Scalar Vacuum Expectation Values Research Articles

Inflation and Higgs phenomenology in a model unifying the DFSZ axion with the majoron

The Two-Higgs-Doublet-Standard Model-Axion-Seesaw-Higgs-Portal inflation (2hdSMASH) model consisting of two Higgs doublets, a Standard Model (SM) singlet complex scalar and three SM singlet right-handed neutrinos can embed axion dark matter, neutrino masses and address inflation. We report on an investigation of the inflationary aspects of 2hdSMASH and its subsequent impact on low energy phenomenology. In particular, we identify inflationary directions for which the parameter values required for successful inflation do not violate perturbative unitarity and boundedness-from-below conditions. By analyzing the renormalization-group flow of the parameters we identify the necessary and sufficient constraints for running all parameters perturbatively and maintaining stability from the electroweak to the PLANCK scale. We observe that stringent constraints arise on the singlet scalar self coupling from inflationary constraints, i.e., λS ∼ 10-10. Further, we find that all theoretical and experimental constraints are satisfied if the portal couplings are typically in the range (v/vS ) and (v/vS )2 (where v, vS refer to the electroweak and singlet scalar vacuum expectation value respectively). As a consequence, inflation is realized in a variety of field space directions in the effective single field regime. Finally we provide testable benchmark scenarios at colliders.

Interpreting electroweak precision data including the W-mass CDF anomaly

We perform a global fit of electroweak data, finding that the anomaly in the W mass claimed by the CDF collaboration can be reproduced as a universal new-physics correction to the T parameter or |H†DμH|2 operator. Contributions at tree-level from multi-TeV new physics can fit the anomaly compatibly with collider bounds: we explore which scalar vacuum expectation values (such as a triplet with zero hypercharge), Z′ vectors (such as a Z′ coupled to the Higgs only), little-Higgs models or higher-dimensional geometries provide good global fits. On the other hand, new physics that contributes at loop-level must be around the weak scale to fit the anomaly. Thereby it generically conflicts with collider bounds, that can be bypassed assuming special kinematics like quasi-degenerate particles that decay into Dark Matter (such as an inert Higgs doublet or appropriate supersymmetric particles).

Revisiting type-II see-saw: present limits and future prospects at LHC

The type-II see-saw mechanism based on the annexation of the Standard Model by weak gauge triplet scalar field proffers a natural explanation for the very minuteness of neutrino masses. Noting that the phenomenology for the non-degenerate triplet Higgs spectrum is substantially contrasting than that for the degenerate one, we perform a comprehensive study for an extensive model parameter space parametrised by the triplet scalar vacuum expectation value (VEV), the mass-splitting between the triplet-like doubly and singly charged scalars and the mass of the doubly charged scalar. Considering all Drell-Yan production mechanisms for the triplet-like scalars and taking into account the all-encompassing complexity of their decays, we derive the most stringent 95% CL lower limits on the mass of the doubly charged scalar for a vast model parameter space by implementing already existing direct collider searches by CMS and ATLAS. These estimated limits are stronger by approximately 50–230 GeV than those reported by CMS and ATLAS. Strikingly, we also find a specific region of the parameter space that is beyond the reach of the existing LHC search strategies. Then, we forecast future limits by extending an ATLAS search at high-luminosity, and we propose a search strategy that yields improved limits for a part of the parameter space.

Vacuum stability vs. positivity in real singlet scalar extension of the standard model

We assume a generic real singlet scalar extension of the Standard Model living in the vacuum (v,w) at the electroweak scale with v=246 GeV and w being respectively the Higgs and the singlet scalar vacuum expectation values. By requiring absolute vacuum stability for the vacuum (v,w), the positivity condition and the perturbativity up to the Planck scale, we show that the viable space of parameters in the model is strongly constrained for various singlet scalar vacuum expectation values w=0.1,1,10,100 TeV. Also, it turns out that the singlet scalar mass can be from a few GeV up to less than TeV.

