The Higgs particle is a new elementary particle predicted in the Standard Model of the elementary particle physics. It plays a special role in the theory of mass generation of quarks, leptons, and gauge bosons. In this article, theoretical issues on the Higgs mechanism are first discussed, and then experimental prospects on the Higgs particle study at the future collider experiments, LHC and ILC, are reviewed. The Higgs coupling determination is an essential step to establish the mass generation mechanism, which could lead to a deeper understanding of particle physics.
I summarize the prospects for discovering and studying the properties of Higgs particles at future high-energy and high-luminosity $e^+e^-$ linear colliders. I will focus on the Higgs particle of the Standard Model and the Higgs bosons predicted by Supersymmetric theories.
The theoretical status of Higgs boson and supersymmetric particle production at hadron colliders is reviewed with particular emphasis on recent results and open problems.
Non-decoupling properties of additional heavy degrees of freedom in the Higgs sector of the Two-Higgs-doublet extension of the Standard model are discussed in a particular case of production of a pair of longitudinaly polarized W-bosons in the e^+e^- annihilation.
We consider beyond the standard model embedding of inert Higgs doublet fields. We argue that inert Higgs doublets can arise naturally in grand unified theories where the necessary associated $Z_2$ symmetry can occur automatically. Several examples are discussed.
Pair production processes of neutral Higgs particles will allow us to study the trilinear Higgs couplings at future high--energy colliders. Several mechanisms give rise to multi--Higgs final states in hadron interactions. In the present paper we investigate Higgs pair production in gluon--gluon collisions. After recapitulating pair production in the Standard Model, the analysis of the cross sections is carried out in detail for the neutral Higgs particles in the minimal supersymmetric extension.
The Higgs particle can decay dominantly into an invisible channel in the Majoron models. We have explored the prospect of detecting such a Higgs particle at LHC via its associated production with a gluon, Z or W boson. While the signal/background ratio is too small for the first process, the latter two provide viable signatures for detecting such a Higgs particle.
I briefly review the Higgs sector in the Standard Model and its minimal Supersymmetric extension, the MSSM. After summarizing the properties of the Higgs bosons and the present experimental constraints, I will discuss the prospects for discovering these particle at the upgraded Tevatron, the LHC and a high-energy $e^+e^-$ linear collider. The possibility of studying the properties of the Higgs particles will be then summarized.
We discuss the spectrum of Higgs bosons in the framework of the exceptional supersymmetric standard model. The presence of a $Z'$ and exotic particles predicted by the exceptional SUSY model allows the lightest Higgs particle to be significantly heavier than in the MSSM and NMSSM. When the mass of the lightest Higgs boson is larger than $135-140 {GeV}$ the heaviest scalar, pseudoscalar and charged Higgs states lie beyond the ${TeV}$ range.
In this write-up I summarize the main points of our recent work, in which we showed that the large flavor QCD is regarded as a dynamical origin of little Higgs, and that the mass of such a little Higgs becomes actually very small.
We consider Higgs-Higgs scattering in a minimal Higgs model with strong coupling which leads to the Higgs as a bound state. The partial wave amplitude is unitarized using general principles of analyticity. We find lambda(m_H)/16pi is approximately 1/2. A Higgs mass of approximately 1 TeV follows from m_H = sqrt{lambda/2} * v (spontaneous symmetry breaking).
A modification of the standard model of electroweak interactions with the nonlocal Higgs sector is proposed. Proper form of nonlocality makes Higgs particles unobservable after the electroweak symmetry breaking. They appear only as a virtual state because their propagator is an entire function. We discuss some specific consequences of this approach comparing it with the conventional standard model.
The Higgs particle can have dominantly invisible decay in a large class of Majoron models as well as some SUSY models. The LEP signal and mass limit for a Higgs particle undergoing invisible decay are explored. They are found to be very similar to those for the standard model decay. The best signatures and discovery limit for an invisibly decaying Higgs particle at the LHC are also discussed. (Invited Talk at the XXIXth Rencontres de Moriond on QCD and High Energy Hadronic Interactions, Méribel, France, 19--26 March 1994.)
