Electroweak Symmetry Breaking Research Articles

Measurement of the Higgs quartic coupling c 2 v from di-Higgs Vector Boson Fusion in the channel

The Brout Englert Higgs (BEH) mechanism of electroweak symmetry breaking and mass generation was experimentally confirmed after the discovery of the Higgs boson at the Large Hadron Collider in 2012. The BEH mechanism not only predicts the existence of a massive scalar particle, but also requires this scalar particle to couple to itself. Double Higgs production provides a unique handle, since it allows the extraction of the trilinear Higgs self-coupling. VBF di-Higgs production also probes the quartic Higgs bosons to vector bosons coupling (c 2 v). In this topic the effort on setting constraints on c 2 v will be discussed. Event selection and reconstruction will be illustrated as well as a Neural Network designed to identify VBF events.

Breakdown of heavy quasiparticles in a honeycomb Kondo lattice: A quantum Monte Carlo study

We show that for the half-filled Kondo lattice model on the honeycomb lattice a Kondo breakdown occurs at small Kondo couplings $J_k$ within the magnetically ordered phase. Our conclusions are based on auxiliary field quantum Monte Carlo simulations of the so-called composite fermion spectral function. Within a U(1) gauge theory formulation of the Kondo model, it becomes apparent that a Higgs mechanism dictates the weight of the resonance in the spectral function. For the honeycomb lattice we observe that for small $J_k$ the quasiparticle pole gives way to incoherent spectral weight but it remains well defined for the square lattice. Our result provides an explicit example where the magnetic transition and the breakdown of heavy quasiparticles are detached as observed in Yb(Rh$_{0.93}$Co$_{0.07}$)$_2$Si$_2$ [Friedemann et al., Nat. Phys. \textbf{5}, 465 (2009)].

Dark photon Dark Matter without Stueckelberg mass

We study the scenario of dark photon Dark Matter where the mass is generated through the Higgs mechanism rather than the constant Stueckelberg mass. In this construction the dark sector contains necessarily extra degrees of freedom and interactions that lead to non-trivial dynamics including thermalization, phase transitions, cosmic string production. As a consequence the predictions of Stueckelberg theories are vastly modified, strongly depending on the couplings to curvature and on the scale of inflation HI compared to the scale f of spontaneous symmetry breaking. We find in particular that only in extreme regions of parameter space the phenomenology of Stueckelberg dark photon is reproduced. These scenarios are strongly constrained by isocurvature perturbations unless the dark sector is approximately Weyl invariant.

Constrained instanton approximation of Skyrmions with massive pions

We present the idea of using the holonomy along a line of a constrained instanton solution (an approximate solution of the Euclidean equations of motion subject to a constraint) of an SU(2) Yang-Mills-Higgs theory to approximate the Skyrmion solution in a chiral model with massive pions. The fact that the gauge field acquires a nonzero mass due to the Higgs mechanism implies that a constrained instanton decays exponentially far from its center, and so does the Skyrmion configuration that it generates via the Atiyah-Manton construction. This is precisely the desired behavior at large distances for the true Skyrmion solutions when the pion mass is included.

Consistent Theory of Kinetic Mixing and the Higgs Low-Energy Theorem.

Extensions of the standard model of particle physics with new Abelian gauge groups allow for kinetic mixing between the new gauge bosons and the hypercharge gauge boson, resulting in mixing with the photon. In many models, the mixing with the hypercharge gauge boson captures only part of the kinetic mixing term with the photon, since the new gauge bosons can also mix with the neutral component of the SU(2)_{L} gauge bosons. We take these contributions into account and present a consistent description of kinetic mixing for general Abelian gauge groups both in the electroweak symmetric and the broken phase. We identify an effective operator that captures the kinetic mixing with SU(2)_{L} and demonstrate how renormalizable contributions arise if the charged fields only obtain their masses from electroweak symmetry breaking. For the first time, a low-energy theorem for the couplings of novel Abelian gauge bosons with the standard model Higgs boson is derived from the one-loop kinetic mixing amplitudes.

