The processes \gamma\gamma\to Z H in SM and MSSM

The process first arises at the one loop level, and as such it provides us with remarkable tests of the structure of the electroweak Higgs sector. These tests are complementary to those in the gauge sector involving . We show that in the standard model (SM) where , as well as in the supersymmetric case where or , observables exist (like e.g. the energy dependence, angular distribution, photon polarization dependence or final Z polarization) which present rather spectacular properties. Such properties involve strong threshold effects with steps, bumps or peaks, reflecting the type of Higgs and heavy quarks and chargino masses and couplings predicted by the SM and supersymmetric models. Received: 13 March 2001 / Published online: 13 June 2001     

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.     

Wino LSP detection in the light of recent Higgs searches at the LHC

Recent LHC data showed excesses of Higgs-like signals at the Higgs mass of around 125 GeV. This may indicate supersymmetric models with relatively heavy scalar fermions to enhance the Higgs mass. The desired mass spectrum is realized in the anomaly-mediated supersymmetry breaking model, in which the Wino can naturally be the lightest superparticle (LSP). We discuss possibilities for confirming such a scenario, particularly detecting signals from Wino LSP at direct detection experiments, indirect searches at neutrino telescopes and at the LHC.     

Production of scalar Higgs and pseudoscalar Higgs in multi-Higgs doublet models at \gamma\gamma colliders

We present the effects of heavy CP-even (H) and CP-odd (A) Higgs bosons on the production cross section of the process at the energy around the mass poles of the Higgs bosons. It is found that interference between H and A with small mass gap, as well as the ones between Higgs bosons and continuum, contributes to the cross section, if the photon beams are polarized and if we observe the helicity of the top quarks. It is demonstrated in the framework of the minimal supersymmetric extension of the standard model that the H and A contributions can be sizable at future colliders for small values of . The methods to measure the CP-parity of the Higgs boson are also presented. The statistical significance of detecting the Higgs signals and measuring the Higgs CP-parity is evaluated. Received: 16 December 1999 / Revised version: 30 January 2000 / Published online: 6 April 2000     

The heavy neutral Higgs signature in the \gamma \gamma \to Z Z process

If the standard model (SM) Higgs particle is sufficiently heavy, then its contribution to should be largely imaginary, interfering with the also predominantly imaginary SM “background” generated by the W loop. For standard model Higgs masses in the region GeV, this interference is found to be constructive and increasing the Higgs signal. In the minimal SUSY case an interference effect should also appear for the contribution of the heavier CP-even neutral Higgs boson , provided it is sufficiently heavy. The effect is somewhat reduced, though, by the smallness of the width and the and ZZ branching ratios. The interference is again found to be constructive for part of the parameter space corresponding to sfermion masses at the TeV scale and maximal stop mixing. For both the SM and the SUSY case, regions of the parameter space exist, though, where the interference may be destructive. It is therefore essential to take these effects into account when searching for possible scalar Higgs-like candidates. To this aim, we present the complete analytic expressions for both resonance and background amplitudes. Received: 20 October 2000 / Revised version: 5 January 2001 / Published online: 23 February 2001     

Higgs physics with a $\gamma \gamma$ collider based on CLIC 1

We present the machine parameters and physics capabilities of the CLIC Higgs Experiment (CLICHE), a low-energy collider based on CLIC 1, the demonstration project for the higher-energy two-beam accelerator CLIC. CLICHE is conceived as a factory capable of producing around 20,000 light Higgs bosons per year. We discuss the requirements for the CLIC 1 beams and a laser backscattering system capable of producing a total (peak) luminosity of cm^-2s^-1 with GeV. We show how CLICHE could be used to measure accurately the mass, , WW and decays of a light Higgs boson. We illustrate how these measurements may distinguish between the Standard Model Higgs boson and those in supersymmetric and more general two-Higgs-doublet models, complement ing the measurements to be made with other accelerators. We also comment on other prospects in and physics with CLICHE. Received: 20 November 2001 / Published online: 24 March 2003     

Higgs as a pseudo-Goldstone boson, the mu problem and gauge-mediated supersymmetry breaking

