Vector-like States Research Articles

SU(5) × U(1)′ Models with a Vector-like Fermion Family

Motivated by experimental measurements indicating deviations from the Standard Model predictions, we discuss F-theory-inspired models, which, in addition to the three chiral generations, contain a vector-like complete fermion family. The analysis takes place in the context of SU(5)×U(1)′ GUT embedded in an E8 covering group, which is associated with the (highest) geometric singularity of the elliptic fibration. In this context, the U(1)′ is a linear combination of four abelian factors subjected to the appropriate anomaly cancellation conditions. Furthermore, we require universal U(1)′ charges for the three chiral families and different ones for the corresponding fields of the vector-like representations. Under the aforementioned assumptions, we find 192 models that can be classified into five distinct categories with respect to their specific GUT properties. We exhibit representative examples for each such class and construct the superpotential couplings and the fermion mass matrices. We explore the implications of the vector-like states in low-energy phenomenology, including the predictions regarding the B-meson anomalies. The rôle of R-parity violating terms appearing in some particular models of the above construction is also discussed.

Qualities of the axion and LSP in Pati-Salam unification with Z4R×ZN symmetry

In this paper we construct supersymmetric Pati-Salam (PS) models containing the minimal supersymmetric standard model and an invisible axion. The models include two discrete symmetries, ${\mathbb{Z}}_{4}^{R}\ifmmode\times\else\texttimes\fi{}{\mathbb{Z}}_{N}$, which maintain the quality of the accidental Peccei-Quinn (PQ) symmetry and thus the solution to the strong $CP$ problem. We require that the discrete anomaly conditions are satisfied for both ${\mathbb{Z}}_{4}^{R}\ifmmode\times\else\texttimes\fi{}{G}_{\mathrm{PS}}^{2}$ and ${\mathbb{Z}}_{N}\ifmmode\times\else\texttimes\fi{}{G}_{\mathrm{PS}}^{2}$. The vacuum expectation value of the PQ field spontaneously breaks all the discrete symmetries. R-parity is violated if any of the PQ field(s) has an odd charge under ${\mathbb{Z}}_{4}^{R}$. We present two explicit models which we refer to as a minimal model where R-parity violation is extremely suppressed, and a nonminimal model where R-parity violation is significant. In the latter model, the neutralino becomes unstable even if it is the lightest supersymmetric particle (LSP), and, in addition, there are new low-energy vectorlike states. In both examples, R-parity violation is sufficiently suppressed such that the proton is stable.

Dark sector portal with vectorlike leptons and flavor sequestering

We consider models with fermionic dark matter that transforms under a non-Abelian dark gauge group. Exotic, vectorlike leptons that also transform under the dark gauge group can mix with standard model leptons after spontaneous symmetry breaking and serve as a portal between the dark and visible sectors. We show in an explicit, renormalizable model based on a dark SU(2) gauge group how this can lead to adequate dark matter annihilation to a standard model lepton flavor so that the correct relic density is obtained. We identify a discrete symmetry that allows mass mixing between the vectorlike fermions and a single standard model lepton flavor, while preventing mixing between these fields and the remaining standard model leptons. This flavor sequestering avoids unwanted lepton-flavor-violating effects, substantially relaxing constraints on the mass scale of the vectorlike states. We discuss aspects of the phenomenology of the model, including direct detection of the dark matter.

Non-universal Z′ from SO(10) GUTs with vector-like family and the origin of neutrino masses

A Z′ gauge boson with mass around the (few) TeV scale is a popular example of physics beyond the Standard Model (SM) and can be a fascinating remnant of a Grand Unified Theory (GUT). Recently, Z′ models with non-universal couplings to the SM fermions due to extra vector-like states have received attention as potential explanations of the present RK, RK⁎ anomalies; this includes GUT model proposals based on the SO(10) group. In this paper we further develop GUT models with a flavour non-universal low scale Z′ and clarify several outstanding issues within them. First, we successfully incorporate a realistic neutrino sector (with linear and/or inverse low scale seesaw mechanism), which was so far a missing ingredient. Second, we investigate in detail their compatibility with the RK, RK⁎ anomalies; we find that the anomalies do not have a consistent explanation within such models. Third, we demonstrate that these models have other compelling phenomenological features; we study the correlations between the flavour violating processes of μ→3e and μ–e conversion in a muonic atom, showing how a GUT imprint could manifest itself in experiments.

