Composite Higgs Scenario Research Articles

High-temperature electroweak baryogenesis with composite Higgs

Electroweak Baryogenesis (EWBG) paired with the Composite Higgs (CH) scenario provides a well-motivated and testable framework for addressing the questions of the origin of the matter-antimatter asymmetry and the naturalness of the electroweak scale. The appeal of both concepts however experiences increasing pressure from the experimental side, as no conclusive signs of the corresponding new physics have been observed. In this note we present a modification of the minimal CH EWBG model, where electroweak symmetry breaking persists to temperatures far above the usually obtained upper bound of ~ 100 GeV. This allows for an increase of the mass of the main actor of EWBG in this scenario — the dilaton. Such a modification results in relaxing the tension with experimental data, generally modifying the phenomenology, and pointing at collider searches for the heavy dilaton as the main direction for its future tests.

Dilaton at the LHC: complementary probe of composite Higgs

The dilaton is predicted in various extensions of the standard model containing sectors with an approximate spontaneously-broken conformal invariance. As a Goldstone boson of a spontaneously broken symmetry, the dilaton can naturally be one of the lightest new physics particles, and therefore may be the first new physics imprint observed in collider experiments. In particular, it can arise in composite Higgs models which are often assumed to have approximate conformal invariance in the UV. The dilaton is then a composite state, generated by the same sector that produces the Higgs. We continue the exploration of composite dilaton signatures at the LHC, using the latest experimental data and analysing the future detection prospects. We elaborate on the connection of the dilaton properties with the properties of the Higgs potential, clarifying in particular the relation between the scale relevant for electroweak fine tuning and the scale controlling the dilaton couplings. This relation is then used to derive the experimental sensitivity to the dilaton in natural composite Higgs scenarios, which reaches ~ 3 TeV in dilaton mass for generic parameter choices. At the same time, we show that dilaton searches are a complementary direction to probe Higgs boson compositeness, with the sensitivity comparable or exceeding that of Higgs coupling measurements.

A second Higgs near 0.5 TeV from bottom-up holographic modeling of beyond the Standard Model strong sector

One of the simplest extensions of the Standard Model (SM) consists in adding a scalar singlet. This second Higgs boson is able to solve several fundamental problems of SM. Additional scalar particles arise naturally in composite Higgs scenarios in which some confining “strong sector” beyond the SM drives the electroweak symmetry breaking. The underlying strongly coupled gauge theory may be similar to QCD and could be modeled holographically. We construct a bottom-up holographic model for description of the spectrum of composite Higgs particles. The model is based on the holographic Soft Wall model and the Wilson confinement criterion. The constructed model predicts the existence of a second Higgs boson with a mass of about 515 GeV.

Following the muon track of hierarchical sectors at LHCb

This paper reports a study of the experimental signatures of hierarchical sectors beyond the Standard Model characterized by a flavour-violating heavy vector and a set of light pseudo-Goldstone bosons a1,2, spanning a large range of lifetimes. The non-minimal scalar spectrum triggers novel B decays into multiple leptons that would have escaped the reach of current searches. Novel displaced vertex analyses at LHCb are therefore discussed to probe the hierarchical new physics, extending the scope of the tracking system of the detector. Additionally, the reach of the proposed CODEX-b experiment is studied. By exploiting the use of tracks only reconstructed in subsystems of the detector at the high-level LHCb trigger, mathcal{B}left({B}_{s}^{0}to {a}_{1}{a}_{2}right)<{10}^{-8} and ℬ(B+ → K+a1a2) < 10−9 could be reached in the muon channel across seven orders of magnitude in the lifetime of the Goldstone bosons. Correspondingly, heavy-light particle couplings of order ≲ 1 could be tested, potentially ruling out composite Higgs scenarios where the heavy and light sectors couple strongly.

