Searcharxiv⌕ Search

arXiv subjects

Raymond R. Volkas

Publications and source records attributed to Raymond R. Volkas.

At least 55 records · Page 3Linked to original sources
arXiv2017

Asymmetric Dark Matter and the hadronic spectra of hidden QCD

The idea that dark matter may be a composite state of a hidden nonabelian gauge sector has received great attention in recent years. Frameworks such as asymmetric dark matter motivate the idea that dark matter may have similar mass to the proton, while mirror matter and $G \times G$ grand unified theories provide rationales for additional gauge sectors which may have minimal interactions with standard model particles. In this work we explore the hadronic spectra that these dark QCD models can allow. The effects of the number of light colored particles and the value of the confinement scale on the lightest stable state, the dark matter candidate, are examined in the hyperspherical constituent quark model for baryonic and mesonic states.

hep-ph↗
arXiv2017

Reconsidering the One Leptoquark solution: flavor anomalies and neutrino mass

We reconsider a model introducing a scalar leptoquark $ϕ\sim (\mathbf{3}, \mathbf{1}, -1/3)$ to explain recent deviations from the standard model in semileptonic $B$ decays. The leptoquark can accommodate the persistent tension in the decays $\bar{B}\rightarrow D^{(*)}τ\barν$ as long as its mass is lower than approximately $10 \text{ TeV}$, and we show that a sizeable Yukawa coupling to the right-chiral tau lepton is necessary for an acceptable explanation. Agreement with the measured $\bar{B}\rightarrow D^{(*)}τ\barν$ rates is mildly compromised for parameter choices addressing the tensions in $b \to s μμ$, where the model can significantly reduce the discrepancies in angular observables, branching ratios and the lepton-flavor-universality observables $R_K$ and $R_{K^*}$. The leptoquark can also reconcile the predicted and measured value of the anomalous magnetic moment of the muon and appears naturally in models of radiative neutrino mass derived from lepton-number violating effective operators. As a representative example, we incorporate the particle into an existing two-loop neutrino mass scenario derived from a dimension-nine operator. In this specific model, the structure of the neutrino mass matrix provides enough freedom to explain the small masses of the neutrinos in the region of parameter space dictated by agreement with the anomalies in $\bar{B}\rightarrow D^{(*)}τ\barν$, but not the $b \to s$ transition. This is achieved without excessive fine-tuning in the parameters important for neutrino mass.

hep-ph↗
arXiv2016

$ν$DFSZ: a technically natural non-supersymmetric model of neutrino masses, baryogenesis, the strong CP problem, and dark matter

We describe a minimal extension of the standard model by three right-handed neutrinos, a scalar doublet, and a scalar singlet (the "$ν$DFSZ") which serves as an existence proof that weakly coupled high-scale physics can naturally explain phenomenological shortcomings of the SM. The $ν$DFSZ can explain neutrino masses, baryogenesis, the strong CP problem, and dark matter, and remains calculably natural despite a hierarchy of scales up to $\sim 10^{11}$ GeV. It predicts a SM-like Higgs boson, (maximally) TeV-scale scalar states, intermediate-scale hierarchical leptogenesis ($10^5\text{ GeV}\lesssim M_{N}\lesssim 10^7\text{ GeV}$), and axionic dark matter.

hep-ph↗
arXiv2016

TeV-scale Pseudo-Dirac Seesaw Mechanisms in an E$_6$ Inspired Model

TeV-scale seesaw mechanisms are interesting due to their potential testability at existing collider experiments. Herein we propose an E$_6$-inspired model allowing a TeV-scale pseudo-Dirac singlet neutrino seesaw mechanism with naturally sizeable Yukawa couplings of $\mathcal{O}(10^{-2})$. The model also contains new U(1) gauge interactions (and associated $Z^\prime$ bosons with which the singlet neutrinos can be produced at colliders), typically long-lived colour triplet fermions, SU(2) doublet fermions, and a complex scalar - potentially all at the TeV-scale. Additionally we identify three possible explanations for the 750 GeV di-photon excess.

hep-ph↗
arXiv2015

Asymmetric Dark Matter and CP Violating Scatterings in a UV Complete Model

We explore possible asymmetric dark matter models using CP violating scatterings to generate an asymmetry. In particular, we introduce a new model, based on DM fields coupling to the SM Higgs and lepton doublets, a neutrino portal, and explore its UV completions. We study the CP violation and asymmetry formation of this model, to demonstrate that it is capable of producing the correct abundance of dark matter and the observed matter-antimatter asymmetry. Crucial to achieving this is the introduction of interactions which violate CP with a $T^{2}$ dependence.