How planar superconductors cure their infrared divergences

Planar superconductors, emerging in thin films with thickness comparable to the superconducting coherence length, differ crucially from their bulk counterparts. Coulomb interactions between charges are logarithmic up to distances comparable to typical sample sizes and the Anderson-Higgs mechanism is ineffective to screen the infrared divergences of the resulting (2+1)-dimensional QED because the Pearl length screening the vortex interactions is also typically larger than the sample size. As a result, the system decomposes into superconducting droplets with the typical size of order of superconducting coherence length. We show that two possible phases of the film match the two known mechanisms for curing the (2+1)-dimensional QED infrared divergences, either by generating a mixed topological Chern-Simons mass or by magnetic monopole instantons. The former mechanism works in superconductors, the latter one governs mirror-dual superinsulators. Planar superconductors are thus described by a topological Chern-Simons gauge (TCSG) theory that replaces the Ginzburg-Landau model in two dimensions. In the TCSG model, the Higgs field is absent. Accordingly, in planar superconductors Abrikosov vortices do not form, and only Josephson vortices without normal core do exist.

Application of Machine Learning Algorithms for Searching BSM Higgs Bosons Decaying to a Pair of Bottom Quarks

Although the discovery of the 125 GeV Higgs boson confirms the Higgs mechanism of the Standard Model (SM), many theories beyond the SM have been introduced to address several phenomena yet to be explained by the SM. For instance, the 2-Higgs Doublet Models is the simplest extension of the SM Higgs sector and predicting the existence of additional Higgs bosons at different states. The aim of this study is to search for machine learning (ML) algorithms which have been widely used in High Energy Physics. This will improve the sensitivity of the search for BSM Higgs bosons produced in association with a bottom quark () that then decays into a pair of bottom quarks (); the predominant decay channel of the Higgs boson, though, buried by a large multi-jet background process. In this study, we train 2 different ML algorithms: Tree-based models and Neural Networks, to classify signal and background events collected by the Compact Muon Solenoid detector from proton-proton collisions at 13 TeV. The evaluation metrics are calculated to provide classification efficiencies from different models. The results show that the classification of signal and background processes can be improved using ML techniques. Neural Networks reported the highest AUC score of 0.951 which is comparable with Adaptive Boosting model, while Decision Trees (DTs) and Random Forest models slightly underperformed by 2 - 3 %. We therefore can make use of the trained models as signal vs background classifiers to perform further statistical analysis searches for BSM Higgs bosons. GRAPHICAL ABSTRACT

Vortex String Formation in Black Hole Superradiance of a Dark Photon with the Higgs Mechanism.

Black hole superradiance, which only relies on gravitational interactions, can provide a powerful probe of the existence of ultralight bosons that are weakly coupled to ordinary matter. However, as a boson cloud grows through superradiance, nonlinear effects from interactions with itself or other fields may become important. As a representative example of this, we use nonlinear evolutions to study black hole superradiance of a vector boson that attains a mass, via a coupling to a complex scalar, through the Higgs mechanism. For the cases considered, we find that the superradiant instability can lead to a transient period where the scalar field reaches its symmetry restoration value, leading to the formation of closed vortex strings, the temporary disruption of the exponential growth of the cloud, and an explosive outburst of energy. After the cloud loses sufficient mass, the superradiant growth resumes, and the cycle repeats. Thus, the black hole will be spun down but, potentially, at a much lower rate compared to when nonlinear effects are unimportant and with the liberated energy going primarily into bosonic radiation instead of gravitational waves.

Neutrino seesaw models at one-loop matching: discrimination by effective operators

Using the functional method, one-loop matching of the type-I, -II and -III see-saw models are investigated and the results are presented in both the Green’s and the Warsaw bases. Although these models generate the same dimension-5 Weinberg operator, they could induce quite different types of dimension-6 effective operators that can be utilized for model discrimination. We also find the threshold effects from one-loop matching could be significant, which turn out to allow triggering electroweak symmetry breaking radiatively in type-II seesaw while forbid that in type-I/-III models. An analytical criterion for such radiative symmetry breaking is also derived in type-II seesaw. Finally, we investigate the indirect signatures from different types of dimension-6 operators at high-energy colliders, low-energy precision experiments and forward physics facilities for model discrimination.

Mass Generation and Gravity

Despite the success of the Higgs mechanism to account for the generation of masses of the Standard Model (SM) elementary particles, the ultimate nature and origin of “mass” remain open questions in contemporary physics. From a foundational perspective, mass should be fundamentally related to the gravitational interaction and, according to Mach, with the structure of the Universe. In the present letter, a fully dynamical mass generation mechanism induced by higher-order corrections in the GR Lagrangian is discussed. Notably, the vacuum energy plays a key role in this process, which applies to both vector bosons and fermions. We show that, at the classical level, the Higgs mechanism can be though of as a particular case of the present mechanism.

