arXiv subjects

Claudia Hagedorn

Publications and source records attributed to Claudia Hagedorn.

At least 19 recordsLinked to original sources

arXiv2025

Selection rules for charged lepton flavour violating processes from residual flavour groups

We systematically investigate the possible phenomenological impact of residual flavour groups in the charged lepton sector. We consider all possible flavour charge assignments for abelian residual symmetries up to Z8. The allowed flavour structures of operators in Standard Model Effective Field Theory (up to dimension six) lead to distinctive and observable patterns of charged lepton flavour violating processes. We illustrate the relevance of such selection rules displaying the current bounds on and the future sensitivities to the new physics scale. These results demonstrate, in particular, the importance and discriminating power of searches for lepton flavour violating tau lepton decays and muonium to antimuonium conversion.

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arXiv2024

Low-scale seesaw with flavour and CP symmetries $\unicode{x2013}$ from colliders to leptogenesis

We consider an extension of the Standard Model with three right-handed neutrinos, endowed with a flavour symmetry $G_f$, $G_f=Δ(3 \, n^2)$ or $G_f=Δ(6 \, n^2)$, $n \geq 2$, and CP. For large active-sterile mixing, we study the properties of the (nearly mass-degenerate) heavy neutrinos, such as their lifetimes and branching ratios. In doing so, we examine the four different cases, called Case 1) through Case 3 b.1), that lead to distinct lepton mixing patterns, all potentially compatible with current data. Furthermore, we comprehensively explore for each case the region of parameter space in which a sufficient amount of baryon asymmetry of the Universe can be generated via leptogenesis, while being testable at accelerator-based and potentially also precision flavour experiments.

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arXiv2023

Flavon vacuum alignment beyond SUSY

In flavor models the vacuum alignment of flavons is typically achieved via the $F$-terms of certain fields in the supersymmetric limit. We propose a method for preserving such alignments, up to a rescaling of the vacuum expectation values, even after supersymmetry (and the flavor symmetry) are softly broken, facilitating the vacuum alignment in models which are non-supersymmetric at low energies. Examples of models with different flavor groups, namely $A_4$, $T_7$, $S_4$ and $Δ(27)$, are discussed.

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arXiv2023

Flavour anomalies meet flavour symmetry

We construct an extension of the Standard Model with a scalar leptoquark $ϕ\sim (3,1,-\tfrac13)$ and the discrete flavour symmetry $G_f=D_{17}\times Z_{17}$ to explain anomalies observed in charged-current semi-leptonic $B$ meson decays and in the muon anomalous magnetic moment, together with the charged fermion masses and quark mixing. The symmetry $Z_{17}^{\rm diag}$, contained in $G_f$, remains preserved by the leptoquark couplings, at leading order, and efficiently suppresses couplings of the leptoquark to the first generation of quarks and/or electrons, thus avoiding many stringent experimental bounds. The strongest constraints on the parameter space are imposed by the radiative charged lepton flavour violating decays $τ\toμγ$ and $μ\to eγ$. A detailed analytical and numerical study demonstrates the feasibility to simultaneously explain the data on the lepton flavour universality ratios $R(D)$ and $R(D^\star)$ and the muon anomalous magnetic moment, while passing the experimental bounds from all other considered flavour observables.

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arXiv2022

Low-scale leptogenesis with flavour and CP symmetries

We consider a type-I seesaw framework with a flavour symmetry from the series of non-abelian groups $Δ(3 n^2)$ and $Δ(6 n^2)$ and a CP symmetry. Breaking these symmetries non-trivially results in the right-handed neutrinos being degenerate in mass up to possible (further symmetry-breaking) splittings $κ$ and $λ$, while the neutrino Yukawa coupling matrix encodes the entire flavour structure in the neutrino sector. For a fixed combination of flavour and CP symmetry and residual groups, this matrix has five real free parameters. Four of them are set by light neutrino masses and data on lepton mixing, while $θ_R$ is related to right-handed neutrinos. We scrutinise for all four lepton mixing patterns, grouped into Case 1) through Case 3 b.1), the potential to generate the baryon asymmetry of the Universe (BAU) through low-scale leptogenesis numerically and analytically. The main results are: a) the possible correlation of the BAU and the Majorana phases of the PMNS mixing matrix in certain instances; b) the possibility to generate the correct amount of BAU for zero $κ$ and $λ$ as well as for large $κ$, depending on the case and choice of group theory parameters; c) the chance to produce sufficient BAU for large active-sterile mixing angles, enabling direct experimental tests at current and future facilities, for $θ_R$ close to a special value, potentially protected by an enhanced residual symmetry. We elucidate these results with representative examples of flavour and CP symmetries, which all lead to a good agreement with the measured lepton mixing angles and, possibly, the current hint of the CP phase $δ$. We identify the CP-violating combinations relevant for low-scale leptogenesis, and show that the parametric dependence of the BAU found numerically can be understood well with their help.

