arXiv subjects

Simon J. D. King

Publications and source records attributed to Simon J. D. King.

15 recordsLinked to original sources

arXiv2021

Solving the electron and muon $g-2$ anomalies in $Z'$ models

We consider simultaneous explanations of the electron and muon $g-2$ anomalies through a single $Z'$ of a $U(1)'$ extension to the Standard Model (SM). We first perform a model-independent analysis of the viable flavour-dependent $Z'$ couplings to leptons, which are subject to various strict experimental constraints. We show that only a narrow region of parameter space with an MeV-scale $Z'$ can account for the two anomalies. Following the conclusions of this analysis, we then explore the ability of different classes of $Z'$ models to realise these couplings, including the SM$+U(1)'$, the $N$-Higgs Doublet Model$+U(1)'$, and a Froggatt-Nielsen style scenario. In each case, the necessary combination of couplings cannot be obtained, owing to additional relations between the $Z'$ couplings to charged leptons and neutrinos induced by the gauge structure, and to the stringency of neutrino scattering bounds. Hence, we conclude that no $U(1)'$ extension can resolve both anomalies unless other new fields are also introduced. While most of our study assumes the Caesium $(g-2)_e$ measurement, our findings in fact also hold in the case of the Rubidium measurement, despite the tension between the two.

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arXiv2021

Discovering the Origin of Yukawa Couplings at the LHC with a Singlet Higgs and Vector-like Quarks

Although the 125 GeV Higgs boson discovered at the LHC is often heralded as the origin of mass, it may not in fact be the origin of Yukawa couplings. In alternative models, Yukawa couplings may instead arise from a seesaw type mechanism involving the mixing of Standard Model (SM) chiral fermions with new vector-like fermions, controlled by the vacuum expectation value (VEV) of a new complex Higgs singlet field $\langle Φ\rangle$. For example, the largest third family $(t,b)$ quark Yukawa couplings may be forbidden by a $U(1)'$ gauge or global symmetry, broken by $\langle Φ\rangle$, and generated effectively via mixing with a vector-like fourth family quark doublet $(T,B)$. Such theories predict a new physical Higgs singlet $ϕ$, which we refer to as the Yukon, resulting from $\langle Φ\rangle$, in the same way that the Higgs boson $h^0$ results from $\langle H\rangle$. In a simplified model we discuss the prospects for discovering the Yukon $ϕ$ in gluon-gluon fusion production, with $(t,b)$ and $(T,B)$ quarks in the loops, and decaying in the channels $ϕ\rightarrow γγ, Zγ$ and $ϕ\rightarrow tT\rightarrow tth^0,ttZ$. The potential for discovery of the Yukon $ϕ$ is studied at present or future hadron colliders such as the LHC (Run 3), HL-LHC, HE-LHC and/or FCC. For example, we find that a 300-350 GeV Yukon $ϕ$ could be accessed at LHC Run 3 in the di-photon channel in the global model, providing a smoking gun signature of the origin of Yukawa couplings. The $tth^0,ttZ$ channels are more involved and warrant a more sophisticated analysis.

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arXiv2020

Fermion Mass Hierarchies from Modular Symmetry

We show how quark and lepton mass hierarchies can be reproduced in the framework of modular symmetry. The mechanism is analogous to the Froggatt-Nielsen (FN) mechanism, but without requiring any Abelian symmetry to be introduced, nor any Standard Model (SM) singlet flavon to break it. The modular weights of fermion fields play the role of FN charges, and SM singlet fields with non-zero modular weight called weightons play the role of flavons. We illustrate the mechanism by analysing $A_4$ (modular level 3) models of quark and lepton (including neutrino) masses and mixing, with a single modulus field. We discuss two examples in some detail, both numerically and analytically, showing how both fermion mass and mixing hierarchies emerge from different aspects of the modular symmetry.

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arXiv2020

$R_K$ and $R_{K^*}$ in an Aligned 2HDM with Right-Handed Neutrinos

We consider the possibility to explain the recent $R_K$ and $R_{K ^*}$ anomalies in a 2-Higgs Doublet Model, known as Aligned, combined with a low scale seesaw mechanism generating light neutrino masses and mixings. In this class of models, a large Yukawa coupling allows for significant non-universal leptonic contributions, through box diagrams mediated by charged Higgs bosons and right-handed neutrinos, to the $b \to s \ell^+ \ell^-$ transition that can then account for both $R_K$ and $R_{K^*}$ anomalies.