Z2 non-restoration and composite Higgs: singlet-assisted baryogenesis w/o topological defects

Simple scalar-singlet extensions of the Standard Model with a (spontaneously broken) Z2 symmetry allow for a strong first order electroweak phase transition, as sought in order to realize electroweak baryogenesis. However they generically also lead to the emergence of phenomenologically problematic domain walls. Here we present a framework with a real scalar singlet that features a different thermal history that avoids this problem by never restoring the Z2 symmetry in the early universe. This is accomplished by considering D > 4 operators that emerge on general grounds, understanding the model as the low energy tail of a more complete theory, like for example in composite Higgs scenarios. Sticking to the latter framework, we present a concrete SO(6)/SO(5) composite realization of the idea. To this end, we additionally provide a complete classification of the structure of the Higgs potential (and the Yukawa couplings) in SO(6)/SO(5) models with fermions in the 1, 6, 15 or 20′ of SO(6).

Search for new physics in the lepton plus missing transverse momentum final state in proton-proton collisions at sqrt{s} = 13 TeV

A search for physics beyond the standard model (SM) in final states with an electron or muon and missing transverse momentum is presented. The analysis uses data from proton-proton collisions at a centre-of-mass energy of 13 TeV, collected with the CMS detector at the LHC in 2016–2018 and corresponding to an integrated luminosity of 138 fb−1. No significant deviation from the SM prediction is observed. Model-independent limits are set on the production cross section of W’ bosons decaying into lepton-plus-neutrino final states. Within the framework of the sequential standard model, with the combined results from the electron and muon decay channels a W’ boson with mass less than 5.7 TeV is excluded at 95% confidence level. Results on a SM precision test, the determination of the oblique electroweak W parameter, are presented using LHC data for the first time. These results together with those from the direct W’ resonance search are used to extend existing constraints on composite Higgs scenarios. This is the first experimental exclusion on compositeness parameters using results from LHC data other than Higgs boson measurements.

On the Impact of the LHC Run 2 Data on General Composite Higgs Scenarios

We study the impact of Run 2 LHC data on general composite Higgs scenarios, where nonlinear effects, mixing with additional scalars, and new fermionic degrees of freedom could simultaneously contribute to the modification of Higgs properties. We obtain new experimental limits on the scale of compositeness, the mixing with singlets and doublets with the Higgs, and the mass and mixing angle of top-partners. We also show that for scenarios where new fermionic degrees of freedom are involved in electroweak symmetry breaking, there is an interesting interplay among Higgs coupling measurements, boosted Higgs properties, SMEFT global analyses, and direct searches for single and double production of vector-like quarks.

Holographic models of composite Higgs in the Veneziano limit. Part I. Bosonic sector

We study strongly-coupled, approximately scale-invariant gauge theories, which develop a mass gap in the infrared. We argue that a large number of fermion flavours is most suitable to provide an ultraviolet completion for the composite Higgs scenario. The holographic approach allows to describe the qualitative features of the non-perturbative dynamics in the Veneziano limit. We introduce new bottom-up holographic models, which incorporate the backreaction of flavour on the geometry, and show that this can correlate the mass gap to the scale of flavour-symmetry breaking. We compute the mass spectrum for the various composite bosonic states, and study its dependence on the scaling dimension of the symmetry-breaking operators, as well as on the number of flavours. The different regions with a light dilaton are critically surveyed. We carefully assess the domain of validity of the holographic approach, and compare it with lattice simulations and the Nambu-Jona-Lasinio model.

Lattice simulations of a gauge theory with mixed adjoint-fundamental matter

In this paper, we summarize our efforts in simulating Yang-Mills theories coupled to matter fields transforming under the fundamental and adjoint representations of the gauge group. In the context of composite Higgs scenarios, gauge theories with mixed representation fields have been suggested to describe the fundamental interactions well beyond the electroweak unification scale, and they are also closely related to supersymmetric QCD. In addition, they are studied as deformations of theories with pure adjoint matter in the context of adiabatic continuity. We provide some first results for bare parameter tuning and interdependence of the two representations. We also investigate how the chiral symmetry breaking or a conformal scenario can be realized and checked in such theories.

Masses of two Higgs doublets within effective theory with four-quark interactions

The composite Higgs scenario in the context of the effective model with SU(2) L × U(1) R symmetric four-fermion interaction proposed by Miransky, Tanabashi and Yamawaki (MTY) is considered. The low-energy dynamics of the model is described by the effective Lagrangian obtained by the Schwinger-DeWitt technique. The questions of the Nambu sum rule and the spectrum of Higgs states are addressed in detail.