Two--photon decays of Higgs bosons are important channels for the search of these particles in the intermediate mass range at the $pp$ colliders LHC and SSC. Dynamical aspects of the Higgs coupling to two photons can also be studied by means of the $γγ$ fusion of Higgs particles at high--energy e$^+$e$^-$ linear colliders. Extending earlier analyses which had been restricted to the Standard Model, we present in this note the QCD radiative corrections to the $γγ$ decay widths of scalar and pseudoscalar Higgs particles in multi--doublet extensions of the Higgs sector, as realized for instance in…
Among the possible decay modes of Higgs particles into supersymmetric states, neutralino and chargino decays play a prominent rôle. The experimental opportunities of observing such decay modes at LEP2 and at future e+e- linear colliders are analyzed within the frame of the Minimal Supersymmetric extension of the Standard Model. For heavy Higgs particles, the chargino/neutralino decay modes can be very important, while only a small window is open for the lightest CP-even Higgs particle. If charginos/neutralinos are found at LEP2, such decay modes can be searched for in a small area of the param…
We show how to systematically analyze what may be inferred should a new scalar particle be discovered in collider experiments. Our approach is systematic in the sense that we perform the analysis in a manner which minimizes apriori theoretical assumptions as to the nature of the scalar particle. For instance, we do not immediately make the common assumption that a new scalar particle is a Higgs boson, and so must interact with a strength proportional to the mass of the particles with which it couples. We show how to compare different observables, and so to develop a decision tree from which th…
The recently reported observation of a new particle with mass about 125 GeV and couplings generally resembling those of the Standard Model Higgs boson provides a potential probe of the physics of electroweak symmetry breaking. Although the current data only provides hints, we suggest a particular combination of Higgs couplings as an assay for new charged particles connected with electroweak symmetry breaking, and construct a simple model with charge 5/3 quarks as a demonstration of its use.
Many new physics models contain new particles that interact with the Higgs boson. These particles could be produced at the LHC via gluon-gluon fusion with an off-shell Higgs, as well as via the Drell-Yan process if charged under a gauge group. We consider in this paper simplified scenarios where the Standard Model is extended by one scalar or fermionic field that interacts with the Higgs boson and we evaluate the impact of the Higgs interaction on the production of the exotic particles at the LHC. This analysis applies in particular to TeV scale seesaw scenarios of neutrino mass generation.
It is shown that the effective Standard Model theory with the physical Higgs mass lighter than $\sim 200$ GeV takes the form of an unbroken electroweak theory already at moderately high, $O(1 TeV)$, energy scales. No such transitional scale exists for the heavier Higgses. This result is independently obtained by two standard regularization methods.
The prospects for discovering Higgs particles and studying their fundamental properties at future high-energy electron-positron and hadron colliders are reviewed. Both the Standard Model Higgs boson and the Higgs particles of its minimal supersymmetric extension are discussed. We update various results by taking into account the new value of the top quark mass obtained by the CDF Collaboration and by including radiative corrections some of which have been calculated only recently.
We analyze the prospects of measuring the parity of Higgs particles in the Standard Model and its supersymmetric extensions. Higgs decays are discussed in this context as well as production processes including, in particular, the fusion of Higgs particles in linearly polarized photon--photon collisions.