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arXiv2020

Lepton and Quark Masses and Mixing in a SUSY Model with Delta(384) and CP

We construct a supersymmetric model for leptons and quarks with the flavor symmetry Delta(384) and CP. The peculiar features of lepton and quark mixing are accomplished by the stepwise breaking of the flavor and CP symmetry. The correct description of lepton mixing angles requires two steps of symmetry breaking, where tri-bimaximal mixing arises after the first step. In the quark sector the Cabibbo angle theta_C equals sin pi/16 = 0.195 after the first step of symmetry breaking and it is brought into full agreement with experimental data after the second step. The two remaining quark mixing angles are generated after the third step of symmetry breaking. All three leptonic CP phases are predicted, sin delta^l = -0.936, |sin alpha|=|sin beta|=1/sqrt{2}. The amount of CP violation in the quark sector turns out to be maximal at the lowest order and is correctly accounted for, when higher order effects are included. Charged fermion masses are reproduced with the help of operators with different numbers of flavor (and CP) symmetry breaking fields. Light neutrino masses, arising from the type-I seesaw mechanism, can accommodate both mass orderings, normal and inverted. The vacuum alignment of the flavor (and CP) symmetry breaking fields is discussed at leading and at higher order.

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arXiv2019

Lepton and Quark Mixing from Stepwise Breaking of Flavor and CP

We explain all features of lepton and quark mixing in a scenario with the flavor symmetry Delta (384) and a CP symmetry, where these are broken in several steps. The residual symmetry in the neutrino and up quark sector is a Klein group and CP, while a Z_3 and a Z_{16} symmetry are preserved among charged leptons and down quarks, respectively. If the Klein group in the neutrino sector is further broken down to a single Z_2 symmetry, we obtain predictions for all lepton mixing parameters in terms of one real quantity, whose size is determined by the value of the reactor mixing angle. The Dirac and Majorana phases are fixed, in particular sin (delta) = -0.936. A sum rule, relating these CP phases and the reactor and atmospheric mixing angles, theta_{13} and theta_{23}, is given. In the quark sector, we have for the Cabibbo angle theta_C= sin (pi/16) = 0.195 after the first step of symmetry breaking. This is brought into full accordance with experimental data with the second step of symmetry breaking, where either the Z_{16} group is broken to a Z_8 symmetry in the down quark sector or the Klein group to one Z_2 symmetry only among up quarks. The other two quark mixing angles are generated in the third and last symmetry breaking step, when the residual symmetries in the up and/or down quark sector are further broken. If this step occurs among both up and down quarks, the amount of CP violation in the quark sector is determined by the lepton sector and explaining the current neutrino oscillation data entails that the Jarlskog invariant J_{CP}^q is in very good agreement with experimental findings. Lastly, a sum rule is derived that contains the CP phase delta^q and theta_C of the quark sector and the lepton mixing parameters theta_{13}, theta_{23} and delta.

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arXiv2019

Long-Lived Particles at the Energy Frontier: The MATHUSLA Physics Case

We examine the theoretical motivations for long-lived particle (LLP) signals at the LHC in a comprehensive survey of Standard Model (SM) extensions. LLPs are a common prediction of a wide range of theories that address unsolved fundamental mysteries such as naturalness, dark matter, baryogenesis and neutrino masses, and represent a natural and generic possibility for physics beyond the SM (BSM). In most cases the LLP lifetime can be treated as a free parameter from the $μ$m scale up to the Big Bang Nucleosynthesis limit of $\sim 10^7$m. Neutral LLPs with lifetimes above $\sim$ 100m are particularly difficult to probe, as the sensitivity of the LHC main detectors is limited by challenging backgrounds, triggers, and small acceptances. MATHUSLA is a proposal for a minimally instrumented, large-volume surface detector near ATLAS or CMS. It would search for neutral LLPs produced in HL-LHC collisions by reconstructing displaced vertices (DVs) in a low-background environment, extending the sensitivity of the main detectors by orders of magnitude in the long-lifetime regime. In this white paper we study the LLP physics opportunities afforded by a MATHUSLA-like detector at the HL-LHC. We develop a model-independent approach to describe the sensitivity of MATHUSLA to BSM LLP signals, and compare it to DV and missing energy searches at ATLAS or CMS. We then explore the BSM motivations for LLPs in considerable detail, presenting a large number of new sensitivity studies. While our discussion is especially oriented towards the long-lifetime regime at MATHUSLA, this survey underlines the importance of a varied LLP search program at the LHC in general. By synthesizing these results into a general discussion of the top-down and bottom-up motivations for LLP searches, it is our aim to demonstrate the exceptional strength and breadth of the physics case for the construction of the MATHUSLA detector.