Heterotic line bundle models on elliptically fibered Calabi-Yau three-folds

We analyze heterotic line bundle models on elliptically fibered Calabi-Yau three-folds over weak Fano bases. In order to facilitate Wilson line breaking to the standard model group, we focus on elliptically fibered three-folds with a second section and a freely-acting involution. Specifically, we consider toric weak Fano surfaces as base manifolds and identify six such manifolds with the required properties. The requisite mathematical tools for the construction of line bundle models on these spaces, including the calculation of line bundle cohomology, are developed. A computer scan leads to more than 400 line bundle models with the right number of families and an SU(5) GUT group which could descend to standard-like models after taking the ℤ2 quotient. A common and surprising feature of these models is the presence of a large number of vector-like states.

Drell–Yan constraints on new electroweak states: LHC as a pp → ℓ+ℓ− precision machine

The Standard Model (SM) extensions with vector-like states which have either zero hypercharge or zero weak isospin are rather poorly constrained by the electroweak precision measurements. Such new states would however modify the running of the gauge couplings at high energies. As a result, the Drell–Yan process [Formula: see text] at the LHC places useful constraints on these models. The relevant observables include both the dilepton invariant mass distribution [Formula: see text] and the forward–backward asymmetry [Formula: see text]. We find that the LHC Run 1 data and the initial data from Run 2 surpass the sensitivity of LEP and already put meaningful constraints on the existence of such particles, which will become progressively stronger with more data.

Bounds on supersymmetric effective operators from heavy diphoton searches

We identify the bounds on supersymmetric effective operators beyond minimal supersymmetric models (MSSM, etc.), from heavy diphoton resonance (X) negative searches at the LHC, where X is identified with the neutral CP-even (odd), H(A) or both (mass degenerate). While MSSM-like models comply with the data, a leading effective operator of d = 6 can contribute significantly to diphoton production [Formula: see text] 1 fb, well above its MSSM value and in conflict with recent data. Both the [Formula: see text] and gg production mechanisms of H and A can contribute comparably to this. We examine the dependence of the diphoton cross-section [Formula: see text] on the values of [Formula: see text], [Formula: see text] and tan [Formula: see text], under the experimental constraints from the SM-like Higgs couplings hgg and h[Formula: see text] (due to mixing) and from the [Formula: see text] and [Formula: see text] searches. These give [Formula: see text] larger than [Formula: see text] 5 TeV for [Formula: see text] in the range 0.5–1 TeV. We show how to generate the d = 6 effective operator from microscopic (renormalizable) models. This demands the presence of vector-like states beyond the MSSM spectrum (and eventually, but not necessarily a gauge singlet), of mass near [Formula: see text] and thus outside the LHC reach.

LHC di-photon excess and gauge coupling unification in extra $$Z^\prime $$ Z ′ heterotic-string derived models

The di-photon excess observed at the LHC can be explained as a Standard Model singlet that is produced and decays by heavy vector-like colour triplets and electroweak doublets in one-loop diagrams. The characteristics of the required spectrum are well motivated in heterotic-string constructions that allow for a light $Z^\prime$. Anomaly cancellation of the $U(1)_{Z^\prime}$ symmetry requires the existence of the Standard Model singlet and vector-like states in the vicinity of the $U(1)_{Z^\prime}$ breaking scale. In this paper we show that the agreement with the gauge coupling data at one-loop is identical to the case of the Minimal Supersymmetric Standard Model, owing to cancellations between the additional states. We further show that effects arising from heavy thresholds may push the supersymmetric spectrum beyond the reach of the LHC, while maintaining the agreement with the gauge coupling data. We show that the string inspired model can indeed account for the observed signal and discuss the feasibility of obtaining viable scalar mass spectrum.

