arXiv subjects

Ye-Ling Zhou

Publications and source records attributed to Ye-Ling Zhou.

At least 55 records · Page 3Linked to original sources

arXiv2020

Modular Invariant Models of Leptons at Level 7

We consider for the first time level 7 modular invariant flavour models where the lepton mixing originates from the breaking of modular symmetry and couplings responsible for lepton masses are modular forms. The latter are decomposed into irreducible multiplets of the finite modular group $Γ_7$, which is isomorphic to $PSL(2,Z_{7})$, the projective special linear group of two dimensional matrices over the finite Galois field of seven elements, containing 168 elements, sometimes written as $PSL_2(7)$ or $Σ(168)$. At weight 2, there are 26 linearly independent modular forms, organised into a triplet, a septet and two octets of $Γ_7$. A full list of modular forms up to weight 8 are provided. Assuming the absence of flavons, the simplest modular-invariant models based on $Γ_7$ are constructed, in which neutrinos gain masses via either the Weinberg operator or the type-I seesaw mechanism, and their predictions compared to experiment.

hep-ph↗

arXiv2020

CP violation and circular polarisation in neutrino radiative decay

The radiative decay of neutral fermions has been studied for decades but $CP$ violation induced within such a paradigm has evaded attention. $CP$ violation in these processes can produce an asymmetry between circularly polarised directions of the radiated photons and produces an important source of net circular polarisation in particle and astroparticle physics observables. The results presented in this work outlines the general connection between $CP$ violation and circular polarisation for both Dirac and Majorana fermions and can be used for any class of models that produce such radiative decays. The total $CP$ violation is calculated based on a widely studied Yukawa interaction considered in both active and sterile neutrino radiative decay scenarios as well as searches for dark matter via direct detection and collider signatures. Finally, the phenomenological implications of the formalism on keV sterile neutrino decay, leptogenesis-induced right-handed neutrino radiative decay and IceCube-driven heavy dark matter decay are discussed.

hep-ph↗

arXiv2020

Multiple modular symmetries as the origin of flavour

We develop a general formalism for multiple moduli and their associated modular symmetries. We apply this formalism to an example based on three moduli with finite modular symmetries $S_4^A$, $S_4^B$ and $S_4^C$, associated with two right-handed neutrinos and the charged lepton sector, respectively. The symmetry is broken by two bi-triplet scalars to the diagonal $S_4$ subgroup. The low energy effective theory involves the three independent moduli fields $τ_A$, $τ_B$ and $τ_C$, which preserve the residual modular subgroups $Z_3^A$, $Z_2^B$ and $Z_3^C$, in their respective sectors, leading to trimaximal TM$_1$ lepton mixing, consistent with current data, without flavons.

hep-ph↗

arXiv2020

Trimaximal TM$_1$ mixing with two modular $S_4$ groups

We discuss a minimal flavour model with twin modular symmetries, leading to trimaximal TM$_1$ lepton mixing in which the first column of the tri-bimaximal lepton mixing matrix is preserved. The model involves two modular $S_4$ groups, one acting in the neutrino sector, associated with a modulus field value $τ_{SU}$ with residual $Z^{SU}_2$ symmetry, and one acting in the charged lepton sector, associated with a modulus field value $τ_{T}$ with residual $Z^{T}_3$ symmetry. Apart from the predictions of TM$_1$ mixing, the model leads to a new neutrino mass sum rule which implies lower bounds on neutrino masses close to current limits from neutrinoless double beta decay experiments and cosmology.

hep-ph↗

arXiv2019

Leptogenesis via Varying Weinberg Operator: the Closed-Time-Path Approach

In this work we provide a detailed study of the CP violating phase transition (CPPT) which is a new mechanism proposed to produce a baryon asymmetry. This mechanism exploits the Weinberg operator whose coefficient is dynamically realised from the vacuum expectation values (VEVs) of new scalars. In the specific case of the first order phase transition, the scalar VEVs vary in the bubble wall which separates the two phases. This results in a spacetime varying coefficient for the Weinberg operator. The interference of two Weinberg operators at different spacetime points generates a CP asymmetry between lepton and anti-lepton production/annihilation processes, which eventually results in an asymmetry between baryon and anti-baryon number densities in the early Universe. We present the calculation of the lepton asymmetry, based on non-equilibrium quantum field theory methods, in full. We consider the influence of the bubble wall characteristics and the impact of thermal effects on the lepton asymmetry and draw a comparison between the CPPT mechanism and electroweak baryogenesis.

hep-ph↗

arXiv2019

Littlest mu-tau seesaw

We propose a $μ-τ$ reflection symmetric Littlest Seesaw ($μτ$-LSS) model. In this model the two mass parameters of the LSS model are fixed to be in a special ratio by symmetry, so that the resulting neutrino mass matrix in the flavour basis (after the seesaw mechanism has been applied) satisfies $μ-τ$ reflection symmetry and has only one free adjustable parameter, namely an overall free mass scale. However the physical low energy predictions of the neutrino masses and lepton mixing angles and CP phases are subject to renormalisation group (RG) corrections, which introduces further parameters. Although the high energy model is rather complicated, involving $(S_4\times U(1))^2$ and supersymmetry, with many flavons and driving fields, the low energy neutrino mass matrix has ultimate simplicity.