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arXiv2018

Phenomenology of the Generalised Scotogenic Model with Fermionic Dark Matter

We study a simple extension of the Standard Model that accounts for neutrino masses and dark matter. The Standard Model is augmented by two Higgs doublets and one Dirac singlet fermion, all charged under a new dark global symmetry. It is a generalised version of the Scotogenic Model with Dirac fermion dark matter. Masses for two neutrinos are generated radiatively at one-loop level. We study the case where the singlet fermion constitutes the dark matter of the Universe. We study in depth the phenomenology of the model, in particular the complementarity between dark matter direct detection and charged lepton flavour violation observables. Due to the strong limits from the latter, dark matter annihilations are suppressed and the relic abundance is set by coannihilations with (and annihilations of) the new scalars if the latter and the Dirac fermion are sufficiently degenerate in mass. We discuss how different ratios of charged lepton flavour violating processes can be used to test the model. We also discuss the detection prospects of the charged scalars at colliders. In some cases these leave ionising tracks and in others have prompt decays, depending on the flavour in the final state and neutrino mass orderings.

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arXiv2018

Asymmetric dark matter, baryon asymmetry and lepton number violation

We study the effect of lepton number violation (LNV) on baryon asymmetry, generated in the early Universe, in the presence of a dark sector with a global symmetry $U(1)_X$, featuring asymmetric dark matter (ADM). We show that in general LNV, observable at the LHC or in neutrinoless double beta decay experiments, cannot wash out a baryon asymmetry generated at higher scales, unlike in scenarios without such dark sector. An observation of LNV at the TeV scale may thus support ADM scenarios. Considering several models with different types of dark matter (DM), we find that the DM mass is of the order of a few GeV or below in our scenario.

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arXiv2017

Theories of Leptonic Flavor

I discuss different theories of leptonic flavor and their capability of describing the features of the lepton sector, namely charged lepton masses, neutrino masses, lepton mixing angles and leptonic (low and high energy) CP phases. In particular, I show examples of theories with an abelian flavor symmetry G_f, with a non-abelian G_f as well as theories with non-abelian G_f and CP.

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arXiv2017

Flavor and CP symmetries for leptogenesis and 0nubb decay

We perform a comprehensive analysis of the phenomenology of leptonic low and high energy CP phases in a scenario with three heavy right-handed neutrinos in which a flavor and a CP symmetry are non-trivially broken. All CP phases as well as lepton mixing angles are determined by the properties of the flavor and CP symmetry and one free real parameter. We focus on the generation of the baryon asymmetry Y_B of the Universe via unflavored leptogenesis and the predictions of m_ee, the quantity measurable in neutrinoless double beta decay. We show that the sign of Y_B can be fixed and the allowed parameter range of m_ee can be strongly constrained. We argue on general grounds that the CP asymmetries epsilon_i are dominated by the contribution associated with one Majorana phase and that in cases in which only the Dirac phase is non-trivial the sign of Y_B depends on further parameters. In addition, we comment on the case of flavored leptogenesis where in general the knowledge of the CP phases and light neutrino mass spectrum is also not sufficient in order to fix the sign of the CP asymmetries. As examples we discuss the series of flavor groups Delta (3 n^2) and Delta (6 n^2), n >= 2 integer, and several classes of CP transformations.

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arXiv2016

Unification of Gauge Couplings in Radiative Neutrino Mass Models

We investigate the possibility of gauge coupling unification in various radiative neutrino mass models, which generate neutrino masses at one- and/or two-loop level. Renormalization group running of gauge couplings is performed analytically and numerically at one- and two-loop order, respectively. We study three different classes of neutrino mass models: (I) minimal ultraviolet completions of the dimension-7 $ΔL=2$ operators which generate neutrino masses at one- and/or two-loop level without and with dark matter candidates, (II) models with dark matter which lead to neutrino masses at one-loop level and (III) models with particles in the adjoint representation of $\mathrm{SU}(3)$. In class (I), gauge couplings unify in a few models and adding dark matter amplifies the chances for unification. In class (II), about a quarter of the models admit gauge coupling unification. In class (III), none of the models leads to gauge coupling unification. Regarding the scale of unification, we find values between $10^{14}$ GeV and $10^{16}$ GeV for models belonging to class (I) without dark matter, whereas models in class (I) with dark matter as well as models of class (II) prefer values in the range $5 \cdot 10^{10}-5 \cdot 10^{14}$ GeV.