If the Einstein field equations are employed at the tree level, then the decay of the standard model Higgs particle into two gravitons is shown to be independent of the gravitational coupling strength G. The result follows from the physical equivalence between the Higgs induced ``inertial mass'' and the ``gravitational mass'' of general relativity. If the Higgs mass lies well between the mass of a bottom quark anti-quark pair and the mass of a top quark anti-quark pair, then the Higgs decay into two gravitons will dominate both the QED induced two photon decay and the QCD induced two jet decay…
In this paper, applying general gauge field theory, we will construct an electroweak model. In this new electroweak model, Higgs mechanism is not used, so no Higgs particle exists in the model. In order to keep the masses of intermediate gauge bosons non-zero, we will introduce two sets of gauge fields. We need a vacuum potential to introduce symmetry breaking and to introduce the masses of all fields. Except for theose terms concern of Higgs particle, the fundamental dynamical properties of this model are similar to those of the standard model. And in a proper limit, this model will approxima…
We initiate the study of exotic Higgs decays to long-lived particles (LLPs) at proposed future lepton colliders, focusing on scenarios with displaced hadronic final states. Our analysis entails a realistic tracker-based search strategy involving the reconstruction of displaced secondary vertices and the imposition of selection cuts appropriate for eliminating the largest irreducible backgrounds. The projected sensitivity is broadly competitive with that of the LHC and potentially superior at lower LLP masses. In addition to forecasting branching ratio limits, which may be freely interpreted in…
We generalize the basis of CP-even chiral effective operators describing a dynamical Higgs sector, to the case in which the Higgs-like particle is light. Gauge and gauge-Higgs operators are considered up to mass dimension five. This analysis completes the tool needed to explore at leading order the connection between linear realizations of the electroweak symmetry breaking mechanism - whose extreme case is the Standard Model - and non-linear realizations with a light Higgs-like particle present. It may also provide a model-independent guideline to explore which exotic gauge-Higgs couplings may…
An overview of hints for new physics outside the Standard Model and the status of sparticle and Higgs searches is given. The present limits on Higgs bosons of about 90 GeV as well as the $\bsg$ rate and cosmological constraints severely restrict the available parameter space of the MSSM.
LHC has found hints for a Higgs particle of 125 GeV. We investigate the possibility that such a particle is a mixture of scalar and pseudoscalar states. For definiteness, we concentrate on a two Higgs doublet model with explicit CP violation and soft Z2 violation. Including all Higgs production mechanisms, we determine the current constraints obtained by comparing h -> gamma gamma with h -> V V*, and comment on the information which can be gained by measurements of h -> b \bar{b}. We find bounds |s_2| < 0.83 at one sigma, where |s_2|=0 (|s_2|=1) corresponds to a pure scalar (pure pseudoscalar)…
In spite of the great success of LHC experiments, we do not know whether the discovered "standard model-like" Higgs particle is really what the standard model predicts or a particle some new physics has in its low energy effective theory. Also the long-standing problems concerning the property of Higgs and its interactions are still there, and we still do not have any conclusive argument of the origin of the Higgs itself. In this article we focus on higher-dimensional theories as new physics. First we give a brief review of their representative scenarios and closely related 4-dimensional scena…
In the Standard Model, all massive elementary particles acquire their masses by coupling to a background Higgs field with a non-zero vacuum expectation value. What is often overlooked is that each massive particle is also a source of the Higgs field. A given particle can in principle shift the mass of a neighboring particle. The mass shift effect goes beyond the usual perturbative Feynman diagram calculations which implicitly assume that the mass of each particle is rigidly fixed. Local mass shifts offer a unique handle on Higgs physics since they do not require the production of on-shell Higg…
Report of the Higgs Working Group to appear in the Proceedings of the Workshop "Physics with $\ee$ Linear Colliders", Annecy-Gran Sasso-Hamburg, Feb. 4 - Sept. 1, 1995, P.M. Zerwas (editor).
In SU(3) simplest little Higgs model, a characteristic particle is the light pseudoscalar boson eta, it leads interesting signals in the LHC/ILC and is studied in literature with different parameterizations. In this work, we show that these different descriptions for eta particle are equivalent up to some SU(3) rotations as long as we suitably redefine the pseudo Goldstone boson fields. We evaluate the necessary SU(3) rotations and built up explicit expressions for redefined fields.
