arXiv subjects

Raymond R. Volkas

Publications and source records attributed to Raymond R. Volkas.

At least 19 recordsLinked to original sources

arXiv2026

Leptogenesis Determined By Low Energy Parameters

We study thermal leptogenesis in three predictive type-I seesaw models in which the neutrino Dirac mass matrix is equal to the mass matrix of up-type quarks, or down-type quarks, or charged leptons. In this framework, the seesaw relation permits a full reconstruction of the heavy right-handed neutrino mass matrix from low-energy neutrino parameters, which greatly reduces the parameter freedom. A systematic numerical scan based on density matrix Boltzmann equations is performed to examine whether the observed baryon asymmetry of the Universe can be obtained. Successful leptogenesis occurs for normal ordering of light neutrino masses with nonzero Majorana phases. In this case, viable solutions are found in model B, associated with down-type quarks, and model C, associated with charged leptons. Both point to a close-mass pair of heavy neutrinos satisfying $|M_i-M_j|/M_i<10^{-3}$, while remaining outside the conventional quasi-degenerate resonant regime. Four representative benchmark points are selected to show the evolution of the asymmetry and the impact of different treatments of spectator effects. Neutrinoless double beta decay is further studied for all parameter points that can generate an acceptable baryon asymmetry $η_B = (6.12 \pm 0.20)\times 10^{-10}$. The predicted effective Majorana mass for certain cases can be probed by next generation experiments with sub-10 meV sensitivity, such as LEGEND-1000, nEXO, JUNO 50 tons, and CUPID-1T. This framework therefore provides clear targets for future searches.

hep-ph↗

arXiv2026

Minimal Flavoured DFSZ Axion Models: Predictive Yukawa Textures and Flavour Constraints

We study a predictive class of flavoured DFSZ axion models in which the Peccei--Quinn symmetry is fully horizontal and the domain wall number is unity. These Minimal F-DFSZ models simultaneously resolve the strong CP problem and avoid the post-inflationary domain wall problem, while enforcing a predictive set of texture-zero quark mass matrices. Remarkably, the entire quark Yukawa sector can be reconstructed in terms of the measured quark masses and Cabibbo--Kobayashi--Maskawa parameters. We derive analytic expressions for the resulting flavour-violating Higgs and axion couplings, thereby making explicit how the new-physics effects are determined by Standard Model observables. We then analyse the full flavour phenomenology of the framework and obtain bounds on the scale of new physics from precision flavour observables.

hep-ph↗

arXiv2026

Classification of minimal models producing b, c and tau masses at one-loop level

The Standard Model neither provides a dynamical explanation for the quark and lepton mass hierarchies, nor a rationale for why all of these masses save that of the top quark are suppressed compared to the electroweak scale. Motivated by this, we explore the alternative radiative mass generation hypothesis, specialising to the generation of the bottom, charm, and tau masses at one-loop level. A classification of all minimal models that use exotic scalars and exotic fermions only is presented, resulting in 25 possibilities featuring four exotic multiplets. As a bonus, some of the models produce a one-loop neutrino Dirac mass if a right-handed neutrino field is included, and some feature WIMP-like dark matter candidates. By way of example, we analyse the phenomenology of a benchmark model chosen from the set of 25 candidates and find that it is capable of exactly replicating the predicted Standard Model Yukawa couplings within the permitted parameter space.

hep-ph↗

arXiv2026

Higgs Yukawa coupling constraints on a benchmark one-loop radiative mass model for the bottom, charm and tau

Measurements of Higgs boson Yukawa couplings to Standard Model (SM) fermions constrain radiative mass models for those species. Such models, motivated by the observed fermion mass hierarchy, act as foils for the SM tree-level mechanism: we cannot claim to have verified the standard mechanism if other possibilities also fit the data well. We construct a benchmark model which generates the top mass at tree level, and the bottom, charm, tau, and tau Dirac neutrino masses at one-loop level. Current theoretical and experimental constraints on the model, including from Higgs decays, demonstrate it possesses viable regions of parameter space. We show that future improvements to measurements will not be able to rule out the model, only increase the scale of new physics required, illustrating how difficult it will be to verify the SM fermion mass generation mechanism with great precision. As a bonus, a dark matter candidate is shown to be capable of reproducing the correct relic density within the permitted parameter space.