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arXiv2019

Modular Invariant Models of Lepton Masses at Levels 4 and 5

We explore alternative descriptions of the charged lepton sector in modular invariant models of lepton masses and mixing angles. In addition to the modulus, the symmetry breaking sector of our models includes ordinary flavons. Neutrino mass terms depend only on the modulus and are tailored to minimize the number of free parameters. The charged lepton Yukawa couplings rely upon the flavons alone. We build modular invariant models at levels 4 and 5, where neutrino masses are described both in terms of the Weinberg operator or through a type I seesaw mechanism. At level 4, our models reproduce the hierarchy among electron, muon and tau masses by letting the weights play the role of Froggatt-Nielsen charges. At level 5, our setup allows the treatment of left and right handed charged leptons on the same footing. We have optimized the free parameters of our models in order to match the experimental data, obtaining a good degree of compatibility and predictions for the absolute neutrino masses and the $CP$ violating phases. At a more fundamental level, the whole lepton sector could be correctly described by the simultaneous presence of several moduli. Our examples are meant to make a first step in this direction.

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arXiv2019

New Physics Suggested by Atomki Anomaly

We consider several extensions of the Standard Model (SM) which can explain the anomalies observed by the Atomki collaboration in the decay of excited states of Beryllium via a new boson with a mass around 17 MeV yielding $e^+e^-$ pairs. We show how both spin-0 and 1 solutions are possible and describe the Beyond the SM (BSM) scenarios that can accommodate these. They include BSM frameworks with either an enlarged Higgs, or gauge sector, or both.

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arXiv2019

${}^8$Be Decay Anomaly and Light $Z'$

In this proceedings, we discuss a light (17 MeV) $Z'$ solution to the anomaly observed in the decay of Beryllium-8 by the Atomki collaboration. We detail an anomaly free model with minimal particle content which can satisfy all other experimental constraints with gauge couplings $\mathcal{O}(10^{-4})$.

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arXiv2018

Atomki Anomaly in Family-Dependent $U(1)'$ Extension of the Standard Model

In the context of a gauge invariant, non-anomalous and family-dependent (non-universal) $U(1)'$ extension of the Standard Model, wherein a new high scale mechanism generates masses and couplings for the first two fermion generations and the standard Higgs mechanism does so for the third one, we find solutions to the anomaly observed by the Atomki collaboration in the decay of excited states of Beryllium, in the form of a very light $Z'$ state, stemming from the $U(1)'$ symmetry breaking, with significant axial couplings so as to evade a variety of low scale experimental constraints.

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arXiv2018

${SO(10)}$ inspired $Z'$ models at the LHC

We study and compare various $Z'$ models arising from $SO(10)$, focussing in particular on the Abelian subgroup $U(1)_{R} \times U(1)_{B-L}$, broken at the TeV scale to Standard Model hypercharge $U(1)_{Y}$. The gauge group $U(1)_{R} \times U(1)_{B-L}$, which is equivalent to the $U(1)_{Y}\times U(1)_χ$ in a different basis, is well motivated from $SO(10)$ breaking and allows neutrino mass via the linear seesaw mechanism. Assuming supersymmetry, we consider single step gauge unification to predict the gauge couplings, then consider the detection and characterisation prospects of the resulting $Z'$ at the LHC by studying its possible decay modes into di-leptons as well as into Higgs bosons. The main new result here is to analyse in detail the expected leptonic forward-backward asymmetry at the high luminosity LHC and show that it may be used to discriminate the $U(1)_{R} \times U(1)_{B-L}$ model from the usual $B-L$ model based on $U(1)_{Y}\times U(1)_{B-L}$.

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arXiv2018

Prospects for Sneutrino Dark Matter in the BLSSM

The $(B-L)$ Supersymmetric Standard Model (BLSSM) motivates several Dark Matter (DM) candidates beyond the Minimally Supersymmetric Standard Model (MSSM). We assess the comparative naturalness of the two models and discuss the potential detection properties of a particular candidate, the Right-Handed (RH) sneutrino.

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arXiv2018

Sneutrino Dark Matter, Constraints and Perspectives

The prevalent Dark Matter (DM) candidate of the (B-L) Supersymmetric Standard Model (BLSSM) is the Right Handed (RH) sneutrino. In this work we assess the ability of ground and space based experiments to establish the nature of this particle, through indirect and collider detection.