We study the heavy Higgs sector of the MSSM composed of the H^\pm, H^0 and A^0 particles in the so-called decoupling limit where m_{A^0} >> m_Z. By integrating out these heavy Higgs particles to one-loop, we compute the effective action for the electroweak gauge bosons and find out that, in the decoupling limit, all the heavy Higgs effects can be absorbed into redefinitions of the Standard Model electroweak parameters. This demonstrates explicitely that the decoupling theorem works for the heavy MSSM Higgs particles. This is also compared with the paradigmatic and different case of the Standar…
We show that simple extensions of two Higgs doublet models consisting of new heavy neutrinos and a singly charged Higgs boson singlet can successfully explain the experimental data on muon and electron anomalous magnetic moments thanks to large chirally-enhanced one-loop level contributions. These contributions arise from the large couplings of inverse seesaw neutrinos with singly charged Higgs bosons and right-handed charged leptons. The regions of parameter space satisfying the experimental data on $(g-2)_{e,μ}$ anomalies allow heavy singly charged Higgs boson masses above the TeV scale, pro…
The couplings of Higgs particles to two photons are analyzed in the threshold region, where the Higgs mass is about twice the loop--particle mass leading to possible bound state formation. The amplitude of the pseudoscalar decay $A\to γγ$ for pseudoscalar masses in the range of $(t\bar t)$--bound state masses is calculated in leading and next--to--leading order in $α_s$. It is shown that the real and imaginary parts of the form factor are significantly different from the lowest order perturbative one.
We compute the particle currents induced on a bubble wall background at finite temperature in a model with CP violation in the Higgs sector. Using a field theory approach we show that fermionic currents arise at one loop, so that a suppression factor ${\cal O}(h_t ϕ/πT)^2$ with respect to previous computations is found. The contributions to the Higgs currents are also derived and their relevancy for the spontaneous baryogenesis mechanism is discussed.
After commenting on the present value of the Higgs particle mass from radiative corrections, we explore the phenomenological implications of an alternative, non-perturbative renormalization of the scalar sector where the mass of the Higgs particle does not represent a measure of observable interactions at the Higgs mass scale. In this approach the Higgs particle could be very heavy, even heavier than 1 TeV, and remain nevertheless a relatively narrow resonance.
Three extensions of the scalar sector of the Standard Model are considered: one extra isosinglet, one extra isotriplet, two extra isotriplets (the Georgi-Machacek model). Double Higgs boson production cross section is calculated in all these extensions. Bounds from electroweak precision observables, signal strength measurements and custodial symmetry violation are estimated.
We address the prediction for the mass of the SM-like Higgs boson in NMSSM scenarios where all BSM particles, including the singlets, have masses at the TeV scale. We provide a full one-loop computation of the matching condition for the quartic Higgs coupling in the NMSSM, supplemented with the two-loop contributions that involve the strong gauge coupling. We discuss the impact of the one- and two-loop corrections that are specific to the NMSSM on the prediction for the Higgs mass, and propose a method to estimate of the uncertainty associated with the uncomputed higher-order terms. Finally, w…
We use the most recent data on the Higgs-like resonance h observed at 125 GeV to derive information about the mass of the heavier Higgs particles predicted by Minimal Supersymmetry. We treat as independent parameters the couplings of h to top quark, beauty and massive vector bosons and, in this three dimensional space, we locate the point realizing the best fit to data and compare it to the position of the Standard Model point and to the region of coupling values accommodating heavier Higgs particles in Minimal Supersymmetry. We conclude that mass values 320< M_H< 360 GeV are compatible at 2si…
We construct for the first time a flavor model, based on the smallest discrete symmetry Delta(27) that implements spontaneous CP violation with a complex phase of geometric origin, which can actually reproduce all quark masses and mixing data. We show that its scalar sector has exotic properties that can be tested at the LHC.