Multiscalar-Metric Gravity: Cosmological Constant Screening and Emergence of Massive-Graviton Dark Components of the Universe

In the multiscalar-metric frameworks, the issues of the vacuum energy/cosmological constant (CC) screening due to Weyl-scale enhancement of the Diff gauge symmetry, along with emergence of the massive dark gravity components through the gravitational Higgs mechanism are considered. A generic dark gravity model is developed, with two extreme versions of the model of particular interest based on general relativity (GR) and its classically equivalent Weyl transverse alternative, being compared and argued to be, generally, inequivalent. The so constructed spontaneously broken Weyl Transverse Relativity (WTR) is proposed as a viable beyond-GR effective field theory of gravity, with screening of the Lagrangian CC, superseded by the induced one, and emergence of the massive tensor and scalar gravitons as dark gravity components. A basic concept with the spontaneously broken Diff gauge symmetry/relativity—in particular, WTR vs. GR—as a principle source of the emergent dark gravity components of the Universe is put forward.

Gluon condensates and effective gluon mass

Lattice simulations along with studies in continuum QCD indicate that non-perturbative quantum fluctuations lead to an infrared regularisation of the gluon propagator in covariant gauges in the form of an effective mass-like behaviour. In the present work we propose an analytic understanding of this phenomenon in terms of gluon condensation through a dynamical version of the Higgs mechanism, leading to the emergence of color condensates. Within the functional renormalisation group approach we compute the effective potential of covariantly constant field strengths, whose non-trivial minimum is related to the color condensates. In the physical case of an $SU(3)$ gauge group this is an octet condensate. The value of the gluon mass obtained through this procedure compares very well to lattice results and the mass gap arising from alternative dynamical scenarios.

On the role of chirality flips for the muon magnetic moment and its relation to the muon mass

The muon mass and the anomalous magnetic moment aμ are quantities which require chirality flips, i.e., transitions between left- and right-handed muons. Muon chirality flips are connected to electroweak symmetry breaking and Yukawa couplings. Scenarios for physics beyond the Standard Model motivated by the quest to understand electroweak symmetry breaking and/or the origin of flavour often introduce new sources of chirality flips; they hence provide potentially large contributions to aμ, and the current aμ measurement provides relevant constraints on such scenarios. This connection between aμ, chirality flips, and the muon mass generation mechanism is important and underlies much of the current research on aμ. The present article provides a brief pedagogical introduction to this role of chirality flips and an overview of general relationships. The general statements are illustrated with several concrete models involving e.g., leptoquarks and supersymmetry.

The Hasse diagram of the moduli space of instantons

Hasse diagrams (or phase diagrams) for moduli spaces of supersymmetric field theories have been intensively studied in recent years, and many tools to compute them have been developed. The moduli space of instantons, despite being well studied, has proven difficult to deal with. In this note we explore the Hasse diagram of this moduli space from several perspectives — using the partial Higgs mechanism, using brane systems and using quiver subtraction — having to refine previously developed techniques. In particular we introduce the new concept of decorated quiver, which allows to deal with a large class of unitary quivers, including those with adjoint matter.

Radiative effects in the scalar sector of vector leptoquark models

Gauge models with massive vector leptoquarks at the TeV scale provide a successful framework for addressing the B-physics anomalies. Among them, the 4321 model has been considered as the low-energy limit of some complete theories of flavor. In this work, we study the renormalization group evolution of this model, laying particular emphasis on the scalar sector. We find that, despite the asymptotic freedom of the gauge couplings, Landau poles can arise at relatively low scales due to the fast running of quartic couplings. Moreover, we discuss the possibility of radiative electroweak symmetry breaking and characterize the fine-tuning associated with the hierarchy between the electroweak scale and the additional TeV-scale scalars. Finally, the idea of scalar fields unification is explored, motivated by ultraviolet embeddings of the 4321 model.

