arXiv subjects

C. C. Nishi

Publications and source records attributed to C. C. Nishi.

At least 19 recordsLinked to original sources

arXiv2026

Nelson-Barr Models with Vector-Like Quark Doublets

We investigate Nelson--Barr solutions to the strong CP problem in which spontaneous CP violation is transmitted to the Standard Model through mixing with a vector-like 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 \barθ until three loops, naturally suppressing hadronic CP violation. We outline the main phenomenological constraints, including future EDM experiments, as well as the main differences between these scenarios and generic models with doublet vector-like quarks.

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arXiv2025

Aligned Zee-Grimus-Neufeld model for $(g-2)_μ$

The recent result of the Fermilab Muon g-2 experiment is still in conflict with the 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 righthanded neutrino. Neutrino masses will be generated by a combination of mechanisms: 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 CLFV.

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arXiv2024

Seesaw Limit of the Nelson-Barr Mechanism

We investigate how the solution to the strong CP problem and the explanation for the observed fermion mass hierarchies can be intrinsically related. Specifically, we explore the Nelson-Barr mechanism and identify its "seesaw limit", where light quark masses are suppressed by large CP-violating terms. Upon adding three (two) vector-like quarks that mix with the down-type (up-type) quark sector of the Standard Model, we demonstrate how the lack of CP violation in the strong sector and the observed quark mass hierarchy can be simultaneously achieved.

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arXiv2024

Flavor invariants for the SM with one singlet vector-like quark

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.

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arXiv2024

Vector-like Singlet Quarks: a Roadmap

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.

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arXiv2023

Leptonic CP violation from the seesaw

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.

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arXiv2023

Connecting $(g-2)_μ$ to neutrino mass in the extended neutrinophilic 2HDM

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.

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arXiv2023

Explicit parametrization of more than one vector-like quark of Nelson-Barr type

Nelson-Barr models solve the strong CP problem based on spontaneous CP violation and generically requires vector-like quarks (VLQs) mixing with standard quarks to transmit the CP violation. We devise an explicit parametrization for the case of two VLQs of either down-type or up-type and quantitatively study several aspects including the hierarchy of the VLQ Yukawas and their irreducible contribution to $\barθ$. In particular, with the use of the parametrization, we show that a big portion of the parameter space for two up-type VLQs at the TeV scale is still allowed by the constraint on $\barθ$, although this case had been previously shown to be very restricted based on estimates.

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arXiv2022

Leptonic CP violation from a vector-like lepton

Leptonic CP violation is yet to be confirmed as an additional source of CP violation in fundamental interactions. We study the case where leptonic CP violation is spontaneous and is induced by the mixing with a heavy charged vector-like lepton (VLL). We show that the non-decoupling of this VLL is linked with the presence of CP violation and its coupling with the SM leptons are partly fixed from the SM Yukawas. Due to the large leptonic mixing angles, these couplings are typically of the same order and there is no flavor preference. Strong but not definitive constraints come from charged lepton flavor violating processes because the VLL can decouple from one or two leptonic flavors in very special points of parameter space. These special points are very sensitive to the neutrino Majorana phases.

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arXiv2021

Flavor constraints for a Vector-like quark of Nelson-Barr type

The Nelson-Barr (NB) mechanism to solve the strong CP problem assumes CP conservation, arranges vanishing $\barθ$ at tree-level and requires vector-like quarks (VLQs) to transmit the CP breaking to the SM. We analyze the flavor constraints coming from the presence of one such down type VLQ of NB type by performing a global fit on the relevant flavor observables. A comparison is made to the case of one generic VLQ. We find that the allowed parameter space for the VLQ Yukawa couplings and the mixing to the SM are confined to a region much smaller than in the generic case, making the NB case falsifiable in principle.

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arXiv2020

Consequences of vector-like quarks of Nelson-Barr type

The Nelson-Barr mechanism to solve the strong CP problem requires vector-like quarks (VLQs) to transmit the spontaneous CP breaking to the SM. We study the scenario where only these VLQs are within reach at the TeV scale while the spontaneous CP breaking sector is inaccessible. We investigate how these VLQs of Nelson-Barr type differ from generic VLQs and find from parameter counting that one less parameter is needed. In particular, for one VLQ of Nelson-Barr type, there is only one CP odd quantity that is responsible for all CP violation. In this case, we solve the technical problem of parametrizing only the new physics parameters while keeping the SM parameters as independent inputs. For one down-type VLQ, the model is largely flavor safe because the VLQ couplings to the SM up quarks and the $W$ are hierarchically smaller for lighter quarks.

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arXiv2020

Leptogenesis in the $μτ$ basis

We formulate three-flavor type-I leptogenesis in the $μτ$ basis which is convenient because in the three-flavor regime, both $μ$ and $τ$ charged lepton Yukawa interactions are in thermal equilibrium and the thermal bath is symmetric under the exchange $μ\leftrightarrow τ$. We apply this formalism to models with $μτ$-reflection $\mathsf{CP}^{μτ}$ symmetry. We confirm the previous result that leptogenesis fails in the three-flavor regime with exact $\mathsf{CP}^{μτ}$ symmetry. Allowing $\mathsf{CP}^{μτ}$ symmetry to be broken to various degrees, we show that leptogenesis can be successful in the three-flavor regime only in certain tuned parameter space, which could further imply additional symmetry is at play. As a bonus, we derive analytical expressions which could be utilized whenever the branching ratios for the decays to $μ$ and $τ$ flavors are equal or approximately so.