We analyze the cascade decays of the scalar quarks and gluinos of the Minimal Supersymmetric extension of the Standard Model, which are abundantly produced at the Large Hadron Collider, into heavier charginos and neutralinos which then decay into the lighter ones and charged Higgs particles, and show that they can have substantial branching fractions. The production rates of these Higgs bosons can be much larger than those from the direct production mechanisms, in particular for intermediate values of the parameter $\tan β$, and could therefore allow for the detection of these particles. We al…
We calculate decays of a Standard Model Higgs boson to a virtual massive particle and discuss how this depends on the massive particle total width. If the partial width of Higgs to a virtual massive particle can be measured this gives a measurement of that massive particle's width. We discuss how one would go about measuring these partial widths of a Higgs experimentally, and how this could lead to a measurement of the $W$ boson and $t$ quark width. For the latter extreme dependence on the Higgs mass and the small $H\to tt^*$ branching ratios mean that little can be learnt about the $t$ quark …
The first run of the Large Hadron Collider at CERN brought the discovery of the Higgs boson, an apparently elementary scalar particle with a mass of 125 GeV, the avatar of the mechanism that hides the electroweak symmetry. A new round of experimentation is beginning, with the energy of the proton--proton colliding beams raised to 6.5 TeV per beam, from 4 TeV at the end of the first run. This article summarizes what we have learned about the Higgs boson, and calls attention to some issues that will be among our central concerns in the near future.
The very large K-factor for Higgs-boson production at hadron colliders is shown to result from enhanced perturbative corrections of the form (C_Aπα_s)^n, which arise in the analytic continuation of the gluon form factor to time-like momentum transfer. These terms are resummed to all orders in perturbation theory using the renormalization group. After the resummation, the K-factor for the production of a light Higgs boson at the LHC is reduced to a value close to 1.3.
The thermal relic abundance of Dark Matter motivates the existence of new electroweak scale particles, independent of naturalness considerations. However, most unnatural Dark Matter models do not ensure the presence of new particles charged under SU(3)_C, resulting in challenging LHC phenomenology. Here, we present a class of models with scalar electroweak doublet Dark Matter that require a host of colored particles at the TeV scale. In these models, the Higgs boson is apparently fine-tuned, but the Dark Matter doublet is kept light without any additional fine-tuning.
The standard model of the elementary particles is built on several assumptions. The Higgs is assumed to be a scalar, a boson, with the charges of a fermion (in the fundamental representations of the charge groups). No explanation is offered for the existence of families of fermions -- quarks and leptons-- for the charges of these family members, for the appearance of the Yukawas which take care of fermion properties. The theory explaining the origin of families predicts that several scalar fields with the boson kind of the charges (in the adjoint representations of the charge groups) manifest …
Motivated by cosmological applications like electroweak baryogenesis, we develop a field theoretic approach to the computation of particle currents on a space-time dependent and CP-violating Higgs background. We consider the Standard Model model with two Higgs doublets and CP violation in the scalar sector, and compute both fermionic and Higgs currents by means of an expansion in the background fields. We discuss the gauge dependence of the results and the renormalization of the current operators, showing that in the limit of local equilibrium, no extra renormalization conditions are needed in…
We analyze the behavior of Higgs to diphoton rate and Higgs gluon-gluon production cross section in minimal extensions of the Standard Model comprising new colored vector-like fermions that do not mix with the ordinary ones. We compare these information with constraints coming from electroweak precision measurements. We compute pair production cross sections for the lightest fermion and discuss the LHC bounds. Finally, we study the phenomenology of possible quarkonium states composed by these new colored fermions.
In this talk we assume SO(10) boundary conditions at the GUT scale, including unification for the third generation Yukawa couplings $λ_t = λ_b = λ_τ$. We find that this assumption is only consistent with the low energy data in a narrow region of soft SUSY breaking parameter space. We discuss the consequences of this result for Higgs and SUSY searches.
In a more general electroweak theory, there could be Higgs particles that are odd under $CP$. Correlations among momenta of the initial electron and final-state fermions are in the Bjorken process sensitive to the $CP$ parity. Monte Carlo data on the expected efficiency demonstrate that it should be possible to verify the scalar character of an intermediate-mass Standard Model Higgs boson after three years of data taking at a future linear collider. This is most likely not possible at LEP2. Signals of possible presence of $CP$ violation in the Higgs sector are briefly discussed.