Top condensation model: a step towards the correct prediction of the Higgs mass

A realization of the composite Higgs scenario in the context of the effective model with the SU(2)_Ltimes U(1)_R symmetric four-Fermi interactions proposed by Miransky, Tanabashi and Yamawaki is studied. The model implements Nambu’s mechanism of dynamical electroweak symmetry breaking leading to the formation of {{bar{t}}}t and {{bar{b}}}b quark condensates. We explore the vacuum structure and spectrum of the model by using the Schwinger proper-time method. As a direct consequence of this mechanism, the Higgs acquires a mass in accord with its experimental value. The present prediction essentially differs from the known overestimated value, m_chi = 2m_t, making more favourable the top condensation scenario presented here. The mass formulas for the members of the second Higgs doublet are also obtained. The Nambu sum rule is discussed. It is shown that the anomalous U(1)_A symmetry breaking modifies this rule at next to leading order in 1/N_c.

Automated predictions from polarized matrix elements

The anticipated experimental resolution and data cache of the High Luminosity Large Hadron Collider will enable precision investigations of polarization in multiboson processes. This includes, for the first time, vector boson scattering. To facilitate such studies, we report the automation of polarized matrix element computations in the publicly available Monte Carlo tool suite, MadGraph5_aMC@NLO. This enables scattering and decay simulations involving helicity-polarized asymptotic or intermediate states, preserving both spin-correlation and off-shell effects. As demonstrations of the method, we investigate the leading order production and decay of polarized weak gauge bosons in the process ppto {jjW}_{lambda}^{+}{W}_{lambda prime}^{-} , with helicity eigenstates (λ, λ′) defined in various reference frames. We consider the Standard Model at both mathcal{O} (α4) and mathcal{O} (α2 {alpha}_s^2 ) as well as a benchmark composite Higgs scenario. We report good agreement with polarization studies based on the On-Shell Projection (OSP) technique. Future capabilities are discussed.

Composite Higgs at high transverse momentum

In this paper we explore composite Higgs scenarios through the effects of light top-partners in Higgs+Jet production at the LHC. The pseudo-Goldstone boson nature of the Higgs field means that single-Higgs production via gluon fusion is insensitive to the mass spectrum of the top-partners. However in associated production this is not the case, and new physics scales may be probed. In the course of the work we consider scenarios with both one and two light top-partner multiplets in the spectrum of composite states. In compliance with perturbativity and experimental constraints, we study corrections to the Higgs couplings and the effects that the light top-partner multiplets have on the transverse momentum spectrum of the Higgs. Interestingly, we find that the corrections to the Standard Model expectation depend strongly on the representation of the top-partners in the global symmetry.

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.

Relaxation of the composite Higgs little hierarchy

We describe a composite Higgs scenario in which a cosmological relaxation mechanism naturally gives rise to a hierarchy between the weak scale and the scale of spontaneous global symmetry breaking. This is achieved through the scanning of sources of explicit global symmetry breaking by a relaxion field during an exponentially long period of inflation in the early universe. We explore this mechanism in detail in a specific composite Higgs scenario with QCD-like dynamics, based on an ultraviolet SU(N )TC ‘technicolor’ confining gauge theory with three Dirac technifermion flavors. We find that we can successfully generate a hierarchy of scales ξ≡〈h〉2/Fπ2 ≳ 1.2 × 10− 4 (i.e., compositeness scales Fπ ∼ 20 TeV) without tuning. This evades all current electroweak precision bounds on our (custodial violating) model. While directly observing the heavy composite states in this model will be challenging, a future electroweak precision measurement program can probe most of the natural parameter space for the model. We also highlight signatures of more general composite Higgs models in the cosmological relaxation framework, including some implications for flavor and dark matter.