Crossed two-Higgs-doublet models: Reduction of Yukawa parameters in the low-scale limit of left-right symmetry and other avatars

We present new variants of the Two Higgs-Doublet Model where all Yukawa couplings with physical Higgs bosons are controlled by the quark mixing matrices of both chiralities, as well as, in one case, the ratio between the two scalar doublets' vacuum expectation values. We obtain these by imposing approximate symmetries on the Lagrangian which, in one of the cases, clearly reveals the model to be the electroweak remnant of the Minimal Left-Right Symmetric Model. We also argue for the benefits of the bidoublet notation in the Two Higgs-Doublet Model context for uncovering new models.

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.

Inverse seesaw mechanism and portal dark matter

We study the phenomenology of the inverse seesaw mechanism in the scalar-Higgs portal dark matter model. The model is an extension of the Standard Model including two additional neutrinos, a singlet scalar and a fermionic dark matter. We consider the inverse seesaw mechanism where the mass of 2 additional neutrinos are made dynamic by the singlet scalar. We found that the natural scale for the scalar vacuum expectation value is naturally close to the weak scale. Motivating by this fact, we focus on the possibility of the singlet scalar connecting with dark matter, i.e., the scalar is also the mediator between dark sector and the Standard Model. We perform a numerical analysis over the parameter space subject to the indirect and direct detection constraints. The feasible region of the parameter space will be discussed.

Precise prediction for the W boson mass in the MRSSM

The mass of the W boson, MW, plays a central role for high-precision tests of the electroweak theory. Confronting precise theoretical predictions with the accurately measured experimental value provides a high sensitivity to quantum effects of the theory entering via loop contributions. The currently most accurate prediction for the W boson mass in the Minimal R-symmetric Supersymmetric Standard Model (MRSSM) is presented. Employing the on-shell scheme, it combines all numerically relevant contributions that are known in the Standard Model (SM) in a consistent way with all MRSSM one-loop corrections. Special care is taken in the treatment of the triplet scalar vacuum expectation value vT that enters the prediction for MW already at lowest order. In the numerical analysis the decoupling properties of the supersymmetric loop contributions and the comparison with the MSSM are investigated. Potentially large numerical effects of the MRSSM-specific Λ superpotential couplings are highlighted. The comparison with existing results in the literature is discussed.

Searching for Higgs Decays to as Many as 8 Leptons.

We propose a search for Higgs decays with as many as 8 leptons in the final state. This signal can arise in a simple model with a hidden vector (A_{d}) that gets mass via a hidden scalar (h_{d}) vacuum expectation value. The 125GeV Higgs boson can then decay H→h_{d}h_{d}→4A_{d}→8f, where f are standard model fermions. We recast current searches and show that a branching ratio (BR) of H→h_{d}h_{d} as large as 10% is allowed. We also describe a dedicated search that could place bounds on BR(H→h_{d}h_{d}) as low as 10^{-5} using only 36 fb^{-1} of data, with significant improvements coming from greater integrated luminosity.

Realizing the supersymmetric inverse seesaw model in the framework of R-parity violation

Abstract If, on one hand, the inverse seesaw is the paradigm of TeV scale seesaw mechanism, on the other it is a challenge to find scenarios capable of realizing it. In this work we propose a scenario, based on the framework of R-parity violation, that realizes minimally the supersymmetric inverse seesaw mechanism. In it the energy scale parameters involved in the mechanism are recognized as the vacuum expectation values of the scalars that compose the singlet superfields N ˆ C and S ˆ . We develop also the scalar sector of the model and show that the Higgs mass receives a new tree-level contribution that, when combined with the standard contribution plus loop correction, is capable of attaining 125 GeV without resort to heavy stops.

Constraints on a Class of Two-Higgs Doublet Models with tree level FCNC

We analyse a class of two Higgs doublet models where flavour-changing neutral currents (FCNC) are present at tree level in a mixing-suppressed manner. In this class of models, because of a discrete symmetry imposed on the lagrangian, the FCNC couplings in the quark and lepton sector are fixed in terms of the corresponding mixing matrix (CKM or PMNS), the fermion masses and the ratio v2/v1 of the vacuum expectation values of the neutral scalars. A large number of processes, including tree and loop level transitions mediated by the new charged or neutral scalars are used as constraints. It is shown that among the interesting phenomenological prospects for these models, the new scalars may have masses within experimental reach.