We perform frequentist and Bayesian statistical analyses of Higgs- and Z-portal models of dark matter particles with spin 0, 1/2 and 1. Our analyses incorporate data from direct detection and indirect detection experiments, as well as LHC searches for monojet and monophoton events, and we also analyze the potential impacts of future direct detection experiments. We find acceptable regions of the parameter spaces for Higgs-portal models with real scalar, neutral vector, Majorana or Dirac fermion dark matter particles, and Z-portal models with Majorana or Dirac fermion dark matter particles. In …
The Higgs-fermion couplings are sensitive probes of possible new physics behind a stable light Higgs particle. It is then essential to identify the flavour pattern of those interactions. We consider the case in which a strong dynamics lies behind a light Higgs, and explore the implications within the Minimal Flavour Violation ansatz. The dominant effects on flavour-changing Higgs-fermion couplings stem in this context from operators with mass dimension <6, and we analyze all relevant chiral operators up to that order, including loop-corrections induced by 4-dimensional ones. Bounds on the oper…
Present data do not rule out the light neutral Higgs particle h or A with mass below 40--50 GeV in the framework of the general 2HDM ("Model II"). The status of this model in a light of existing LEP I data and a potential of the new muon experiment (g-2), the measurement of photon-gluon and gluon-gluon fusion at HERA as well as photon-photon fusion at low energy gamma gamma NLC is discussed.
A promising solution to the doublet-triplet splitting problem of SUSY GUT's is the Higgs as pseudo-Goldstone boson mechanism. We present two models which naturally implement this idea and extend one of them to include fermion masses. We examine the phenomenological consequences of this mechanism and present the favored parameter region.
A collider probe of the radiative seesaw models are considered. Two key ingredients of these models, the extended Higgs sector and the source of the Majorana mass, although these details differ model by model, would be studied at the TeV-scale electron-positron and electron-electron colliders. The searches and mass determinations in the inert doublet model, which is the extended Higgs sector of the Ma model, are summarized as an example.
Even though the unified theory of electroweak interactions is very successful at low energies, there remains one part to be confirmed. It is the sector involving Higgs particles. Those Higgs particles are expected to be discovered. It has been shown recently that Higgs particles can be viewed as gauge fields in higher dimensional gauge theory. The mass of a Higgs particle and its coupling to other particles are constrained by the gauge principle. In this scenario the mass of a Higgs particle is predicted to be in the range of 120 GeV - 290 GeV, exactly in the range explored at LHC, provided th…
We propose a description of the Higgs boson as top-antitop quark bound state within a nonlocal relativistic quark model of Nambu - Jona-Lasinio type. In contrast to model with local four-fermion interaction, the mass of the scalar bound state can be lighter than the sum of its constituents. This is achieved by adjusting the interaction range and the value of the coupling constant to experimental data, for both the top quark mass and the scalar Higgs boson mass, which can simultaneously be described.
Models with extra gauge bosons often require an extended Higgs sector, which contains a rich spectrum of Higgs bosons with properties that deviate from the Standard Model (SM). Such Higgs bosons could be searched using standard mechanisms similar to the SM. However, the existence of the new gauge bosons could provide new production mechanisms, which could probe the non-standard origin of the Higgs particle. In this paper we study how the Z' from a model with extra U(1)' could be used as a source of Higgs bosons. We study 3-bodies decays of the Z' into a Higgs boson and a top anti-top pair or W…
We explore the ability of three-particle templates to distinguish color neutral objects from QCD background. This method is particularly useful to identify the standard model Higgs, as well as other massive neutral particles. Simple cut-based analysis in the overlap distributions of the signal and background is shown to provide a significant rejection power. By combining with other discriminating variables, such as planar flow, and several variables that depend on the partonic template, three-particle templates are used to characterize the influence of gluon emission and color flow in collider…
