Singlet Vacuum Expectation Value Research Articles

Probing high scale Dirac leptogenesis via gravitational waves from domain walls

We propose a novel way of probing high-scale Dirac leptogenesis, a viable alternative to the canonical leptogenesis scenario where the total lepton number is conserved, keeping light standard model neutrinos purely Dirac. The simplest possible seesaw mechanism for generating light Dirac neutrinos involves heavy singlet Dirac fermions and a singlet scalar. In addition to unbroken global lepton number, a discrete ${Z}_{2}$ symmetry is imposed to forbid direct coupling between right and left chiral parts of light Dirac neutrinos. Generating light Dirac neutrino mass requires the singlet scalar to acquire a vacuum expectation value (VEV) that also breaks the ${Z}_{2}$ symmetry, leading to the formation of domain walls in the early Universe. These walls, if made unstable by introducing a soft ${Z}_{2}$-breaking term, generate gravitational waves (GWs) with a spectrum characterized by the wall tension or the singlet VEV, and the soft symmetry breaking scale. The scale of leptogenesis depends on the ${Z}_{2}$-breaking singlet VEV, which is also responsible for the tension of the domain wall, affecting the amplitude of GWs produced from the collapsing walls. We find that most of the near-future GW observatories will be able to probe Dirac leptogenesis scales all the way up to ${10}^{11}\text{ }\text{ }\mathrm{GeV}$.

CP violation in the extension of SM with a complex singlet scalar and vector quarks

We consider the simplest extension of the SM with a complex singlet and a pair of heavy doublet vector quarks, the so-called cSMCS model. In this model, the CP violation can emerge spontaneously as a consequence of the time-dependent phase of the complex singlet vacuum expectation value and the mass mixing of the SM and heavy vector quarks. In our model, the CP-violating time-dependent phase depends on the Higgs field within the bubble-wall via the Higgs-singlet coupling that can directly explain the observed baryon-to-entropy ratio ∼9×10−11. Additionally, as a result of the mixing between vector quarks and SM quarks, the tree level Flavour Changing Neutral Currents and the charged-current decay channels t′→Wb,Zt,hit and b′→Wt,Zb,hib would arise that are well constrained by experimental data. We investigate the implications of these constraints on the total cross-section for the production of heavy vector quark pairs via pp→t′t′¯ and pp→b′b′¯ channels and obtain the bounds on the heavy quark masses for this model. Accordingly, we show the contribution of these heavy quarks in the Higgs bosons signal strength, Rγγ, as well as corrections to the gauge boson propagators.

Gauge-independent renormalization of the N2HDM

The Next-to-Minimal 2-Higgs-Doublet Model (N2HDM) is an interesting benchmark model for a Higgs sector consisting of two complex doublet and one real singlet fields. Like the Next-to-Minimal Supersymmetric extension (NMSSM) it features light Higgs bosons that could have escaped discovery due to their singlet admixture. Thereby, the model allows for various different Higgs-to-Higgs decay modes. Contrary to the NMSSM, however, the model is not subject to supersymmetric relations restraining its allowed parameter space and its phenomenology. For the correct determination of the allowed parameter space, the correct interpretation of the LHC Higgs data and the possible distinction of beyond-the-Standard Model Higgs sectors higher order corrections to the Higgs boson observables are crucial. This requires not only their computation but also the development of a suitable renormalization scheme. In this paper we have worked out the renormalization of the complete N2HDM and provide a scheme for the gauge-independent renormalization of the mixing angles. We discuss the renormalization of the {mathbb{Z}}_2 soft breaking parameter m122 and the singlet vacuum expectation value vS . Both enter the Higgs self-couplings relevant for Higgs-to-Higgs decays. We apply our renormalization scheme to different sample processes such as Higgs decays into Z bosons and decays into a lighter Higgs pair. Our results show that the corrections may be sizable and have to be taken into account for reliable predictions.