Model of the Neutrino Mass Matrix With δ=-π/2 and θ_(23)=π/4

Experimental data have provided stringent constraints on the neutrino mixing parameters. In the standard parameterization the mixing angle θ_(23) is close to π/4. There is also evidence which shows that the CP violating phase is close to -π/2. We study the neutrino mass matrix reconstructed using this information and find several interesting properties. We show that a theoretical model based on the A_4 symmetry naturally predicts δ = -π/2 and θ_(23) = π/4 when the Yukawa couplings and scalar vacuum expectation values are real, reaching a μ－τ exchange and CP conjugate symmetry limit. In this case CP violation solely comes from the complex group theoretical Clebsh-Gordan coefficients. The model also predicts |V_(e2)| = 1/√3, consistent with the data. With complex Yukawa couplings the values for δ and θ_(23) can significantly deviate away from the symmetry values -π/2 and π/4, respectively. But |V_(e2)| = 1/√3 is not altered. This matrix is an excellent lowest order approximation for theoretical model buildings of the neutrino mass matrix.

Infinite number of MSSMs from heterotic line bundles?

We consider heterotic ${\mathrm{E}}_{8}\ifmmode\times\else\texttimes\fi{}{\mathrm{E}}_{8}$ supergravity compactified on smooth Calabi-Yau manifolds with line bundle gauge backgrounds. Infinite sets of models that satisfy the Bianchi identities and flux quantization conditions can be constructed by letting their background flux quanta grow without bound. Even though we do not have a general proof, we find that all examples are at the boundary of the theory's validity: the Donaldson-Uhlenbeck-Yau equations, which can be thought of as vanishing D-term conditions, cannot be satisfied inside the K\"ahler cone unless a growing number of scalar vacuum expectation values is switched on. As they are charged under various line bundles simultaneously, the gauge background gets deformed by these VEVs to a non-Abelian bundle. In general, our physical expectation is that such infinite sets of models should be impossible, since they never seem to occur in exact conformal field theory constructions.

Naturalness and dimensional transmutation in classically scale-invariant gravity

We discuss the nature of quantum field theories involving gravity that are classically scale-invariant. We show that gravitational radiative corrections are crucial in the determination of the nature of the vacuum state in such theories, which are renormalisable, technically natural, and can be asymptotically free in all dimensionless couplings. In the pure gravity case, we discuss the role of the Gauss-Bonnet term, and we find that Dimensional Transmutation (DT) \`a la Coleman-Weinberg leads to extrema of the effective action corresponding to nonzero values of the curvature, but such that these extrema are local maxima. In even the simplest extension of the theory to include scalar fields, we show that the same phenomenon can lead to extrema that are local minima of the effective action, with both non-zero curvature and non-zero scalar vacuum expectation values, leading to spontaneous generation of the Planck mass. Although we find an asymptotically free (AF) fixed point exists, unfortunately, no running of the couplings connect the region of DT to the basin of attraction of the AF fixed point. We also find there remains a flat direction for one of the conformal modes. We suggest that in more realistic models AF and DT could be compatible, and that the same scalar vacuum expectation values could be responsible both for DT and for spontaneous breaking of a Grand Unified gauge group.

LeptonicCPviolating phase in the Yukawaon model

In the so-called ``Yukawaon'' model, the (effective) Yukawa coupling constants ${Y}_{f}^{\text{eff}}$ are given by vacuum expectation values (VEVs) of scalars ${Y}_{f}$ (Yukawaons) with $3\ifmmode\times\else\texttimes\fi{}3$ components. In this brief article, we change VEV forms $⟨{Y}_{f}⟩$ in the previous paper into a unified form. Therefore, parameter fitting for quark and lepton masses and mixings is revised. Especially, we obtain predicted values of neutrino mixing ${\mathrm{sin}}^{2}2{\ensuremath{\theta}}_{13}$ and a leptonic $CP$ violating phase ${\ensuremath{\delta}}_{CP}^{\ensuremath{\ell}}$ that are consistent with the observed curve in the $({\mathrm{sin}}^{2}2{\ensuremath{\theta}}_{13},{\ensuremath{\delta}}_{CP}^{\ensuremath{\ell}})$ reported by the T2K group recently.