hep-ph↗
arXiv2015

Natural leptogenesis and neutrino masses with two Higgs doublets

The minimal Type I see-saw model cannot explain the observed neutrino masses and the baryon asymmetry of the Universe via hierarchical thermal leptogenesis without ceding naturalness. We show that this conclusion can be avoided by adding a second Higgs doublet with $\tanβ\gtrsim 4$. The models considered naturally accommodate a SM-like Higgs boson, and predict TeV-scale scalar states and low- to intermediate-scale hierarchical leptogenesis with $10^3\text{ GeV}\lesssim M_{N_1}\lesssim 10^8\text{ GeV}$.

hep-ph↗
arXiv2015

Grand unified hidden-sector dark matter

We explore GxG unified theories with the visible and the hidden or dark sectors paired under a Z2 symmetry. Developing a system of 'asymmetric symmetry breaking' we motivate such models on the basis of their ability to generate dark baryons that are confined with a mass scale just above that of the proton, as motivated by asymmetric dark matter. This difference is achieved from the distinct but related confinement scales that develop in unified theories that have the two factors of $G$ spontaneously breaking in an asymmetric manner. We show how Higgs potentials that admit different gauge group breaking chains in each sector can be constructed, and demonstrate the capacity for generating different fermion mass scales. Lastly we discuss supersymmetric extensions of such schemes.

hep-ph↗
arXiv2015

Electroweak naturalness in three-flavour Type I see-saw and implications for leptogenesis

In the Type I see-saw model, the naturalness requirement that corrections to the electroweak $μ$ parameter not exceed 1 TeV results in a rough bound on the lightest right-handed neutrino mass, $M_{N_1}\lesssim 3\times 10^7$ GeV. In this letter we derive generic bounds applicable in any three-flavour Type I see-saw model. We find $M_{N_1}\lesssim 4\times 10^7$ GeV and $M_{N_2}\lesssim 7\times 10^7$ GeV. In the limit of one massless neutrino, there is no naturalness bound on $M_{N_3}$ in the Poincare protected decoupling limit. Our results confirm that no Type I see-saw model can explain the observed neutrino masses and baryogenesis via hierarchical ($N_1$-, $N_2$-, or $N_3$-dominated) thermal leptogenesis while remaining completely natural.

hep-ph↗
arXiv2015

Testing Radiative Neutrino Mass Models at the LHC

The Large Hadron Collider provides us new opportunities to search for the origin of neutrino mass. Beyond the minimal see-saw models a plethora of models exist which realise neutrino mass at tree- or loop-level, and it is important to be sure that these possibilities are satisfactorily covered by searches. The purpose of this paper is to advance a systematic approach to this problem. Majorana neutrino mass models can be organised by SM-gauge-invariant operators which violate lepton number by two units. In this paper we write down the minimal ultraviolet completions for all of the mass-dimension 7 operators. We predict vector-like quarks, vector-like leptons, scalar leptoquarks, a charged scalar, and a scalar doublet, whose properties are constrained by neutrino oscillation data. A detailed collider study is presented for $O_3=LLQ\bar dH$ and $O_8 = L\bar d\bar e^\dagger \bar u^\dagger H$ completions with a vector-like quark $χ\sim(3, 2, -\frac{5}{6})$ and a leptoquark $ϕ\sim(\bar 3,1,\frac{1}{3})$. The existing LHC limits extracted from searches for vector-like fermions and sbottoms/stops are $m_χ\gtrsim 620$ GeV and $m_ϕ\gtrsim 600$ GeV.

hep-ph↗
arXiv2014

The role of CP violating scatterings in baryogenesis - case study of the neutron portal

Many baryogenesis scenarios invoke the charge parity (CP) violating out-of-equilibrium decay of a heavy particle in order to explain the baryon asymmetry. Such scenarios will in general also allow CP violating scatterings. We study the effect of these CP violating scatterings on the final asymmetry in a neutron portal scenario. We solve the Boltzmann equations governing the evolution of the baryon number numerically and show that the CP violating scatterings play a dominant role in a significant portion of the parameter space.

hep-ph↗
arXiv2014

Particle-antiparticle asymmetries from annihilations

An extensively studied mechanism to create particle-antiparticle asymmetries is the out-of-equilibrium and CP violating decay of a heavy particle. Here we instead examine how asymmetries can arise purely from 2 <-> 2 annihilations rather than from the usual 1 <-> 2 decays and inverse decays. We review the general conditions on the reaction rates that arise from S-matrix unitarity and CPT invariance, and show how these are implemented in the context of a simple toy model. We formulate the Boltzmann equations for this model, and present an example solution.

hep-ph↗
arXiv2014

Testable two-loop radiative neutrino mass model based on an $LLQd^cQd^c$ effective operator

A new two-loop radiative Majorana neutrino mass model is constructed from the gauge-invariant effective operator $L^i L^j Q^k d^c Q^l d^c ε_{ik} ε_{jl}$ that violates lepton number conservation by two units. The ultraviolet completion features two scalar leptoquark flavors and a color-octet Majorana fermion. We show that there exists a region of parameter space where the neutrino oscillation data can be fitted while simultaneously meeting flavor-violation and collider bounds. The model is testable through lepton flavor-violating processes such as $μ \to eγ$, $μ\to eee$, and $μN \to eN$ conversion, as well as collider searches for the scalar leptoquarks and color-octet fermion. We computed and compiled a list of necessary Passarino-Veltman integrals up to boxes in the approximation of vanishing external momenta and made them available as a Mathematica package, denoted as ANT.