hep-ph↗

arXiv2019

Leptogenesis via Varying Weinberg Operator: a Semi-Classical Approach

In this Letter, we introduce leptogenesis via a varying Weinberg operator from a semi-classical perspective. This mechanism is motivated by the breaking of an underlying symmetry which triggers a phase transition that causes the coupling of the Weinberg operator to become dynamical. Consequently, a lepton anti-lepton asymmetry arises from the interference of the Weinberg operator at two different spacetime points. Using this semi-classical approach, we treat the Higgs as a background field and show a reflection asymmetry between the leptons and anti-leptons is generated in the vicinity of the bubble wall. We solve the equations of motion of the lepton and anti-lepton quasiparticles to obtain the final lepton asymmetry.

hep-ph↗

arXiv2019

Constraining $A_4$ Leptonic Flavour Model Parameters at Colliders and Beyond

The observed pattern of mixing in the neutrino sector may be explained by the presence of a non-Abelian, discrete flavour symmetry broken into residual subgroups at low energies. Many flavour models require the presence of Standard Model singlet scalars which can promptly decay to charged leptons in a flavour-violating manner. We constrain the model parameters of a generic $A_4$ leptonic flavour model using a synergy of experimental data including limits from charged lepton flavour conversion, an 8 TeV collider analysis and constraints from the anomalous magnetic moment of the muon. The most powerful constraints derive from the MEG collaborations' limit on Br$\left(μ\to eγ\right)$ and the reinterpretation of an 8 TeV ATLAS search for anomalous productions of multi-leptonic final states. We quantify the exclusionary power of each of these experiments and identify regions where the constraints from collider and MEG experimental data are complementary.

hep-ph↗

arXiv2019

Effective alignments and the landscape of $S_4$ flavour models

We explore the concept of effective alignments: contractions of multiple flavour symmetry breaking flavon fields. These contractions give rise to directions that are hard or impossible to obtain directly by breaking the flavour symmetry. Within this context, and using $S_4$ as the flavour symmetry to exemplify, we perform a phenomenological check of lepton flavour models built from pairing any two effective alignments up to order 2 (in flavon contractions). The check is performed for each pair of effective alignments in a framework with models of constrained sequential dominance type, in a basis where the charged leptons are diagonal. We thus obtain an indication of which effective alignments are interesting for model building, within this so-called $S_4$ landscape. We find three types of viable topologies and provide examples of models realizing this strategy for each topology.

hep-ph↗

arXiv2019

Neutrino non-standard interactions as a portal to test flavour symmetries

Imposing non-Abelian discrete flavour symmetries to neutrino non-standard interactions (NSIs) is discussed for the first time. For definiteness, we choose $A_4$ as the flavour symmetry, which is subsequently broken to the residual symmetry $Z_2$ in the neutrino sector. We provide a general discussion on flavour structures of NSIs from higher-dimensional operators ($d\leqslant8$) without inducing unnecessary tree-level 4-charged-fermion interactions. Both $A_4$- and $Z_2$-motivated NSI textures are obtained. UV completions of higher-dimensional operators lead to extra experimental constraints on NSI textures. We study the implementation of matter-effect NSIs in DUNE from phenomenological point of view, and discover that DUNE can test $A_4$ with a high level of statistics. We also present exclusion limits of sum rules suggested by UV-complete models. Our result shows that the NSI effects, though predicted to be small for DUNE, could provide useful information that might extend our understanding of the flavour symmetry.

hep-ph↗

arXiv2018

Spontaneous breaking of $SO(3)$ to finite family symmetries with supersymmetry - an $A_4$ model

We discuss the breaking of $SO(3)$ down to finite family symmetries such as $A_4$, $S_4$ and $A_5$ using supersymmetric potentials for the first time. We analyse in detail the case of supersymmetric $A_4$ and its finite subgroups $Z_3$ and $Z_2$. We then propose a supersymmetric $A_4$ model of leptons along these lines, originating from $SO(3)\times U(1)$, which leads to a phenomenologically acceptable pattern of lepton mixing and masses once subleading corrections are taken into account. We also discuss the phenomenological consequences of having a gauged $SO(3)$, leading to massive gauge bosons, and show that all domain wall problems are resolved in this model.

hep-ph↗

arXiv2018

On Neutrino Mixing in Matter and CP and T Violation Effects in Neutrino Oscillations

