Electroweak Symmetry Breaking Research Articles

Electroweak breaking and Higgs boson profile in the simplest linear seesaw model

We examine the simplest realization of the linear seesaw mechanism within the Standard Model gauge structure. Besides the standard scalar doublet, there are two lepton-number-carrying scalars, a nearly inert SU(2)L doublet and a singlet. Neutrino masses result from the spontaneous violation of lepton number, implying the existence of a Nambu-Goldstone boson. Such “majoron” would be copiously produced in stars, leading to stringent astrophysical constraints. We study the profile of the Higgs bosons in this model, including their effective couplings to the vector bosons and their invisible decay branching ratios. A consistent electroweak symmetry breaking pattern emerges with a compressed spectrum of scalars in which the “Standard Model” Higgs boson can have a sizeable invisible decay into the invisible majorons.

ν’s in Particle Physics and Cosmology

We propose a minimal scale-invariant (SI) scotogenic model and study its phenomenological implications. The SI scotogenic predicts a singlet scalar (dilaton) that triggers electroweak symmetry breaking and sources the lepton number violation that allows radiative neutrino mass. A viable parameter space is found for dark matter candidate and the observed neutrino oscillation. The model can be probed in lepton flavor violating processes, future dark matter direct detection experiments, and collider searches.

Gauge-invariant microscopic kinetic theory of superconductivity: Application to the optical response of Nambu-Goldstone and Higgs modes

We show that the gauge-invariant kinetic equation of superconductivity provides an efficient approach to study the electromagnetic response of the gapless Nambu-Goldstone and gapful Higgs modes on an equal footing. We prove that the Fock energy in the kinetic equation is equivalent to the generalized Ward's identity. Hence, the gauge invariance directly leads to the charge conservation. Both linear and second-order responses are investigated. The linear response of the Higgs mode vanishes in the long-wave limit. Whereas the linear response of the Nambu-Goldstone mode interacts with the long-range Coulomb interaction, causing the original gapless spectrum lifted up to the plasma frequency as a result of the Anderson-Higgs mechanism, in consistency with the previous works. The second-order response exhibits interesting physics. On one hand, a finite second-order response of the Higgs mode is obtained in the long-wave limit. We reveal that this response, which has been experimentally observed, is attributed solely to the drive effect rather than the widely considered Anderson-pump effect. On the other hand, the second-order response of the Nambu-Goldstone mode, free from the influence of the long-range Coulomb interaction and hence the Anderson-Higgs mechanism, is predicted. We find that both Anderson-pump and drive effects play important role in this response. A tentative scheme to detect this second-order response is proposed.

Low scale leptogenesis in a hybrid model of the scotogenic type I and III seesaw models

The scotogenic type I and type III seesaw models are good candidates to explain the existence of neutrino masses and dark matter simultaneously. However, since triplet fermions have SU(2) gauge interaction, they cannot be out of equilibrium before the electroweak symmetry breaking. Thus, leptogenesis seems to be difficult within a framework of the pure type III seesaw model. Some extension seems to be required to solve this fault. A model extended by introducing a singlet fermion could be such a simple example. If the singlet fermion is in the thermal equilibrium even for its extremely small neutrino Yukawa coupling, leptogenesis could be shown to occur successfully for a rather low mass of the singlet fermion. The required mass could be lowered to $10^4$~GeV.

Mass Gap in Weakly Coupled Abelian Higgs on a Unit Lattice

The proof of Higgs mechanism in a weakly coupled lattice gauge theory in $d \geqslant 2$ is revisited. A new power series cluster expansion is applied and the mass gap is shown to exist for the observable $F_{\mu\nu}$.

Dark matter and LHC phenomenology of a scale-invariant scotogenic model * * Supported in part by The National Key Research and Development Program of China (2017YFA0402200), the Grants (NSFC-11575089, NSFC-11025525) and the CAS Center for Excellence in Particle Physics (CCEPP)

We study the phenomenology of a model that addresses the neutrino mass, dark matter, and generation of the electroweak scale in a single framework. Electroweak symmetry breaking is realized via the Coleman-Weinberg mechanism in a classically scale invariant theory, while the neutrino mass is generated radiatively through interactions with dark matter in a typically scotogenic manner. The model introduces a scalar triplet and singlet and a vector-like fermion doublet that carry an odd parity of , and an even parity scalar singlet that helps preserve classical scale invariance. We sample over the parameter space by taking into account various experimental constraints from the dark matter relic density and direct detection, direct scalar searches, neutrino mass, and charged lepton flavor violating decays. We then examine by detailed simulations possible signatures at the LHC to find some benchmark points of the free parameters. We find that the future high-luminosity LHC will have a significant potential in detecting new physics signals in the dilepton channel.