Improving the tunings of the MSSM by adding triplets and singlet

We study an extension of the minimal supersymmetric standard model (MSSM) which includes both new $SU(2)$ triplets with hypercharge $\ifmmode\pm\else\textpm\fi{}1$ and a standard model gauge singlet (\`a la the next-to-minimal supersymmetric standard model [NMSSM]) which are coupled to each other. We are motivated by the little hierarchy problem, as well as by the $\ensuremath{\mu}$ problem of the MSSM. We show that the NMSSM and the triplet-extended MSSM can successfully solve problems of one another: while triplets are responsible for large correction to the lightest physical Higgs mass, the singlet's vacuum expectation value (VEV) explains why the $\ensuremath{\mu}$ terms (for the Higgs doublets and the new triplets) are naturally of order the electroweak (EW) scale. We also show that singlet-triplet coupling significantly changes the renormalization group evolution of the singlet mass squared, helping to render this mass squared negative, as required for the singlet to acquire a VEV. We analyze constraints on this scenario from EW precision measurements and find that a relatively large region of the parameter space of this model is viable, especially with the triplet fermions (including doubly charged) being light.

Two hundred heterotic standard models on smooth Calabi-Yau threefolds

We construct heterotic standard models by compactifying on smooth Calabi-Yau three-folds in the presence of purely Abelian internal gauge fields. A systematic search over complete intersection Calabi-Yau manifolds with less than six K\"ahler parameters leads to over 200 such models which we present. Each of these models has precisely the matter spectrum of the minimal supersymmetric standard model, at least one pair of Higgs doublets, the standard model gauge group, and no exotics. For about 100 of these models there are four additional $U(1)$ symmetries which are Green-Schwarz anomalous and, hence, massive. In the remaining cases, three $U(1)$ symmetries are anomalous, while the fourth, massless one can be spontaneously broken by singlet vacuum expectation values. The presence of additional global $U(1)$ symmetries, together with the possibility of switching on singlet vacuum expectation values, leads to a rich phenomenology which is illustrated for a particular example. Our database of standard models, which can be further enlarged by simply extending the computer-based search, allows for a detailed and systematic phenomenological analysis of string standard models, covering issues such as the structure of Yukawa couplings, $R$-parity violation, proton stability, and neutrino masses.

SU(7) unification of SU(3) C × SU(4) W × U(1) B−L

We propose the SUSY SU(7) unification of the SU(3)_C* SU(4)_W* U(1)_{B-L} model. Such unification scenario has rich symmetry breaking chains in a five-dimensional orbifold. We study in detail the SUSY SU(7) symmetry breaking into SU(3)_C* SU(4)_W* U(1)_{B-L} by boundary conditions in a Randall-Sundrum background and its AdS/CFT interpretation. We find that successful gauge coupling unification can be achieved in our scenario. Gauge unification favors low left-right and unification scales with tree-level \sin^2\theta_W=0.15. We use the AdS/CFT dual of the conformal supersymmetry breaking scenario to break the remaining N=1 supersymmetry. We employ AdS/CFT to reproduce the NSVZ formula and obtain the structure of the Seiberg duality in the strong coupling region for 3/2N_c<N_F<3N_C. We show that supersymmetry is indeed broken in the conformal supersymmetry breaking scenario with a vanishing singlet vacuum expectation value.

Displaced vertices in extended supersymmetric models

In extended supersymmetric models with additional singlet Higgs fields displaced vertices could be observed if the decay width of the next-to-lightest supersymmetric particle becomes very small due to a singlino dominated LSP. We study the supersymmetric parameter space where displaced vertices of the second lightest neutralino exist in the NMSSM and an E 6 inspired model. For a mass difference between LSP and NLSP of more than 10 GeV the singlet vacuum expectation value has to be at least of the order of 100 TeV in order to obtain a lightest neutralino with a singlino component large enough for displaced vertices.