Aspects of 3-neutrino mixing and oscillations in vacuum and in matter with constant density are investigated working with a real form of the neutrino Hamiltonian. We find the (approximate) equalities $θ^m_{23} = θ_{23}$ and $δ^m = δ$, $θ_{23}$ ($θ^m_{23}$) and $δ$ ($δ^m$) being respectively the atmospheric neutrino mixing angle and the Dirac CP violation phase in vacuum (in matter) of the neutrino mixing matrix, which are shown to represent excellent approximations for the conditions of the T2K (T2HK), T2HKK, NO$ν$A and DUNE neutrino oscillation experiments. A new derivation of the known relation $\sin2θ^m_{23} \sinδ^m = \sin2θ_{23} \sinδ$ is presented and it is used to obtain a correlation between the shifts of $θ_{23}$ and $δ$ due to the matter effect. A derivation of the relation between the rephasing invariants which determine the magnitude of CP and T violating effects in 3-flavour neutrino oscillations in vacuum, $J_{\rm CP}$, and of the T violating effects in matter with constant density, $J^{m}_{\rm T} \equiv J^{m}$, reported in \cite{Krastev:1988yu} without a proof, is presented. It is shown that the function $F$ which appears in this relation, $J^{m} = J_{\rm CP}\,F$, and whose explicit form was given in \cite{Krastev:1988yu}, coincides with the function $\tilde{F}$ in the similar relation $J^{m} = J_{\rm CP}\tilde{F}$ derived in \cite{Naumov:1991ju}, although $F$ and $\tilde{F}$ are expressed in terms of different sets of neutrino mass and mixing parameters and have completely different forms.

hep-ph↗

arXiv2018

Effective alignments as building blocks of flavour models

Flavour models typically rely on flavons - scalars that break the family symmetry by acquiring vacuum expectation values in specific directions. We develop the idea of effective alignments, i.e.\ cases where the contractions of multiple flavons give rise to directions that are hard or impossible to obtain directly by breaking the family symmetry. Focusing on the example where the symmetry is $S_4$, we list the effective alignments that can be obtained from flavons vacuum expectation values that arise naturally from $S_4$. Using those effective alignments as building blocks, it is possible to construct flavour models, for example by using the effective alignments in constrained sequential dominance models. We illustrate how to obtain several of the mixing schemes in the literature, and explicitly construct renormalizable models for three viable cases, two of which lead to trimaximal mixing scenarios.

hep-ph↗

arXiv2018

Renormalization-Group Equations of Neutrino Masses and Flavor Mixing Parameters in Matter

We borrow the general idea of renormalization-group equations (RGEs) to understand how neutrino masses and flavor mixing parameters evolve when neutrinos propagate in a medium, highlighting a meaningful possibility that the genuine flavor quantities in vacuum can be extrapolated from their matter-corrected counterparts to be measured in some realistic neutrino oscillation experiments. Taking the matter parameter $a \equiv 2\sqrt{2} \ G^{}_{\rm F} N^{}_e E$ to be an arbitrary scale-like variable with $N^{}_e$ being the net electron number density and $E$ being the neutrino beam energy, we derive a complete set of differential equations for the effective neutrino mixing matrix $V$ and the effective neutrino masses $\widetilde{m}^{}_i$ (for $i = 1, 2, 3$). Given the standard parametrization of $V$, the RGEs for $\{\widetildeθ^{}_{12}, \widetildeθ^{}_{13}, \widetildeθ^{}_{23}, \widetildeδ\}$ in matter are formulated for the first time. We demonstrate some useful differential invariants which retain the same form from vacuum to matter, including the well-known Naumov and Toshev relations. The RGEs of the partial $μ$-$τ$ asymmetries, the off-diagonal asymmetries and the sides of unitarity triangles of $V$ are also obtained as a by-product.

hep-ph↗

arXiv2018

Baryogenesis via leptonic CP-violating phase transition

We propose a new mechanism to generate a lepton asymmetry based on the vacuum CP-violating phase transition (CPPT). This approach differs from classical thermal leptogenesis as a specific seesaw model, and its UV completion, need not be specified. The lepton asymmetry is generated via the dynamically realised coupling of the Weinberg operator during the phase transition. This mechanism provides a connection with low-energy neutrino observables.

hep-ph↗

arXiv2017

Flavour symmetric connections with CLFV

Flavons are crucial for understanding lepton mixing in models with non-Abelian discrete symmetries. They also result in charged lepton flavour violation (CLFV) via the couplings with leptons. I emphasise that the flavon-triggered CLFV succeeds strong connections with lepton flavour mixing. Relations between branching ratios of CLFV decays and mixing angles are discussed, and CLFV sum rules are obtained. Flavons with masses around hundreds of GeV are consistent by current CLFV measurements.

hep-ph↗

arXiv2017

Creating the Baryon Asymmetry from Lepto-Bubbles

We propose a new mechanism of baryogenesis which proceeds via a CP-violating phase transition. During this phase transition, the coupling of the Weinberg operator is dynamically realised and subsequently a lepton asymmetry is generated via the non-zero interference of this operator at different times. This new scenario of leptogenesis provides a direct connection between the baryon asymmetry, low energy neutrino parameters and leptonic flavour models.

hep-ph↗

arXiv2016

Theoretical motivations for precision measurements of oscillation parameters

We discuss two theoretical motivations of precision measurements of oscillation parameters. One is the guidance for flavour model building with flavour symmetries, and the other is the connection with the origin of matter-antimatter asymmetry. We also present our recent progress in these aspects, a new approach of flavour model building based on cross coupling in the flavon potential and a novel mechanism of leptogenesis via phase transition.

hep-ph↗