hep-ph↗
arXiv2014

Phenomenology of a very light scalar (100 MeV $<m_h<$ 10 GeV) mixing with the SM Higgs

In this paper we investigate the phenomenology of a very light scalar, $h$, with mass 100 MeV $<m_h<$ 10 GeV, mixing with the SM Higgs. As a benchmark model we take the real singlet scalar extension of the SM. We point out apparently unresolved uncertainties in the branching ratios and lifetime of $h$ in a crucial region of parameter space for LHC phenomenology. Bounds from LEP, meson decays and fixed target experiments are reviewed. We also examine prospects at the LHC. For $m_h \lesssim m_B$ the dominant production mechanism is via meson decay; our main result is the calculation of the differential $p_T$ spectrum of $h$ scalars originating from B mesons and the subsequent prediction of up to thousands of moderate (triggerable) $p_T$ displaced dimuons possibly hiding in the existing dataset at ATLAS/CMS or at LHCb. We also demonstrate that the subdominant $Vh$ production channel has the best sensitivity for $m_h \gtrsim m_B$ and that future bounds in this region could conceivably compete with those of LEP.

hep-ph↗
arXiv2013

A Clash-of-Symmetries Mechanism from Intersecting Domain-Wall Branes

We present a new Clash-of-Symmetries mechanism in the context of an intersecting domain-wall brane model in 5+1-dimensional Minkowskian spacetime recently proposed by the authors. This new application of the Dvali-Shifman idea is designed for localizing gauge fields on a domain-wall intersection and we employ it by adding a gauge group $G$ and giving the scalar fields which form lump-like profiles gauge charges. These fields in turn break $G$ to two different subgroups $H_{1}$ and $H_{2}$ on each domain wall, and the gauge fields of these subgroups are taken to be localized to the respective walls by the confinement dynamics of $G$. There is then a further breaking on the domain-wall intersection to $H_{1}\cap{}H_{2}$ and gauge fields of this overlap group can then be localized to the intersection if they belong inside non-Abelian subgroups of both $H_{1}$ and $H_{2}$ which are spontaneously broken on the intersection and confining in the 4+1D bulks of the respective domain-wall branes. This mechanism has some similarities to the Clash-of-Symmetries mechanism on a single domain wall except that in this case $H_{1}$ and $H_{2}$ need not be isomorphic. We then give some interesting examples of the mechanism in an SU(7) gauge theory, several of which result in the localization of the Standard Model gauge group.

hep-ph↗
arXiv2013

Poincare Protection for a Natural Electroweak Scale

We discuss a class of technically-natural UV extensions of the Standard Model in which the electroweak scale is shielded from large radiative corrections from heavy UV physics due to an enhanced Poincare symmetry. Such heavy sectors can be invoked to provide solutions to known shortcomings of the Standard Model, such the strong-CP problem, the absence of dark matter, and the lack of neutrino masses. We discuss the relationship to scale-invariant models.

hep-ph↗
arXiv2013

Two radiative inverse seesaw models, dark matter, and baryogenesis

The inverse seesaw mechanism allows the neutrino masses to be generated by new physics at an experimentally accessible scale, even with O(1) Yukawa couplings. In the inverse seesaw scenario, the smallness of neutrino masses is linked to the smallness of a lepton number violating parameter. This parameter may arise radiatively. In this paper, we study the cosmological implications of two contrasting radiative inverse seesaw models, one due to Ma and the other to Law and McDonald. The former features spontaneous, the latter explicit lepton number violation. First, we examine the effect of the lepton-number violating interactions introduced in these models on the baryon asymmetry of the universe. We investigate under what conditions a pre-existing baryon asymmetry does not get washed out. While both models allow a baryon asymmetry to survive only once the temperature has dropped below the mass of their heaviest fields, the Ma model can create the baryon asymmetry through resonant leptogenesis. Then we investigate the viability of the dark matter candidates arising within these models, and explore the prospects for direct detection. We find that the Law/McDonald model allows a simple dark matter scenario similar to the Higgs portal, while in the Ma model the simplest cold dark matter scenario would tend to overclose the universe.

hep-ph↗
arXiv2013

Review of asymmetric dark matter

Asymmetric dark matter models are based on the hypothesis that the present-day abundance of dark matter has the same origin as the abundance of ordinary or visible matter: an asymmetry in the number densities of particles and antiparticles. They are largely motivated by the observed similarity in the mass densities of dark and visible matter, with the former observed to be about five times the latter. This review discusses the construction of asymmetric dark matter models, summarizes cosmological and astrophysical implications and bounds, and touches on direct detection prospects and collider signatures.

hep-ph↗