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arXiv2019

Solving the strong CP problem with non-conventional CP

A very simple model is presented where all CP violation in Nature is spontaneous in origin. The CKM phase is generated unsuppressed and the strong CP problem is solved with only moderately small couplings between the SM and the CP violation sector or mediator sector because corrections to $\barθ$ arise only at two loops. The latter feature follows from an underlying unconventional CP symmetry of order 4 imposed in the sectors beyond the SM composed of only two vector-like quarks of charge $-1/3$ and one complex scalar singlet. No additional symmetry is necessary to implement the Nelson-Barr mechanism.

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arXiv2018

Fermion Mass Hierarchy and Double Seesaw Mechanism in a 3-3-1 Model with an Axion

We present a model based on the $SU(3)_C\otimes SU(3)_L\otimes U(1)_X$ gauge symmetry that relates the mass hierarchy of the fermions with the solution to the strong CP problem through the $U(1)_{PQ}$ Peccei-Quinn symmetry. This last symmetry arises accidentally with the imposition of a discrete $Z_9$ symmetry, which also secludes the different scales in the double seesaw mechanism taking place in the neutrino sector. The symmetry breakdown is performed by three scalar triplets plus a scalar singlet hosting an axion field, whose particle excitation can be a component of dark matter. We show a mechanism where a small effective vev is generated for a scalar triplet which is supposed to have a bare mass above the energy scale where the $SU(3)_L\otimes U(1)_X$ symmetry is broken. Combined with the energy scale in which the $U(1)_{PQ}$ is broken, such a mechanism gives rise to a natural hierarchy to the fermions. Beyond the Standard Model particle content, the model predicts an invisible axion, $a$, three GeV neutrinos, $N_{iL}$, plus several new particles at the TeV scale which are: five vector bosons, $U^\pm$, $V^0$, $V^{0\dagger}$, and $Z^\prime$; one up-type $U$, and two down-type $D_a$ quarks; and at least a CP-even, $H_1$, plus non-hermitian neutral, $ϕ^0$, $ϕ^{0\dagger}$, scalar bosons. The model may be tested by looking for the possible production of such particles at the LHC.

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arXiv2018

Mu-tau reflection symmetry with a high scale texture-zero

The $μτ$-reflection symmetric neutrino mass matrix can accommodate all known neutrino mixing angles, with maximal atmospheric angle fixed, and predicts all the unknown CP phases of the lepton sector but is unable to predict the absolute neutrino mass scale. Here we present a highly predictive scenario where $μτ$-reflection is combined with a discrete abelian symmetry to enforce a texture-zero in the mass matrix of the heavy right-handed neutrinos that generate the light neutrino masses. Such a restriction reduces the free parameters of the low energy theory to zero and the absolute neutrino mass scale is restricted to few discrete regions, three in the few meV range and one extending up to around 30 meV. The heavy neutrino sector is dependent only on two free parameters which are further restricted to small regions from the requirement of successful leptogenesis. Mass degenerate heavy neutrinos are possible in one case but there is no resonant enhancement of the CP asymmetry.

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arXiv2018

Hierarchical Fermions and Detectable $Z^\prime$ from Effective Two-Higgs-Triplet 3-3-1 Model

We develop a ${\rm SU}(3)_C\otimes {\rm SU}(3)_L\otimes {\rm U}(1)_X$ model where the number of fermion generations is fixed by cancellation of gauge anomalies, being a type of 3-3-1 model with new charged leptons. Similarly to the economical 3-3-1 models, symmetry breaking is achieved effectively with two scalar triplets so that the spectrum of scalar particles at the TeV scale contains just two CP even scalars, one of which is the recently discovered Higgs boson, plus a charged scalar. Such a scalar sector is simpler than the one in the Two Higgs Doublet Model, hence more attractive for phenomenological studies, and has no FCNC mediated by scalars except for the ones induced by the mixing of SM fermions with heavy fermions. We identify a global residual symmetry of the model which guarantees mass degeneracies and some massless fermions whose masses need to be generated by the introduction of effective operators. The fermion masses so generated require less fine-tuning for most of the SM fermions and FCNC are naturally suppressed by the small mixing between the third family of quarks and the rest. The effective setting is justified by an ultraviolet completion of the model from which the effective operators emerge naturally. A detailed particle mass spectrum is presented, and an analysis of the $Z^\prime$ production at the LHC run II is performed to show that it could be easily detected by considering the invariant mass and transverse momentum distributions in the dimuon channel.

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arXiv2018

CP Violation in the Lepton Sector and Implications for Leptogenesis

We review the current status of the data on neutrino masses and lepton mixing and the prospects for measuring the CP-violating phases in the lepton sector. The possible connection between low energy CP violation encoded in the Dirac and Majorana phases of the Pontecorvo-Maki-Nakagawa-Sakata mixing matrix and successful leptogenesis is emphasized in the context of seesaw extensions of the Standard Model with a flavor symmetry Gf (and CP symmetry).

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arXiv2017

One-loop considerations for coexisting vacua in the CP conserving 2HDM

The Two-Higgs-Doublet model (2HDM) is a simple and viable extension of the Standard Model (SM) with a scalar potential complex enough that two minima may coexist. In this work we investigate if the procedure to identify our vacuum as the global minimum by tree-level formulas carries over to the one-loop corrected potential. In the CP conserving case, we identify two distinct types of coexisting minima --- the regular ones (moderate $\tanβ$) and the non-regular ones (small or large $\tanβ$) --- and conclude that the tree level expectation fails only for the non-regular type of coexisting minima. For the regular type, the sign of $m^2_{12}$ already precisely indicates which minima is the global one, even at one-loop.

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