Particle spectrum in the non-minimal supersymmetric standard model with tan [formula omitted

We present a detailed analysis of the particle spectrum of the non-minimal supersymmetric standard model (NMSSM), containing two Higgs doublets and a singlet, in the limit tan β ⋍ m t m b . This is compared with the corresponding particle spectrum of the minimal supersymmetric standard model (MSSM). In this limit the singlet vacuum expectation value is forced to be large, of the order of 10 TeV, and the singlet decouples from the lightest scalar Higgs boson and the neutralinos. With the exception of the lightest Higgs boson, the particle spectrum in the model turns out to be heavy. The radiatively corrected lightest Higgs boson mass is in the neighbourhood of ∼ 130 GeV.

UPPER BOUND ON THE MASS OF LIGHTEST HIGGS BOSON IN THE SUPERSYMMETRIC SINGLET MAJORAN MODEL

We calculate an upper bound on the lightest Higgs boson mass in the supersymmetric singlet majoran model containing SU(2)×U(1) singlet right-handed neutrino fields and a chiral singlet having two units of lepton number. We show that although the singlet vacuum expectation values and supersymmetry breaking masses do not decouple from the upper bound, even at the tree level, their effect is small, thereby reducing the bound to the corresponding bound in the minimal supersymmetric standard model. Radiative corrections to the bound are similar to those in the minimal model. We also show that the couplings of the lightest Higgs boson to the majorans are suppressed in this model.

The non-minimal supersymmetric standard model with tan [formula omitted

We consider the supersymmetric extension of the standard model with an additional singlet S, the non-minimal super-symmetric standard model (NMSSM), in the limit tan β ∼- m t m b . We embed this model in a supergravity framework with universal boundary conditions and analyze the renormalization group improved tree-level potential. We examine the relationship between this model and the minimial supersymmetric standard model (MSSM), and discuss the novel connections between the two when tan β is large. Strong correlations between the free parameters of the nonminimal model are found and the reasons for these discussed. The singlet vacuum expectation value is forced to be large, of the order of 10 TeV. The radiatively corrected mass of the lightest Higgs boson is found to be 〈 140 GeV.

Mass bounds for the neutral Higgs bosons in the Next-To-Minimal Supersymmetric Standard Model

In the Next-To-Minimal Supersymmetric Standard Model (NMSSM), the Higgs and neutralino/chargino sectors are strongly correlated by four common parameters at tree level. Therefore we analyze the experimental data from both the search for Higgs bosons as well as for neutralinos and charginos at LEP 100 in order to constrain the parameter space and the masses of the neutral Higgs particles in the NMSSM. We find that small singlet vacuum expectation values are ruled out, but a massless neutral Higgs scalar and pseudoscalar is not excluded for most of the parameter space of the NMSSM. Improved limits from the neutralino/chargino search at LEP 200, however, may lead to nonvanishing lower Higgs mass bounds.

Invisible Higgs decays and neutrino physics

A wide class of neutrino physics motivated models are characterized by the spontaneous violation of a global U(1) lepton number symmetry at or below the electroweak scale by an SU(2)⊗U(1) singlet vacuum expectation value 〈 σ〉 ≲ O(1) TeV. In all these models the main Higgs decay channel is likely to be “invisible”, e.g. h → JJ, where J denotes the associated weakly interacting pseudoscalar Goldstone boson — the majoron. This leads to events with large missing energy that could be observable at LEP and affect the Higgs mass bounds obtained, as well as lead to novel ways to search for Higgs bosons and high-energy supercolliders such as the LHC/SSC.

17 keV NEUTRINO AND MAJORON MODELS

A model for the 17 keV Dirac neutrino is considered in the framework of the SU(2) × U(1) theory. No right-handed neutrinos are introduced. The Dirac mass for the neutrino arises from the Le+Lτ–Lμ invariant couplings of the left-handed neutrinos to an SU(2) triplet. An SU(2) singlet field is introduced to suppress the Majoron coupling to the Z. This makes the model consistent with the LEP results on the invisible Z width. The singlet vacuum expectation value ω is constrained to be ≤O(80 MeV) from cosmological considerations. For, ω≈80 MeV, the 17 keV neutrino is shown to provide the bulk of the dark matter.