The 750 GeV di-photon LHC excess and extra $$Z^\prime $$ Z ′ s in heterotic-string derived models

The ATLAS and CMS collaborations recently recorded possible di-photon excess at 750 GeV and a less significant di-boson excess around 1.9 TeV. Such excesses may be produced in heterotic-string derived Z' models, where the di-photon excess may be connected with the Standard Model singlet scalar responsible for the Z' symmetry breaking, whereas the di-boson excess arises from production of the extra vector boson. Additional vector-like states in the string Z' model are instrumental to explain the relatively large width of the di-photon events and mandated by anomaly cancellation to be in the vicinity of the Z' breaking scale. Wilson line breaking of the non-Abelian gauge symmetries in the string models naturally gives rise to dark matter candidates. Future collider experiments will discriminate between the high-scale heterotic-string models, which preserve the perturbative unification paradigm indicated by the Standard Model data, versus the low scale string models. We also discuss the possibility for the production of the di-photon events with high scale $U(1)_{Z^\prime}$ breaking.

A constrained supersymmetric left-right model

We present a supersymmetric left-right model which predicts gauge coupling unification close to the string scale and extra vector bosons at the TeV scale. The subtleties in constructing a model which is in agreement with the measured quark masses and mixing for such a low left-right breaking scale are discussed. It is shown that in the constrained version of this model radiative breaking of the gauge symmetries is possible and a SM-like Higgs is obtained. Additional CP-even scalars of a similar mass or even much lighter are possible. The expected mass hierarchies for the supersymmetric states differ clearly from those of the constrained MSSM. In particular, the lightest down-type squark, which is a mixture of the sbottom and extra vector-like states, is always lighter than the stop. We also comment on the model's capability to explain current anomalies observed at the LHC.

Light vectorlike fermions in a minimalSU(5)setup

The Standard Model fermion sector is enlarged by either one light singlet vectorlike down-type quark or one light vectorlike lepton doublet, which might be accommodated within a five-dimensional representation of $SU(5)$. At low energies the inclusion of these states affects precisely measured observables in flavor physics, as well as electroweak precision measurements. These experimental results strongly constrain couplings of vectorlike states to the Standard Model particles. Having these bounds, we investigate the impact of vectorlike fermions on the mass matrices for down-type quarks and charged leptons in an $SU(5)$ setting. We find that unitary transformations relating an arbitrary flavor basis to the mass eigenstate basis depend only on three free parameters. Then we discuss the parameter space constrained by low-energy data, assuming vectorlike quark and vectorlike lepton masses to be 800 GeV and 400 GeV, respectively. We demonstrate that these two scenarios generate unique patterns for relevant proton decay widths. A further improvement of experimental bounds on proton decay modes would thus differentiate the allowed parameter space. We finally present two full-fledged $SU(5)$ models that allow for gauge coupling unification with light vectorlike fermions under consideration and discuss their viability.

Vector-like leptons: Higgs decays and collider phenomenology

We study the impact of heavy vector-like leptons on several observables in collider and low-energy physics. These states, present in many well-motivated extensions of the Standard Model, can induce lepton flavour violation and non-standard Higgs decays. We study these effects in an effective model inspired by the composite Higgs scenario. After deriving bounds on the mass and production cross-section of the vector-like states using recent LHC data on multilepton searches, we discuss the modification of the Higgs decays to dilepton, diphoton and $Z\gamma$ final states as well as low-energy observables like radiative lepton decays, the anomalous magnetic moment of the muon and the electric dipole moment of the electron. We find several interesting correlations. In particular, we show that branching fractions of lepton flavour-violating Higgs decays at an observable level are prohibited due to the strong bounds on the radiative lepton decays.