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arXiv2015

Lepton mixing from the interplay of the alternating group A5 and CP

Assuming three generations of Majorana neutrinos, we study the different mixing patterns that arise from the non-trivial breaking of the flavor group A5 and CP to the residual symmetries Z3, Z5 or Z2 x Z2 in the charged lepton and to Z2 x CP in the neutrino sector. All patterns contain only one free parameter theta and thus mixing angles as well as the Dirac and the two Majorana phases are strongly correlated. We perform an analytical and a numerical study of all possible mixing patterns. It turns out that only four patterns can describe the experimentally measured values of the mixing angles for a particular choice of theta well. All of them predict trivial Majorana phases, while the Dirac phase delta is maximal for two patterns and trivial for the two remaining ones. If delta is maximal, also the atmospheric mixing angle is fixed to be maximal.

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arXiv2014

A Realistic Pattern of Lepton Mixing and Masses from S4 and CP

We present a supersymmetric model with the flavour symmetry S4 x Z3 and a CP symmetry which are broken to a Z3 subgroup of the flavour symmetry in the charged lepton sector and to Z2 x CP (x Z3) in the neutrino one at leading order. This model implements an approach, capable of predicting lepton mixing angles and Dirac as well as Majorana phases in terms of one free parameter. This parameter, directly related to the size of the reactor mixing angle theta_{13}, can be naturally of the correct order in our model. Atmospheric mixing is maximal, while sin^2 theta_{12} is larger than 1/3. All three phases are predicted: the Dirac phase is maximal, whereas the two Majorana phases are trivial. The neutrino mass matrix contains only three real parameters at leading order and neutrino masses effectively only depend on two of them. As a consequence, they have to be normally ordered and the absolute neutrino mass scale and the sum of the neutrino masses are predicted. The vacuum of the flavons can be correctly aligned. We study subleading corrections to the leading order results and show that they are small.

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arXiv2014

Mixing Patterns from the Groups Sigma (n phi)

We survey the mixing patterns which can be derived from the discrete groups Sigma (36 x 3), Sigma (72 x 3), Sigma (216 x 3) and Sigma (360 x 3), if these are broken to abelian subgroups Ge and Gnu in the charged lepton and neutrino sector, respectively. Since only Sigma (360 x 3) possesses Klein subgroups, only this group allows neutrinos to be Majorana particles. We find a few patterns that can agree well with the experimental data on lepton mixing in scenarios with small corrections and that predict the reactor mixing angle theta_{13} to be 0.1 <= theta_{13} <= 0.2. All these patterns lead to a trivial Dirac phase. Patterns which instead reveal CP violation tend to accommodate the data not well. We also comment on the outer automorphisms of the discussed groups, since they can be useful for relating inequivalent representations of these groups.

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arXiv2013

Lepton Mixing Parameters from Discrete and CP Symmetries

We consider a scenario with three Majorana neutrinos in which a discrete, finite flavour group G_f is combined with a generalized CP transformation. We derive conditions for consistently defining such a setup. We show that in general lepton mixing angles and CP phases (Dirac as well as Majorana) only depend on one single parameter theta which can take values between 0 and pi, if the residual symmetries are G_e contained in G_f in the charged lepton and G_nu=Z_2 x CP in the neutrino sector. We perform a comprehensive study for G_f=S_4 and find five cases which are phenomenologically interesting. They naturally lead to a non-zero reactor mixing angle and all mixing parameters are strongly correlated. Some of the patterns predict maximal atmospheric mixing and maximal Dirac phase, while others predict trivial Dirac and Majorana phases.

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arXiv2012

SUSY S4 x SU(5) revisited

Following the recent results from Daya Bay and RENO, which measure the lepton mixing angle theta^l_13 ~ 0.15, we revisit a supersymmetric S4 x SU(5) model, which predicts tri-bimaximal (TB) mixing in the neutrino sector with theta^l_13 being too small in its original version. We show that introducing one additional S4 singlet flavon into the model gives rise to a sizable theta^l_13 via an operator which leads to the breaking of one of the two Z2 symmetries preserved in the neutrino sector at leading order. The results of the original model for fermion masses, quark mixing and the solar mixing angle are maintained to good precision. The atmospheric and solar mixing angle deviations from TB mixing are subject to simple sum rule bounds.

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