hep-ph↗

arXiv2026

Muonphilic asymmetric dark matter at a future muon collider

We explore phenomenological constraints on, and future muon collider sensitivities to, the parameter spaces of various muonphilic portals to fermionic asymmetric dark matter (ADM). Both WEFT-level dimension-6 effective operators and two UV models based on gauged $L_μ- L_τ$ are considered. One of the latter features a vector coupling to the dark matter and the other an axial vector coupling. The ADM criterion that at least $99\%$ of the dark matter relic density is asymmetric is also imposed. We identify which of these scenarios are currently allowed by direct detection and collider constraints, and then determine how much more of the parameter space could be probed by 3 and 10 TeV muon colliders with 1 ab$^{-1}$ of data. For the UV models, the constraints from $g-2$ of the muon are included. The future sensitivity curves due to neutron star heating considerations are also depicted. We present results for both the few-GeV dark matter mass regime motivated by ADM approaches to the $Ω_b \simeq Ω_\text{DM}/5$ coincidence problem, and for larger masses in the context of more general ADM.

hep-ph↗

arXiv2026

Dark Matter Freeze-in from a $Z^\prime$ Reheaton

We consider the Standard Model (SM) extended by a secluded $U(1)_D$ gauge sector encompassing a Dirac fermion ($χ$) dark matter (DM), an abelian gauge boson $Z^\prime$ and a SM-singlet complex-scalar field $Φ$, whose radial component drives cosmic inflation. When the Higgs portal coupling is small, the $Z^\prime$ then acts as a {\it ``reheaton''}, dominating the energy budget of the Universe before finally yielding the SM bath, with reheating temperature $< O(10)$ TeV, through the gauge portal interaction. We explore the possibility that DM freezes-in via non-thermal $Z^\prime$ decays before reheating ends, giving rise to substantial viable parameter space. We account for non-perturbative effects, relevant during the initial stages of reheating, using lattice simulations. We additionally show how the cosmological gravitational wave (GW) background produced by preheating and inflation allow for a direct probe of the reheating mechanism.

hep-ph↗

arXiv2026

A new idea for relating the asymmetric dark matter mass scale to the proton mass

Asymmetric dark matter is a well-motivated approach to explain the apparent coincidence between the relic densities of visible and dark matter, $Ω_D \simeq 5.4Ω_b$. A complete explanation requires two components, a relation between the particle masses of the dark and visible matter, and a second relation between the number densities in each sector. In this work, we propose a new mechanism to address the former. We consider an extended $SU(3)_1 \times SU(3)_2$ colour group in the visible sector, with QCD embedded as the diagonal subgroup. A $\mathbb{Z}_2$ exchange symmetry then relates $SU(3)_2$ to a dark, confining $SU(3)_D$ sector. The dark matter is a composite state of dark fermions transforming in the fundamental representation of $SU(3)_D$. The spontaneously broken $\mathbb{Z}_2$ symmetry ultimately leads to a relation between the QCD and dark gauge couplings which, for suitable field content, gives rise to confinement scales of the same order of magnitude. The mechanism leads to a rich particle spectrum above the TeV scale which could be probed at future experiments. The model also naturally includes an axion solution to the strong CP problem.

hep-ph↗

arXiv2025

The post-inflationary cosmology of the VISH$ν$ axion-majoron model

It was previously shown how several explanatory deficiencies of the Standard Model (including the origin of dark matter, matter-antimatter asymmetry, small active neutrino mass, strong CP-conservation and the seeds for large-scale structure formation) may be economically resolved when an experimentally-accessible QCD axion also plays the role of the majoron, and the scalar partner of the axion is dynamical during inflation. In this paper, we complete this general study of the cosmological history for a unit domain-wall number option of the DFSZ-type, dubbed VISH$ν$, by performing a detailed lattice-informed analysis of the reheating era. In doing so, we make inflationary and leptogenesis predictions more precise through estimates of the reheating temperature and the expansion history. The viable reheating scenarios, which at the same time satisfy strict conditions for naturalness (radiative stability), are also shown to respect dark radiation bounds. We also characterise the high-frequency spectrum of gravitational waves, and mention other phenomenological implications that distinguish VISH$ν$ from alternative proposals.

hep-ph↗

arXiv2025

Z and Higgs boson decays with doubly-charged scalars at one-loop: current constraints, future sensitivities, and application to lepton-triality models

We analyse the $Z$ and Higgs boson decays $Z\to \ell^+ \ell^- $ ($\ell = e, μ,τ$), $H\to γγ$ and $H\to Zγ$ that are induced at one-loop level in models with a doubly-charged isosinglet scalar. After discussing current constraints, we derive the parameter space that will be probed by the HL-LHC and the possible future colliders the ILC, CEPC and FCC. We then apply those constraints to lepton triality models which are based on a discrete $Z_3$ family symmetry and were recently studied in the context of charged-lepton flavour-violating processes at Belle II and the proposed $μ^+ μ^+$ and $μ^+ e^-$ collider known as $μ$TRISTAN. We find that the future constraints that can be imposed by $Z \to \ell^+ \ell^-$ on the lepton flavour conserving couplings of the triality models reduce the viable parameter space to probe lepton flavour violating processes. The constraints from Higgs boson decays are the first on the Higgs portal sector of the triality models.