A UV complete compositeness scenario: LHC constraints meet the lattice

We investigate a recently proposed UV-complete composite Higgs scenario in the light of the first LHC runs. The model is based on a SU(4) gauge group with global flavour symmetry breaking SU(5) → SO(5), giving rise to pseudo Nambu-Goldstone bosons in addition to the Higgs doublet. This includes a real and a complex electroweak triplet with exotic electric charges. Including these, as well as constraints on other exotic states, we show that LHC measurements are not yet sensitive enough to significantly constrain the model’s low energy constants. The Higgs potential is described by two parameters which are on the one hand constrained by the LHC measurement of the Higgs mass and Higgs decay channels and on the other hand can be computed from correlation functions in the UV-complete theory. Hence to exclude the model at least one constant needs to be determined and to validate the Higgs potential both constants need to be reproduced by the UV-theory. Due to its UV-formulation, a certain number of low energy constants can be computed from first principle numerical simulations of the theory formulated on a lattice, which can help in establishing the validity of this model. We assess the potential impact of lattice calculations for phenomenological studies, as a preliminary step towards Monte Carlo simulations.

Same-sign W pair production in composite Higgs models

Non-minimal composite Higgs scenarios can contain exotic Higgs states which, if getting observed at the Large Hadron Collider, will help to constrain the underlying UV structure of the strong dynamics. Doubly charged Higgs bosons are well-motivated scalar degrees of freedom in this context. Their phenomenology in typical composite scenarios can differ from well-established Higgs triplet extensions of the SM. Related search strategies are not necessarily adapted to such a scenario as a consequence. In this paper we discuss the sensitivity reach to doubly charged Higgs bosons with decays into pairs of same-sign $W$ bosons. While production cross sections are small, we show that significant constraints on $H^{\pm\pm}\to W^{\pm}W^{\pm}$ can be obtained, providing a new opportunity to constrain the potential composite structure of the TeV scale up to $m_{H^{\pm\pm}}\simeq 800$~GeV.

LHC vector resonance searches in the toverline{t}Z final state

LHC searches for BSM resonances in l+l−, jj, toverline{t} , γγ and VV final states have so far not resulted in discovery of new physics. Current results set lower limits on mass scales of new physics resonances well into the mathcal{O}(1) TeV range, assuming that the new resonance decays dominantly to a pair of Standard Model particles. While the SM pair searches are a vital probe of possible new physics, it is important to re-examine the scope of new physics scenarios probed with such final states. Scenarios where new resonances decay dominantly to final states other than SM pairs, even though well theoretically motivated, lie beyond the scope of SM pair searches. In this paper we argue that LHC searches for (vector) resonances beyond two particle final states would be useful complementary probes of new physics scenarios. As an example, we consider a class of composite Higgs models, and identify specific model parameter points where the color singlet, electrically neutral vector resonance ρ0 decays dominantly not to a pair of SM particles, but to a fermionic top partner Tf1 and a top quark, with Tf1 → tZ. We show that dominant decays of ρ0 → Tf1t in the context of Composite Higgs models are possible even when the decay channel to a pair of Tf1 is kinematically open. Our analysis deals with scenarios where both mρ and {m}_T{{}_{{}_f}}_{{}_1} are of mathcal{O}(1) TeV, leading to highly boosted toverline{t}Z final state topologies. We show that the particular composite Higgs scenario we consider is discoverable at the LHC13 with as little as 30 fb−1, while being allowed by other existing experimental constraints.

Model building with non-compact cosets

We explore Goldstone boson potentials in non-compact cosets of the form SO(n,1)/SO(n). We employ a geometric approach to find the scalar potential, and focus on the conditions under which it is compact in the large field limit. We show that such a potential is found for a specific misalignment of the vacuum. This result has applications in different contexts, such as in Composite Higgs scenarios and theories for the Early Universe. We work out an example of inflation based on a non-compact coset which makes predictions which are consistent with the current observational data.

Top partners searches and composite Higgs models

Colored fermionic partners of the top quark are well-known signatures of the Composite Higgs scenario and for this reason they have been and will be subject of an intensive experimental study at the LHC. Performing an assessment of the theoretical implications of this experimental effort is the goal of the present paper. We proceed by analyzing a set of simple benchmark models, characterized by simple two-dimensional parameter spaces where the results of the searches are conveniently visualized and their impact quantified. We only draw exclusion contours, in the hypothesis of no signal, but of course our formalism could equally well be used to report discoveries in a theoretically useful format.