Natural TeV cutoff of the Higgs field from a multiplicative Lagrangian

The various types of the non-standard Lagrangian can be added to the standard Lagrangian with the invariant of the equation of motion in the low energy limit. In this paper, we construct the multiplicative Lagrangian of a complex scalar field giving the approximated Klein-Gordon equation from the inverse problem of the calculus of variation. Then, this multiplicative Lagrangian with arbitrary high cutoff is applied to the toy model of the Higgs mechanism in U(1)-gauge symmetry in order to study the simple effects in the Higgs physics. We show that, after spontaneous symmetry breaking happens, the Higgs vev is free from the Fermi-coupling constant and the Higgs field gets the natural cutoff in TeV scale. The other relevant coupling constants, the UV-sensitivity of Higgs mass due to the loop correction, some applications on the strong CP problem as well as anomalous small fermion mass, and the cosmological constant problem are also discussed.

Unified explanation of the anomalies in semileptonic B decays and the W mass

The discrepancies between the measurements of rare (semi-)leptonic $B$ decays and the corresponding Standard Model predictions point convincingly towards the existence of new physics for which a heavy neutral gauge boson ($Z^\prime$) is a prime candidate. However, the effect of the mixing of the $Z^\prime$ with the SM $Z$, even though it cannot be avoided by any symmetry, is usually assumed to be small and thus neglected in phenomenological analyses. In this letter we point out that a mixing of the naturally expected size leads to lepton flavour universal contributions, providing a very good fit to $B$ data. Furthermore, the global electroweak fit is affected by $Z-Z^\prime$ mixing where the tension in the $W$ mass, recently confirmed and strengthened by the CDF measurement, prefers a non-zero value of it. We find that a $Z^\prime$ boson with a mass between $\approx 1-5\,\rm {TeV}$ can provide a unified explanations of the $B$ anomalies and the $W$ mass. This strongly suggests that the breaking of the new gauge symmetry giving raise to the $Z^\prime$ boson is linked to electroweak symmetry breaking with intriguing consequences for model building.

Portraying double Higgs at the Large Hadron Collider II

The Higgs potential is vital to understand the electroweak symmetry breaking mechanism, and probing the Higgs self-interaction is arguably one of the most important physics targets at current and upcoming collider experiments. In particular, the triple Higgs coupling may be accessible at the HL-LHC by combining results in multiple channels, which motivates to study all possible decay modes for the double Higgs production. In this paper, we revisit the double Higgs production at the HL-LHC in the final state with two b-tagged jets, two leptons and missing transverse momentum. We focus on the performance of various neural network architectures with different input features: low-level (four momenta), high-level (kinematic variables) and image-based. We find it possible to bring a modest increase in the signal sensitivity over existing results via careful optimization of machine learning algorithms making a full use of novel kinematic variables.

Leptogenesis enabled by dark matter

We propose that weak scale leptogenesis via $\ensuremath{\sim}10\text{ }\text{ }\mathrm{TeV}$ scale right-handed neutrinos could be possible if their couplings had transitory larger values in the early Universe. The requisite lifted parameters can be attained if a light scalar $\ensuremath{\phi}$ is displaced a long distance from its origin by the thermal population of fermions $X$ that become massive before electroweak symmetry breaking. The fermion $X$ can be a viable dark matter candidate; for suitable choice of parameters, the light scalar itself can be dark matter through a misalignment mechanism. We find that a two-component dark matter population made up of both $X$ and $\ensuremath{\phi}$ is a typical outcome in our framework.

Gravitational production of dark photon dark matter with mass generated by the Higgs mechanism

We study the gravitational production of dark photon dark matter during inflation, when dark photons acquire mass by the Higgs mechanism. In the previous study, it was assumed that the dark photon has a Stückelberg mass, or a mass generated by the Higgs mechanism with a sufficiently heavy Higgs boson. In this paper we consider a case in which the Higgs boson is not fully decoupled; the Higgs field changes its vacuum expectation value after inflation. Then, the dark photon mass also changes with time after inflation, and the time evolution of the longitudinal mode is different from the case with a Stückelberg mass. Consequently, the spectrum of the dark photon energy density can have two peaks at an intermediate scale and a small scale. We show that the dark photon can explain the dark matter if its current mass is larger than 6 μeV × (HI /1014 GeV)-4 and smaller than 0.8 GeV × (HI /1014 GeV)-3/2, with HI being the Hubble parameter during inflation. A higher mass is required if one considers a larger gauge coupling constant. The result for the Stückelberg mass can be reproduced in the limit of a small gauge coupling constant. We also comment on the constraints set by various conjectures in quantum gravity theory.