hep-ph↗

arXiv2024

Neutrino theory: open questions and future opportunities

The subtitle of my talk is ``The quest for understanding the origin of neutrino masses''. After reviewing why the discovery of neutrino masses is also the discovery of New Physics, the substance of the talk details mechanisms for generating Majorana neutrino masses and implications for experimental searches and/or cosmology. I review high-scale seesaw, low-scale seesaw and radiative mechanisms, asking at every turn how testable the scenario is. While it is clear that determining the origin of neutrino masses -- knowing what Lagrangian to put into textbooks -- is a distant and ambitious goal, I end with experimental advances that we can reasonably hope for that would constitute progress.

hep-ph↗

arXiv2024

Scalar dark matter explanation of the excess in the Belle II $B^+\to K^+ +$ invisible measurement

Recently Belle II reported the first measurement of $B^+\to K^++{\rm invisible (inv)}$, which is $2.7σ$ above the standard model (SM) prediction. If confirmed, this calls for new physics beyond SM. In the SM, the invisible particles are neutrino-anti-neutrino pairs. There are more possibilities when going beyond the SM. In this work, we focus on decays to dark matter (DM) and show that the $B\to K +\mathrm{inv}$ excess from Belle II and DM relic density can be simultaneously explained in a simple extension of the SM. The model introduces a real scalar singlet $ϕ$ acting as a DM candidate, and two heavy vector-like quarks $Q,D$ with the same quantum numbers as the SM left-handed quark doublet and right-handed down-type quark singlet, respectively. All these new particles are odd under a $\mathbb{Z}_2$ symmetry while the SM particles are even. The model can successfully explain the Belle II anomaly and DM relic density for TeV-scale heavy quarks with hierarchical Yukawa couplings involving $b$ and $s$ quarks. At the same time, it can easily satisfy other flavour physics constraints. Direct detection searches utilizing the Migdal effect constrain some of the parameter space.

hep-ph↗

arXiv2024

Explaining the cosmological dark matter coincidence in asymmetric dark QCD

To properly solve the coincidence problem ($Ω_\mathrm{DM} \simeq 5Ω_\mathrm{VM}$) in a model of asymmetric dark matter, one cannot simply relate the number densities of visible and dark matter without also relating their particle masses. Following previous work, we consider a framework where the dark matter is a confined state of a dark QCD gauge group whose confinement scale is dynamically related to the QCD confinement scale by a mechanism utilising infrared fixed points of the two gauge couplings. In this work we present a new, `zero-coupling infrared fixed point' approach, which allows a larger proportion of models in this framework to generically relate the masses of the visible and dark matter particles. Due to the heavy mass scale required for the new field content, we introduce supersymmetry to the theory. We consider how these models may be incorporated in a full theory of asymmetric dark matter, presenting some example leptogenesis-like models. We also discuss the phenomenology of these models; in particular, there are gravitational wave signals which, while weak, may be measurable at future mHz and $μ$Hz detectors.

hep-ph↗

arXiv2024

Classification of three-family flavoured DFSZ axion models that have no domain wall problem

We provide an exhaustive classification of three-family DFSZ axion models that have no cosmological domain wall problem. This result is obtained by making the Peccei-Quinn symmetry flavour dependent in certain specific ways, thus reinforcing a possible connection between the strong CP problem and the flavour puzzle. Known DFSZ flavour variants such as the top-specific model emerge as special cases. Key features of the phenomenology of these models are briefly discussed.

hep-ph↗

arXiv2023

Complementarity of $μ$TRISTAN and Belle II in searches for charged-lepton flavour violation

We analyse the potential of the proposed $μ^+ μ^+$ and $μ^+ e^-$ collider $μ$TRISTAN to complement the searches for charged-lepton flavour-violation (CLFV) that can be carried out by Belle II. $μ$TRISTAN offers the possibility of directly producing and studying new resonances that could mediate CLFV for a certain range of masses. In addition, we find that it can produce competitive bounds to those from Belle II for cases where the new resonance lies beyond direct reach. We illustrate these points with three $Z_3$ "lepton triality" models, where we also find an example that can only be probed by $μ$TRISTAN. These three models feature doubly-charged scalars, denoted $k_{1,2,3}$ respectively, that induce both CLFV and flavour-conserving processes. Tree-level $k_1$ exchange induces the CLFV scattering process $μ^+ e^- \to e^+ τ^-$, while $k_2$ interactions induce $μ^+ μ^+ \to τ^+ e^+$, $μ^+ e^- \to τ^+ μ^-$ and make a non-SM contribution to the flavour-conserving scattering $μ^+ μ^+ \to μ^+ μ^+$. The $k_3$ model has a non-SM contribution to the flavour-conserving process $μ^+ e^- \to μ^+ e^-$. Other scattering processes involving $k_1$, $k_2$ or $k_3$ are not relevant for $μ$TRISTAN and outside the scope of our analysis. We quantify the sensitivity of $μ$TRISTAN for each of these processes. For the $k_1$ and $k_2$ cases we compare the $μ$TRISTAN reach to the expected sensitivity of Belle II to the crossing symmetry related CLFV $τ$ decays.