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arXiv2017

Sneutrino Dark Matter in the BLSSM

In the framework of the $(B-L)$ Supersymmetric Standard Model (BLSSM), we assess the ability of ground and space based experiments to establish the nature of its prevalent Dark Matter (DM) candidate, the sneutrino, which could either be CP-even or -odd. Firstly, by benchmarking this theory construct against the results obtained by the Planck spacecraft, we extract the portions of the BLSSM parameter space compliant with relic density data. Secondly, we show that, based on current sensitivities of the Fermi Large Area Telescope (FermiLAT) and their future projections, the study of high-energy $γ$-ray spectra will eventually enable us to extract evidence of this DM candidate through its annihilations into $W^+W^-$ pairs (in turn emitting photons), in the form of both an integrated flux and a differential energy spectrum which cannot be reconciled with the assumption of DM being fermionic (like, e.g., a neutralino), although it should not be possible to distinguish between the scalar and pseudoscalar hypotheses. Thirdly, we show that, while underground direct detection experiments will have little scope in testing sneutrino DM, the Large Hadron Collider (LHC) may be able to do so in a variety of multi-lepton signatures, with and without accompanying jets (plus missing transverse energy), following data collection during Run 2 and 3.

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arXiv2017

Supersymmetric Gauged B-L Model of Dark Matter and Fine Tuning

We investigate how the Fine-Tuning (FT) in the B-L Supersymmetric Standard Model (BLSSM) compares to the Minimally Supersymmetric Standard Model (MSSM), where both models have universality. This is done for two scales: both low (i.e. collider) and high (i.e. Grand Unified Theory (GUT)) scales. We see this is similar for both models and the two scale regimes. We also study the possible Dark Matter (DM) candidates each model offers in a realistic scenario satisfying relic density constraints. Our findings are that whilst the parameter space for the single MSSM DM candidate is severely constrained, the BLSSM offers multiple candidates in a much wider region.

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arXiv2017

Naturalness and Dark Matter Properties of the BLSSM

In this report, we compare the naturalness and Dark Matter (DM) properties of the Minimal Supersymmetric Standard Model (MSSM) and the $B-L$ Supersymmetric Standard Model (BLSSM), with universality in both cases. We do this by adopting standard measures for the quantitative analysis of the Fine-Tuning (FT), at both low (i.e. supersymmetric (SUSY)) and high (i.e. unification) scales. We will see a similar level of FT for both models in these scenarios, with a slightly better FT for the BLSSM at SUSY scales and MSSM at Grand Unification Theory (GUT) scales. When including DM relic constraints, we drastically confine the MSSM's parameter space, whereas we still find a large parameter space available for the non-minimal scenario.

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arXiv2017

Naturalness and Dark Matter in the BLSSM

We study the naturalness properties of the $B-L$ Supersymmetric Standard Model (BLSSM) and compare them to those of the Minimal Supersymmetric Standard Model (MSSM) at both low (i.e., Large Hadron Collider) energies and high (i.e., unification) scales. By adopting standard measures of naturalness, we assess that, in presence of full unification of the additional gauge couplings and scalar/fermionic masses of the BLSSM, such a scenario reveals a somewhat higher degree of Fine-Tuning (FT) than the MSSM, when the latter is computed at the unification scale and all available theoretical and experimental constraints, but the Dark Matter (DM) ones, are taken into account. Yet, such a difference, driven primarily by the collider limits requiring a high mass for the gauge boson associated to the breaking of the additional $U(1)_{B-L}$ gauge group of the BLSSM in addition to the $SU(3)_C\times SU(2)_L \times U(1)_Y$ of the MSSM, should be regarded as a modest price to pay for the former in relation to the latter, if one notices that the non-minimal scenario offers a significant volume of parameter space where numerous DM solutions of different compositions can be found to the relic density constraints, unlike the case of the minimal structure, wherein only one type of solution is accessible over an ever diminishing parameter space. In fact, this different level of tension within the two SUSY models in complying with current data is well revealed when the FT measure is recomputed in terms of the low energy spectra of the two models, over their allowed regions of parameter space now in presence of all DM bounds, as it is shown that the tendency is now opposite, the BLSSM appearing more natural than the MSSM.

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