Heavy SUSY Higgs bosons at e + e − linear colliders

The production mechanisms and decay modes of the heavy neutral and charged Higgs bosons in the Minimal Supersymmetric Standard Model are investigated at future e ⁺ e ^? colliders in the TeV energy regime. We generate supersymmetric particle spectra by requiring the MSSM Higgs potential to produce correct radiative electroweak symmetry breaking, and we assume a common scalar mass m₀, gaugino mass m_1/2 and trilinear coupling A, as well as gauge and Yukawa coupling unification at the Grand Unification scale. Particular emphasis is put on the low tan β solution in this scenario where decays of the Higgs bosons to Standard Model particles compete with decays to supersymmetric charginos/neutralinos as well as sfermions. In the high tan β case, the supersymmetric spectrum is either too heavy or the supersymmetric decay modes are suppressed, since the Higgs bosons decay almost exclusively into b and τ pairs. The main production mechanisms for the heavy Higgs particles are the associated AH production and H ⁺H^? pair production with cross sections of the order of a few fb.     

Flavor changing neutral currents in the Lee–Wick Standard Model

Recently an extension of the Standard Model (the Lee–Wick Standard Model) based on ideas of Lee and Wick (LW) was introduced. It does not contain quadratic divergences in the Higgs mass and hence solves the hierarchy puzzle. The LW Standard Model contains new heavy LW-resonances at the TeV scale that decay to ordinary particles. In this Letter we examine in more detail the flavor structure of the theory. We integrate out the heavy LW-fermions at tree level and find that this induces flavor changing Z-boson couplings. However, these flavor changing neutral currents are acceptably small since they are automatically suppressed by small Yukawa couplings. This is the case even though the theory does not satisfy the principle of minimal flavor violation. New couplings of the charged W-bosons to quarks and leptons are also induced. We also integrate out the LW–Higgs and examine the four-fermion operators induced.     

Two-and three-body decay modes of SUSY Higgs particles

We summarize the dominant decay modes of the neutral and charged Higgs bosons in the Minimal Supersymmetric extension of the Standard Model. While two-body decays are in general dominating, the branching ratios for three-body decays of the heavy scalar, pseudoscalar and charged Higgs bosons can be large below the thresholds if top quarks, W/Z bosons or heavy scalar bosons are involved. Analytical expressions have been derived for the partial decay widths and the physical implications of these decay modes are discussed.     

Chargino decays in the complex MSSM: a full one-loop analysis

We evaluate two-body decay modes of charginos in the Minimal Supersymmetric Standard Model with complex parameters (cMSSM). Assuming heavy scalar quarks we take into account all decay channels involving charginos, neutralinos, (scalar) leptons, Higgs bosons and Standard Model gauge bosons. The evaluation of the decay widths is based on a full one-loop calculation including hard and soft QED radiation. Special attention is paid to decays involving the Lightest Supersymmetric Particle (LSP), i.e. the lightest neutralino, or a neutral or charged Higgs boson. The higher-order corrections of the chargino decay widths involving the LSP can easily reach a level of about ±10%, while the corrections to the decays to Higgs bosons are slightly smaller, translating into corrections of similar size in the respective branching ratios. These corrections are important for the correct interpretation of LSP and Higgs production at the LHC and at a future linear e ⁺ e ⁻ collider. The results will be implemented into the Fortran code FeynHiggs.     

Higgs production and decay with a fourth Standard-Model-like fermion generation

State-of-the-art predictions for the Higgs-boson production cross section via gluon fusion and for all relevant Higgs-boson decay channels are presented in the presence of a fourth Standard-Model-like fermion generation. The qualitative features of the most important differences to the genuine Standard Model are pointed out, and the use of the available tools for the predictions is described. For a generic mass scale of 400−600 GeV in the fourth generation explicit numerical results for the cross section and decay widths are presented, revealing extremely large electroweak radiative corrections, e.g., to the cross section and the Higgs decay into WW or ZZ pairs, where they amount to about −50 % or more. This signals the onset of a non-perturbative regime due to the large Yukawa couplings in the fourth generation. An estimate of the respective large theoretical uncertainties is presented as well.     