On symmetry-extended standard model and neutrino masses

An Abelian symmetry extension of the Standard Model and its possible implication for the neutrino sector is considered. The symmetry breaking by a new scalar vacuum expectation value is described and the resulting suppressed neutrino mass terms are determined. From the known data, the strengths of the corresponding suppressing effect are derived and the associated new scales are approached.

Universal bilinear form of quark and lepton mass matrices

In the so-called ``yukawaon" model, the (effective) Yukawa coupling constants $Y_f^{eff}$ are given by vacuum expectation values (VEVs) of scalars $Y_f$ (yukawaons) with $3\times 3$ components. In the present model, all of the VEV matrices $\langle Y_f \rangle$ are given by a bilinear form of VEVs of flavons $\Phi_f$, $\langle Y_f \rangle_i^{\ j} = k_f \langle \Phi_f\rangle_{ik} \langle \bar{\Phi}_f\rangle^{kj}$, where $\Phi_f$ is assigned to ${\bf 6}$ of U(3) family symmetry. As input parameters with family-number dependent values, we use only charged lepton mass values. Under this formulation, we can give reasonable values of quark and lepton masses and their mixings. A $CP$ violating phase $\delta_{CP}^\ell=26^\circ$ in the lepton sector is predicted. The effective Majorana neutrino mass is also predicted.

Yukawaon model with anomaly free set of quarks and leptons in a U(3) family symmetry

In the so-called "yukawaon" model, the (effective) Yukawa coupling constants $Y_f^{eff}$ are given by vacuum expectation values (VEVs) of scalars $Y_f$ (yukawaons) with $3\times 3$ components. So far, yukawaons $Y_f$ have been assigned to ${\bf 6}$ or ${\bf 6}^*$ of U(3) family symmetry, so that quarks and leptons were not anomaly free in U(3). In this paper, yukawaons are assigned to ${\bf 8}+{\bf 1}$ of U(3), so that quarks and leptons are anomaly free. Since VEV relations among yukawaons are also considerably changed, parameter fitting of the model is renewed. After fixing our free parameters by observed mass ratios, we have only two and one remaining free parameters for quark and lepton mixings, respectively. We obtain successful predictions for the quark and lepton mixing parameters including magnitudes of $CP$ violation. The effective Majorana neutrino mass is also predicted.

Minimal tree-level seesaws with a heavy intermediate fermion

There exists a generic minimal tree-level diagram, with two external scalars and a heavy intermediate fermion, that can generate naturally small neutrino masses via a seesaw. This diagram has a mass insertion on the internal fermion line, and the set of such diagrams can be partitioned according to whether the mass insertion is of the Majorana or Dirac type. We show that, once subjected to the demands of naturalness (i.e. precluding small scalar vacuum expectation values that require fine-tuning), this set is finite, and contains a relatively small number of elements. Some of the corresponding models have appeared in the literature. We present the remaining original models, thus generalizing the Type-I and Type-III seesaws, and apparently exhausting the list of their minimal non-tuned variants.

Superpotential de-sequestering in string models

Non-perturbative superpotential cross-couplings between visible sector matter and K\"ahler moduli can lead to significant flavour-changing neutral currents in compactifications of type IIB string theory. Here, we compute corrections to Yukawa couplings in orbifold models with chiral matter localised on D3-branes and non-perturbative effects on distant D7-branes. By evaluating a threshold correction to the D7-brane gauge coupling, we determine conditions under which the non-perturbative corrections to the Yukawa couplings appear. The flavour structure of the induced Yukawa coupling generically fails to be aligned with the tree-flavour structure. We check our results by also evaluating a correlation function of two D7-brane gauginos and a D3-brane Yukawa coupling. Finally, by calculating a string amplitude between n hidden scalars and visible matter we show how non-vanishing vacuum expectation values of distant D7-brane scalars, if present, may correct visible Yukawa couplings with a flavour structure that differs from the tree-level flavour structure.