hep-ph↗

arXiv2023

Constraining Dark Photons with Self-consistent Simulations of Globular Cluster Stars

We revisit stellar constraints on dark photons. We undertake dynamical stellar evolution simulations which incorporate the resonant and off-resonant production of transverse and longitudinal dark photons. We compare our results with observables derived from measurements of globular cluster populations, obtaining new constraints based on the luminosity of the tip of the red-giant branch (RGB), the ratio of populations of RGB to horizontal branch (HB) stars (the $R$-parameter), and the ratio of asymptotic giant branch to HB stars (the $R_2$-parameter). We find that previous bounds derived from static stellar models do not capture the effects of the resonant production of light dark photons leading to overly conservative constraints, and that they over-estimate the effects of heavier dark photons on the RGB-tip luminosity. This leads to differences in the constraints of up to an order of magnitude in the kinetic mixing parameter.

hep-ph↗

arXiv2022

VISH$ν$: a unified solution to five SM shortcomings with a protected electroweak scale

We propose a Standard Model extension, coined VISH$ν$ (Variant-axIon Seesaw Higgs $ν$-trino), that is an $N_{\text{DW}} = 1$ variation of its predecessor, the $ν$DFSZ model. In accounting for the origin of neutrino masses, dark matter and the baryon asymmetry of the universe, VISH$ν$ inherits the explanatory power of $ν$DFSZ while, of course, resolving the strong $CP$ problem. In both models, the electroweak scale is naturally protected from a high seesaw scale that is identified with the Peccei-Quinn (PQ) spontaneous symmetry breaking scale. Through a flavour variant coupling structure, VISH$ν$ evades a domain wall problem, extending the cosmological reach of the $ν$DFSZ to include a viable period of inflation. The primary focus of this paper is on the inflationary dynamics of VISH$ν$ and their naturalness in the sense of radiative stability. We find that non-minimal gravitational couplings, generically developed by the VISH$ν$ scalar fields, naturally support a viable inflaton field which typically has both a PQ scalar and Higgs component. An axion mass window [$40μ\text{eV}, \sim 2\text{meV}$] accessible to forthcoming searches, results for the case where PQ symmetry is restored during the (p)reheating phase.

hep-ph↗

arXiv2022

Advancing Globular Cluster Constraints on the Axion-Photon Coupling

We improve the current upper bound on the axion-photon coupling derived from stellar evolution using the $R_2$ parameter, the ratio of stellar populations on the Asymptotic Giant Branch to Horizontal Branch in Globular Clusters. We compare this with data from simulations using the stellar evolution code MESA which include the effects of axion production. Particular attention is given to quantifying in detail the effects of uncertainties on the $R$ and $R_2$ parameters due to the modelling of convective core boundaries. Using a semiconvective mixing scheme we constrain the axion-photon coupling to be $g_{aγγ} < 0.47 \times 10^{-10}~\mathrm{GeV}^{-1}$. This rules out new regions of QCD axion and axion-like particle parameter space. Complementary evidence from asteroseismology suggests that this could improve to as much as $g_{aγγ} < 0.34 \times 10^{-10}~\mathrm{GeV}^{-1}$ as the uncertainties surrounding mixing across convective boundaries are better understood.

hep-ph↗

arXiv2022

Exploring the cosmological dark matter coincidence using infrared fixed points

The asymmetric dark matter (ADM) paradigm is motivated by the apparent coincidence between the cosmological mass densities of visible and dark matter, $Ω_\mathrm{DM} \simeq 5Ω_\mathrm{VM}$. However, most ADM models only relate the number densities of visible and dark matter, and do not motivate the similarity in their particle masses. One exception is a framework introduced by Bai and Schwaller, where the dark matter is a confined state of a dark QCD-like gauge group, and the confinement scales of visible and dark QCD are related by a dynamical mechanism utilising infrared fixed points of the two gauge couplings. We build upon this framework by properly implementing the dependence of the results on the initial conditions for the gauge couplings in the UV. We then reassess the ability of this framework to naturally explain the cosmological mass density coincidence, and find a reduced number of viable models. We identify features of the viable models that allow them to naturally relate the masses of the dark baryon and the proton while also avoiding collider constraints on the new particle content introduced.

hep-ph↗