Top quark mass in exophobic Pati-Salam heterotic string model

We analyse the phenomenology of an exemplary exophobic Pati-Salam heterotic string vacuum, in which no exotic fractionally charged states exist in the massless string spectrum. Our model also contains the Higgs representations that are needed to break the gauge symmetry to that of the Standard Model and to generate fermion masses at the electroweak scale. We show that the requirement of a leading mass term for the heavy generation, which is not degenerate with the mass terms of the lighter generations, places an additional strong constraint on the viability of the models. In many models a top quark Yukawa may not exist at all, whereas in others two or more generations may obtain a mass term at leading order. In our exemplary model a mass term at leading order exist only for one family. Additionally, we demonstrate the existence of supersymmetric F- and D-flat directions that give heavy mass to all the colour triplets beyond those of the Standard Model and leave one pair of electroweak Higgs doublets light. Hence, below the Pati-Salam breaking scale, the matter states in our model that are charged under the observable gauge symmetries, consist solely of those of the Minimal Supersymmetric Standard Model.     

Strangephilic Higgs bosons in the MSSM

We suggest a new CPX-derived scenario for the search for strangephilic MSSM Higgs bosons at the Tevatron and the LHC, in which all neutral and charged Higgs bosons decay predominantly into pairs of strange quarks and into a strange and a charm quark, respectively. The proposed scenario is realized within a particular region of the MSSM parameter space and requires large values of tan?β, where threshold radiative corrections are significant to render the effective strange-quark Yukawa coupling dominant. Experimental searches for neutral Higgs bosons based on the identification of b-quark jets or τ leptons may miss a strangephilic Higgs boson and its existence could be inferred indirectly by searching for hadronically decaying charged Higgs bosons. Potential strategies and experimental challenges to search for strangephilic Higgs bosons at the Tevatron and the LHC are discussed.     

Enhancement of “CP-odd” Higgs boson production in the minimal supersymmetric Standard Model with explicit CP-violation

We calculate the production cross section of the “CP-odd” Higgs boson via gluon fusion in the minimal supersymmetric Standard Model with explicit CP-violation in the stop sector. We show that there is a parameter region in which the cross section is enhanced by a factor of about 1000, as compared to the case without CP-violation in the stop sector. In the parameter region where the “CP-odd” Higgs boson can decay into a stop pair, the stop pair events will be the important signature of the enhanced “CP-odd” Higgs boson production. In the case where the “CP-odd” Higgs boson cannot decay into any superparticles, the γγ and ττ decay channels could become important for discovering the “CP-odd” Higgs boson. We also discuss the constraints from electric dipole moments of electron, neutron and mercury on the viable parameter space mentioned above.     

Lorentz violation from the Higgs portal

We study bounds and signatures of models where the Higgs doublet has an inhomogeneous mass or vacuum expectation value, being coupled to a hidden sector that breaks Lorentz invariance. This physics is best described by a low-energy effective Lagrangian in which the Higgs speed-of-light is smaller than c; such effect is naturally small because it is suppressed by four powers of the inhomogeneity scale. The Lorentz violation in the Higgs sector is communicated at tree level to fermions (via Yukawa interactions) and to massive gauge bosons, although the most important effect comes from one-loop diagrams for photons and from two-loop diagrams for fermions. We calculate these effects by deriving the renormalization-group equations for the speed-of-light of the Standard Model particles. An interesting feature is that the strong coupling dynamically makes the speed-of-light equal for all colored particles.     

Analytic approach to the MSSM mass spectrum in the large tan β regime

In various unified extensions of the Minimal Supersymmetric Standard Model, the Yukawa couplings of the third generation are predicted to be of the same order. As a result, low energy measured mass ratios require large ratios of the standard model Higgs vacuum expectation values, corresponding to a large value of the parameter tan β. We present analytic solutions for the Yukawa couplings and the Higgs and third generation squark masses, in the case of large top and bottom Yukawa couplings. We examine regions of these Yukawas which give predictions for the top mass compatible with the present experimentally determined top mass and provide useful approximate formulae for the scalars. We discuss the implications on the radiative symmetry breaking mechanism and derive constraints on the undetermined initial conditions of the scalars.     