Effective theories of dark mesons with custodial symmetry

Dark mesons are bosonic composites of a new, strongly-coupled sector beyond the Standard Model. We consider several dark sectors with fermions that transform under the electroweak group, as arise from a variety of models including strongly-coupled theories of dark matter (e.g., stealth dark matter), bosonic technicolor (strongly-coupled indcued electroweak symmetry breaking), vector-like confinement, etc. We consider theories with two and four flavors under an SU(N) strong group that acquire variously chiral, vector-like, and mixed contributions to their masses. We construct the non-linear sigma model describing the dark pions and match the ultraviolet theory onto a low energy effective theory that provides the leading interactions of the lightest dark pions with the Standard Model. We uncover two distinct classes of effective theories that are distinguishable by how the lightest dark pions decay: “Gaugephilic”: where π0 → Zh, π± → W h dominate once kinematically open, and “Gaugephobic”: where {pi}^0to overline{f}f , {pi}^{pm}to overline{f}^{prime }f dominate. Custodial SU(2) plays a critical role in determining the “philic” or “phobic” nature of a model. In dark sectors that preserve custodial SU(2), there is no axial anomaly, and so the decay π0 → γγ is highly suppressed. In a companion paper, we study dark pion production and decay at colliders, obtaining the constraints and sensitsivity at the LHC.

Fermion mass hierarchy in grand gauge-Higgs unification

Abstract Grand gauge-Higgs unification of 5D $SU(6)$ gauge theory on an orbifold $S^1/Z_2$ is discussed. The Standard Model (SM) fermions are introduced on one of the boundaries and some massive bulk fields are also introduced so that they couple to the SM fermions through the mass terms on the boundary. Integrating out the bulk fields generates SM fermion masses with exponentially small bulk mass dependences. The SM fermion masses except for the top quark are shown to be reproduced by mild tuning of the bulk masses. The one-loop Higgs potential is calculated and it is shown that electroweak symmetry breaking occurs by introducing additional bulk fields. The Higgs boson mass is also computed.

Scalar dark matter in the conformally invariant extension of the standard model

We study a classically scale-invariant model with an electroweak singlet scalar mediator together with a scalar dark matter multiplet of global O(N) symmetry. Our most general conformally invariant scalar potential generates the electroweak symmetry breaking via the Coleman-Weinberg mechanism, and the new scalar singlet acquires its mass through radiative corrections of the SM particles and the scalar dark matter. Taking into account the collider bounds, we present the allowed region of new physics parameters satisfying the recent measurement of relic abundance. With the obtained parameter sets, we predict the elastic scattering cross section of the new scalar multiplet into target nuclei for a direct detection of the dark matter. We also perform a full analysis with arbitrary set of parameters for N≥2, and discuss the implication of the constraints from the on-going direct and indirect detections of dark matter.

Probing unitarity violation in the tail of the off-shell Higgs boson in VLVL mode

Off-shell Higgs at the high mass tail may shed light on the underlying mechanism of the electroweak symmetry breaking. Due to the large cancellation in the standard model (SM) between the box and Higgs-mediated triangle diagrams, the $gg\to WW(ZZ)$ process in the SM is dominated by the $V_T V_T$ transverse-mode at the high mass tail. The cancellation does not necessarily hold, when there is a sufficiently large new physics contribution resulting in $V_LV_L$ longitudinal mode, which is commonly the case when the Higgs sector is modified. Thus the $V_LV_L$ final states in the high mass tail can be utilized as a sensitive probe for such models. In the paper we focus on a study of the $gg \to ZZ$ process in the fully leptonic decay modes, proposing to utilize the polarization modes of the off-shell Higgs to probe new physics, whose contribution mainly shows in the longitudinal mode. As examples, we analyze three different Higgs sector new physics cases (Higgs portal with a light scalar, a broad-width scalar that mixes with the Higgs, and quantum critical Higgs models), and demonstrate that the angular information relating to the polarization serves as very sensitive probe for such new physics.

