrino masses and it is accompanied by many CP violating phases. We study the case where all leptonic CP violation arises from the soft breaking in the heavy Majorana mass matrix. Parameter counting reveals that one less parameter is needed to describe this case. This reduction leads to restrictions on the parameter space of heavy neutrinos. We analyze the minimal seesaw case in detail and find that mass degeneracy of heavy neutrinos is not possible for certain values of the CP phases.

One simple way to lower the scale of the seesaw mechanism that generates neutrino masses is to attribute part of their smallness to a suppressed vacuum expectation value of a second Higgs doublet as in the neutrinophilic 2HDM or in the type IB seesaw model. On that structure we add one charged singlet scalar to induce a chirally enhanced contribution to (g – 2)_μ with the same righthanded neutrinos of the seesaw. We discuss the interplay of generating the necessary contribution to the latter with lepton flavor violation which is also necessarily brought to low scale. We show that it is possible to explain (g –2)_μ even for heavy neutrino masses of order of a few TeV.