Measuring hidden Higgs and strongly-interacting Higgs scenarios

Higgs couplings can be affected by physics beyond the Standard Model. We study modifications through interactions with a hidden sector and in specific composite Higgs models accessible at the LHC. Both scenarios give rise to congruent patterns of universal, or partially universal, shifts. In addition, Higgs decays to the hidden sector may lead to invisible decay modes which we also exploit. Experimental bounds on such potential modifications will measure the concordance of an observed Higgs boson with the Standard Model.     

Thermal equilibrium during the electroweak phase transition

The perturbative effective potential for the Standard Model develops a barrier, at temperatures around the electroweak scale, which separates the minimum at zero field and a deeper non-zero minimum. This could create out of equilibrium conditions by inducing the localization of the Higgs field in a metastable state around zero. In this picture vacuum decay would occur through bubble nucleation. I show that there is an upper bound on the Higgs mass for the above scenario to be realized. The barrier must be high enough to prevent thermal fluctuations of the Higgs expectation value from establishing thermal equilibrium between the two minima. The upper bound is estimated to be lower than the experimental lower limit. This also imposes constraints on extensions of the Standard Model constructed in order to generate a strongly first order phase transition.     

Enhanced di-photon Higgs signal in the Next-to-Minimal Supersymmetric Standard Model

In the Next-to-Minimal Supersymmetric Standard Model, CP-even Higgs bosons can have masses in the range of 80–110 GeV in agreement with constraints from LEP due to their sizeable singlet component. Nevertheless their branching ratio into two photons can be more than 10 times larger than the one of a Standard Model Higgs boson of similar mass due to a reduced coupling to b quarks. This can lead to a spectacular enhancement of the Higgs signal rate in the di-photon channel at hadron colliders by a factor 6. Corresponding scenarios can occur in the Next-to-Minimal Supersymmetric Standard Model for a relatively low Susy breaking scale.     

The probable fate of the Standard Model

Extrapolating the Standard Model to high scales using the renormalisation group, three possibilities arise, depending on the mass of the Higgs boson: if the Higgs mass is large enough the Higgs self-coupling may blow up, entailing some new non-perturbative dynamics; if the Higgs mass is small the effective potential of the Standard Model may reveal an instability; or the Standard Model may survive all the way to the Planck scale for an intermediate range of Higgs masses. This latter case does not necessarily require stability at all times, but includes the possibility of a metastable vacuum which has not yet decayed. We evaluate the relative likelihoods of these possibilities, on the basis of a global fit to the Standard Model made using the Gfitter package. This uses the information about the Higgs mass available directly from Higgs searches at LEP and now the Tevatron, and indirectly from precision electroweak data. We find that the ‘blow-up’ scenario is disfavoured at the 99% confidence level (96% without the Tevatron exclusion), whereas the ‘survival’ and possible ‘metastable’ scenarios remain plausible. A future measurement of the mass of the Higgs boson could reveal the fate of the Standard Model.     

Standard Model without elementary scalars and high energy Lorentz violation

If Lorentz symmetry is violated at high energies, interactions that are usually non-renormalizable can become renormalizable by weighted power counting. Recently, a CPT invariant, Lorentz violating extension of the Standard Model containing two scalar-two fermion interactions (which can explain neutrino masses) and four fermion interactions (which can explain proton decay) was proposed. In this paper we consider a variant of this model, obtained suppressing the elementary scalar fields, and argue that it can reproduce the known low-energy physics. In the Nambu–Jona-Lasinio spirit, we show, using a large N _c expansion, that a dynamical symmetry breaking takes place. The effective potential has a Lorentz invariant minimum and the Lorentz violation does not reverberate down to low energies. The mechanism generates fermion masses, gauge-boson masses and scalar bound states, to be identified with composite Higgs bosons. Our approach is not plagued by the ambiguities of approaches based on non-renormalizable vertices. The low-energy effective action is uniquely determined and predicts relations among parameters of the Standard Model.     