Cosmology from multimeasure multifield model

We consider the cosmological application of a (variant of) relatively newly proposed model1 unifying inflation, dark energy, dark matter, and the Higgs mechanism. The model was originally defined using additional non-Riemannian measures, but it can be reformulated into effective quintessential model unifying inflation, dark energy and dark matter. Here, we demonstrate numerically that it is capable of describing the entire evolution of the Universe in a seamless way, but this requires some revision of the model setup. The main reason is that there is a strong effective friction in the model, a feature which has been neglected in the pioneer work. This improves the model potential for proper description of the evolution of the Universe, because the friction ensures a finite time inflation with dynamically maintained low-value slow-roll parameters in the realistic scenarios. In addition, the model predicts the existence of a constant scalar field in late Universe.

The Dynamical Origin of the Graviton Mass in the Non-Linear Theory of Massive Gravity

We compare the standard Higgs mechanism corresponding to the scalar field, with the dynamical origin of the graviton mass inside the scenario of the dRGT theory of massive gravity. We demonstrate that the effective mass perceived locally by different observers depends on how they define the local time with respect to the preferred notion of time defined by the Stückelberg function T 0 ( r , t ) .

Dissection of an SO(5)×U(1) gauge-Higgs unification model

We analyze models of electroweak symmetry breaking in warped 5-dimensional space with gauge bosons and fermions in the bulk. The Higgs boson is identified with the 5th component of a gauge field. We dynamically generate the Higgs potential using a competition between the top quark multiplet and another fermion multiplet to create a little hierarchy characterized by a small parameter s = v/f. Using a Green's function method, we compute the properties of the model systematically as a power series in s. We discuss the constraints on this model from the measured value of the Higgs mass, the masses of top quark partners, and precision electroweak observables.

Higgs and BSM Studies at the LHC

The discovery and study of the Higgs boson at the Large Hadron Collider of CERN has proven the validity of the Brout–Englert–Higgs mechanism of mass creation in the standard model via spontaneous symmetry breaking. The new results obtained by the ATLAS and CMS Collaborations at the LHC show that all measured cross-sections agree within uncertainties with the predictions of the theory. However, the standard model has obvious difficulties (nonzero neutrino masses, hierarchy problem, existence of dark matter, non-existence of antimatter galaxies, etc.), which point towards more possible violated symmetries. We first summarize the present status of the studies of the Higgs boson, including the latest results at 13 TeV p-p collision energy, then enlist some of the problems with possible solutions and the experimental situation regarding them.

Photon masses in the landscape and the swampland

In effective quantum field theory, a spin-1 vector boson can have a technically natural small mass that does not originate from the Higgs mechanism. For such theories, which may be written in Stückelberg form, there is no point in field space at which the mass is exactly zero. I argue that quantum gravity differs from, and constrains, effective field theory: arbitrarily small Stückelberg masses are forbidden. In particular, the limit in which the mass goes to zero lies at infinite distance in field space, and this distance is correlated with a tower of modes becoming light according to the Swampland Distance Conjecture. Application of Tower or Sublattice variants of the Weak Gravity Conjecture makes this statement more precise: for a spin-1 vector boson with coupling constant e and Stückelberg mass m, local quantum field theory breaks down at energies at or below ΛUV = min((mMPl/e)1/2, e1/3MPl). Combined with phenomenological constraints, this argument implies that the Standard Model photon must be exactly massless. It also implies that much of the parameter space for light dark photons, which are the target of many experimental searches, is compatible only with Higgs and not Stückelberg mass terms. This significantly affects the experimental limits and cosmological histories of such theories. I explain various caveats and weak points of the arguments, including loopholes that could be targets for model-building.

Same-sign WW scattering in the HEFT: discoverability vs. EFT validity

Vector boson scatterings are fundamental processes to shed light on the nature of the electroweak symmetry breaking mechanism. Deviations from the Standard Model predictions on the corresponding observables can be interpreted in terms of effective field theories, that however undergo consistency conditions. In this paper, the same-sign WW scattering is considered within the HEFT context and the correct usage of the effective field theory approach is discussed. Regions of the parameters space are identified where a signal of new physics could be measured at HL-LHC with a significance of more than 5σ and the effective field theory description is consistently adopted. These results are then translated into bounds on the ξ parameter in the composite Higgs scenario. The discussion on the agreement with previous literature and the comparison with the equivalent analysis in the SMEFT case are also included.