Yukawa couplings and masses of non-chiral states for the Standard Model on D6-branes on $ {T^{{6}}}/\mathbb{Z}_{{6}}^{\prime } $

The perturbative leading order open string three-point couplings for the Standard Model with hidden USp(6) on fractional D6-branes on T6/Z6' from arXiv:0806.3039 [hep-th], arXiv:0910.0843 [hep-th] are computed. Physical Yukawa couplings consisting of holomorphic Wilsonian superpotential terms times a non-holomorphic prefactor involving the corresponding classical open string Kaehler metrics are given, and mass terms for all non-chiral matter states are derived. The lepton Yukawa interactions are at leading order flavour diagonal, while the quark sector displays a more intricate pattern of mixings. While N=2 supersymmetric sectors acquire masses via only two D6-brane displacements - which also provide the hierarchies between up- and down-type Yukawas within one quark or lepton generation -, the remaining vector-like states receive masses via perturbative three-point couplings to some Standard Model singlet fields with vevs along flat directions. Couplings to the hidden sector and messengers for supersymmetry breaking are briefly discussed.

Froggatt-Nielsen model for leptophilic scalar dark matter decay

We construct a model of decaying, TeV-scale scalar dark matter motivated by data from the PAMELA and Fermi-LAT experiments. By introducing an appropriate Abelian discrete symmetry and an intermediate scale of vector-like states that are responsible for generating lepton Yukawa couplings, we show that Planck-suppressed corrections may lead to decaying dark matter that is leptophilic and has the desired lifetime. The dark matter candidate decays primarily to lepton/anti-lepton pairs, and at a subleading rate to final states with a lepton, anti-lepton and standard model Higgs boson. We show that the model can reproduce the observed positron flux and positron fraction while remaining consistent with the bounds on the cosmic ray antiproton flux.

The μ problem and sneutrino inflation

We consider sneutrino inflation and post-inflation cosmology in the singlet extension of the MSSM with approximate Peccei-Quinn(PQ) symmetry, assuming that supersymmetry breaking is mediated by gauge interaction. The PQ symmetry is broken by the intermediate-scale VEVs of two flaton fields, which are determined by the interplay between radiative flaton soft masses and higher order terms. Then, from the flaton VEVs, we obtain the correct mu term and the right-handed(RH) neutrino masses for see-saw mechanism. We show that the RH sneutrino with non-minimal gravity coupling drives inflation, thanks to the same flaton coupling giving rise to the RH neutrino mass. After inflation, extra vector-like states, that are responsible for the radiative breaking of the PQ symmetry, results in thermal inflation with the flaton field, solving the gravitino problem caused by high reheating temperature. Our model predicts the spectral index to be n_s\simeq 0.96 due to the additional efoldings from thermal inflation. We show that a right dark matter abundance comes from the gravitino of 100 keV mass and a successful baryogenesis is possible via Affleck-Dine leptogenesis.

Perturbativity and a fourth generation in the MSSM

We study an extension of the MSSM with a fourth generation of chiral matter. With this extension no value of tanβ allows the theory to stay perturbative up to the GUT scale. We suggest one model with extra vector-like states at the TeV scale that allows perturbativity all the way up to the GUT scale.

Millions of standard models on ℤ6′?

Using previous results on the statistics of intersecting D6-brane models on T 6 /Zwe analyse solutions that resemble the gauge group and matter content of the supersymmetric standard model. In particular the structure of the hidden sector, the gauge coupling constants and chiral exotic matter content are computed and classified for all possible standard model-like configurations on this background. It turns out that the number of chiral exotics, Higgses and values of gauge couplings are strongly correlated. Some examples with the chiral spectrum of the supersymmetric standard model plus vector-like states and a massless U(1)B−L are discussed in detail.

Supersymmetric standard models on D-branes

Type IIB superstring models with the standard model gauge group on D3-branes and with massless matter associated with open strings joining D3-branes to D3-branes or D3-branes to D7 3-branes are studied. Models with gauge coupling constant unification at an intermediate scale between about 10 10 and 10 12 GeV and consistency with the observed value of sin 2 θ W ( M Z ) are obtained. Extra vector-like states and extra pairs of Higgs doublets play a crucial role.