Dynamical generation of flavour

We propose the generation of Standard Model fermion hierarchy by the extension of renormalizable SO(10) GUT with O(N_g) family gauge symmetry. In this scenario, Higgs representations of SO(10) also carry family indices and are called Yukawons. Vacuum expectation values of these Yukawon fields break GUT and family symmetry and generate MSSM Yukawa couplings dynamically. We have demonstrated this idea using \({\mathbf {10}}\oplus {\mathbf {210}} \oplus {\mathbf {126}} \oplus {\overline {\mathbf {126}}}\) Higgs irrep, ignoring the contribution of 120-plet which is, however, required for complete fitting of fermion mass-mixing data. The effective MSSM matter fermion couplings to the light Higgs pair are determined by the null eigenvectors of the MSSM-type Higgs doublet superfield mass matrix \(\mathcal {H}\). A consistency condition on the doublet ([1,2,±1]) mass matrix (\(\text {Det}(\mathcal {H})=\) 0) is required to keep one pair of Higgs doublets light in the effective MSSM. We show that the Yukawa structure generated by null eigenvectors of \(\mathcal {H}\) are of generic kind required by the MSSM. A hidden sector with a pair of (S_ab; ?_ab) fields breaks supersymmetry and facilitates \(D_{O(N_{g})}\hspace *{-1pt}=\) 0. SUSY breaking is communicated via supergravity. In this scenario, matter fermion Yukawa couplings are reduced from 15 to just 3 parameters in MSGUT with three generations.     

Higgs boson properties in the Standard Model and its supersymmetric extensions

We review the realization of the Brout–Englert–Higgs mechanism in the electroweak theory and describe the experimental and theoretical constraints on the mass of the single Higgs boson expected in the minimal Standard Model. We also discuss the couplings of this Higgs boson and its possible decay modes as functions of its unknown mass. We then review the structure of the Higgs sector in the minimal supersymmetric extension of the Standard Model (MSSM), noting the importance of loop corrections to the masses of its five physical Higgs bosons. Finally, we discuss some non-minimal models. To cite this article: J. Ellis et al., C. R. Physique 8 (2007).     

Search for invisibly decaying Higgs bosons with large decay width using the OPAL detector at LEP

This paper describes a topological search for an invisibly decaying Higgs boson, H, produced via the Bjorken process (e⁺e^-→HZ). The analysis is based on data recorded using the OPAL detector at LEP at centre-of-mass energies from 183 to 209 GeV corresponding to a total integrated luminosity of 629 pb^-1. In the analysis only hadronic decays of the Z boson are considered. A scan over Higgs boson masses from 1 to 120 GeV and decay widths from 1 to 3000 GeV revealed no indication for a signal in the data. From a likelihood ratio of expected signal and standard model background we determine upper limits on cross-section times branching ratio to an invisible final state. For moderate Higgs boson decay widths, these range from about 0.07 pb (M_H=60 GeV) to 0.57 pb (M_H=114 GeV). For decay widths above 200 GeV the upper limits are of the order of 0.15 pb. The results can be interpreted in general scenarios predicting a large invisible decay width of the Higgs boson. As an example we interpret the results in the so-called stealthy Higgs scenario. The limits from this analysis exclude a large part of the parameter range of this scenario experimentally accessible at LEP 2.     

Uplifted supersymmetric Higgs region

We show that the parameter space of the Minimal Supersymmetric Standard Model includes a region where the down-type fermion masses are generated by the loop-induced couplings to the up-type Higgs doublet. In this region the down-type Higgs doublet does not acquire a vacuum expectation value at tree level, and has sizable couplings in the superpotential to the tau leptons and bottom quarks. Besides a light standard-like Higgs boson, the Higgs spectrum includes the nearly degenerate states of a heavy spin-0 doublet which can be produced through their couplings to the b quark and decay predominantly into τ ⁺ τ ^? or τν.     

On the Higgs mass generation mechanism in the Standard Model

The mass-generation mechanism is the most urgent problem of modern particle physics. The discovery and study of the Higgs boson with the Large Hadron Collider at CERN are the highest priority steps to solve the problem. In this paper, the Standard Model Higgs mechanism of elementary particle mass generation is reviewed with pedagogical details. The discussion of the Higgs quadric self-coupling λ parameter and the bounds to the Higgs boson mass are presented. In particular, the unitarity, triviality, and stability constraints on the Higgs boson mass are discussed. The generation of a finite value for the λ parameter due to quantum corrections via effective potential is illustrated. Some simple predictions for the top quark and the Higgs boson masses are given when both the top Yukawa coupling and the Higgs self-coupling λ are equal to 1. The text was submitted by the authors in English.