Phase transition and vacuum stability in the classically conformal B\u2013L model

Within classically conformal models, the spontaneous breaking of scale invariance is usually associated to a strong first order phase transition that results in a gravitational wave background within the reach of future space-based interferometers. In this paper we study the case of the classically conformal gauged B–L model, analysing the impact of this minimal extension of the Standard Model on the dynamics of the electroweak symmetry breaking and derive its gravitational wave signature. Particular attention is paid to the problem of vacuum stability and to the role of the QCD phase transition, which we prove responsible for concluding the symmetry breaking transition in part of the considered parameter space. Finally, we calculate the gravitational wave signal emitted in the process, finding that a large part of the parameter space of the model can be probed by LISA.

The role of singletons in S7 compactifications

We derive the isometry irrep content of squashed seven-sphere compactifications of eleven-dimensional supergravity, i.e., the left-squashed (LS7) with mathcal{N} = 1 and right-squashed (RS7) with mathcal{N} = 0 supersymmetry, in a manner completely independent of the round sphere. Then we compare this result with the spectrum obtained by Higgsing the round sphere spectrum. This way we discover features of the spectra which makes it possible to argue that the only way the round spectrum can be related by a Higgs mechanism to the one of LS7 is if the singletons are included in the round sphere spectrum. For this to work also in the RS7 case it seems that the gravitino of the LS7 spectrum must be replaced by a fermionic singleton present in the RS7 spectrum.

Measuring Higgs self-couplings in the presence of V V H and V V HH at the ILC

The recent discovery of a Higgs boson at LHC, while establishing the Higgs mechanism as the way of electroweak symmetry breaking, started an era of precision measurements involving the Higgs boson. In an effective Lagrangian framework, we consider the [Formula: see text] process, at an ILC running at a center-of-mass energy of 500 GeV to investigate the effect of the [Formula: see text] and [Formula: see text] couplings on the sensitivity of [Formula: see text] coupling on this process. Our results show that the sensitivity of the trilinear Higgs self-couplings on this process has somewhat strong dependence on the Higgs-gauge boson couplings. Single- and two-parameter reaches of ILC with integrated luminosity of 1000 fb[Formula: see text] are obtained on all the effective couplings indicating how these limits are affected by the presence of anomalous [Formula: see text] and [Formula: see text] couplings. The kinematic distributions studied to understand the effect of the anomalous couplings, again, show strong influence of [Formula: see text]–[Formula: see text] couplings on the dependence of these distributions on [Formula: see text] coupling. Similar results are indicated in the case of the process, [Formula: see text], considered at a center-of-mass energy of 2 TeV, where the cross-section is large enough. The effect of [Formula: see text] and [Formula: see text] couplings on the sensitivity of [Formula: see text] coupling is clearly established through our analyses in this process.

Leibniz-Yang-Mills gauge theories and the 2-Higgs mechanism

A quadratic Leibniz algebra $(\mathbb{V},[\ifmmode\cdot\else\textperiodcentered\fi{},\ifmmode\cdot\else\textperiodcentered\fi{}],\ensuremath{\kappa})$ gives rise to a canonical Yang-Mills type functional $S$ over every space-time manifold. The gauge fields consist of 1-forms $A$ taking values in $\mathbb{V}$ and 2-forms $B$ with values in the subspace $\mathbb{W}\ensuremath{\subset}\mathbb{V}$ generated by the symmetric part of the bracket. If the Leibniz bracket is antisymmetric, the quadratic Leibniz algebra reduces to a quadratic Lie algebra, $B\ensuremath{\equiv}0$, and $S$ becomes identical to the usual Yang-Mills action functional. We describe this gauge theory for a general quadratic Leibniz algebra. We then prove its (classical and quantum) equivalence to a Yang-Mills theory for the Lie algebra $\mathfrak{g}=\mathbb{V}/\mathbb{W}$ to which one couples massive 2-form fields living in a $\mathfrak{g}$-representation. Since in the original formulation the $B$-fields have their own gauge symmetry, this equivalence can be used as an elegant mass-generating mechanism for 2-form gauge fields, thus providing a ``higher Higgs mechanism'' for those fields.