We study the interplay between the spontaneous breaking of a global symmetry of the Higgs sector and gauge-mediated supersymmetry breaking, in the framework of a supersymmetric model with global SU(3) symmetry. In addition to solving the supersymmetric flavor problem and alleviating the little hierarchy problem, this scenario automatically triggers the breaking of the global symmetry and provides an elegant solution to the μ/Bμ problem of gauge mediation. We study in detail the processes of global symmetry and electroweak symmetry breaking, including the contributions of the top/stop and gauge-Higgs sectors to the one-loop effective potential of the pseudo-Goldstone Higgs boson. While the joint effect of supersymmetry and of the global symmetry allows in principle the electroweak symmetry to be broken with little fine-tuning, the simplest version of the model fails to bring the Higgs mass above the LEP bound due to a suppressed tree-level quartic coupling. To cure this problem, we consider the possibility of additional SU(3)-breaking contributions to the Higgs potential, which results in a moderate fine-tuning. The model predicts a rather low messenger scale, a small tanβ value, a light Higgs boson with Standard Model-like properties, and heavy higgsinos.     

Rare decay B\rightarrow X_sl^+l^- in a CP spontaneously broken two-Higgs doublet model

The Higgs boson mass spectrum and couplings of the neutral Higgs bosons to the fermions are worked out in a CP spontaneously broken two-Higgs doublet model in the large case. The differential branching ratio, forward-backward asymmetry, CP asymmetry and lepton polarization for are computed. It is shown that the effects of neutral Higgs bosons are quite significant when is large. Especially, the CP violating normal polarization can be as large as several percents. Received: 15 October 2001 / Revised version: 5 March 2002 / Published online: 26 July 2002     

Finite theories before and after the discovery of a Higgs boson at the LHC

Finite Unified Theories (FUTs) are N = 1 supersymmetric Grand Unified Theories (GUTs) which can be made finite to all‐loop orders, based on the principle of reduction of couplings, and therefore are provided with a large predictive power. Confronting the predictions of SU(5) FUTs with the top and bottom quark masses and other low‐energy experimental constraints a light Higgs‐boson mass in the range M_h ∼ 121–126 GeV was predicted, in striking agreement with the recent discovery of a Higgs‐like state around ∼ 125.5 GeV at ATLAS and CMS. Furthermore the favoured model, a finiteness constrained version of the MSSM, naturally predicts a relatively heavy spectrum with coloured supersymmetric particles above ∼ 1.5 TeV, consistent with the non‐observation of those particles at the LHC. Restricting further the best FUT's parameter space according to the discovery of a Higgs‐like state and B‐physics observables we find predictions for the rest of the Higgs masses and the supersymmetric particle spectrum.     

\mu\rightarrow e\gamma decay in the left-right supersymmetric model

We calculate the dipole amplitude for the decay and related processes in the left-right supersymmetric model when parity breaking occurs at a considerably large scale. The low-energy flavor violation in the model originates either from the nonvanishing remnants of the left-right symmetry in the slepton mass matrix or from the direct flavor changing lepton-slepton-neutralino interaction. The result is found to be large and already accessible with current experimental accuracy for supersymmetric masses not far above the electroweak scale. It also provides nontrivial constraints on the lepton mixing in the model. Received: 8 June 1998 / Published online: 15 October 1998     

Soft gluon radiation in Higgs boson production at the LHC

We examine the contributions of soft gluons to the Higgs production cross section at the LHC in the Standard Model and its minimal supersymmetric extension. The soft gluon radiation effects of this reaction share many features with the Drell-Yan process, but arise at lowest order from a purely gluonic initial state. We provide an extension of the conventional soft gluon resummation formalism to include a new class of contributions which we argue to be universal, and resum these and the usual Sudakov effects to all orders. The effect of these new terms is striking: only if they are included, does the expansion of the resummed cross section to next-to-leading order reproduce the exact result to within a few percent for the full range of Higgs boson masses. We use our resummed cross section to derive next-to-next-to-leading order results, and their scale dependence. Moreover, we demonstrate the importance of including the novel contributions in the resummed Drell-Yan process.     

The light Higgs window in the 2HDM at GigaZ

The sensitivity to a light Higgs boson in the general 2HDM (II), with a mass below 40 GeV, is estimated for a future Linear Collider operating with a very high luminosity at the Z peak (GigaZ). We consider possible Higgs boson production via the Bjorken process, the (hA) pair production, the Yukawa process , and the decay . Although the discovery potential is considerably extended compared to the current sensitivities, mainly from LEP, the existence of a h or A even with a mass of a few GeV cannot be excluded with two billion Z decays. The need to study the very light Higgs scenario at a Linear Collider running at several hundred GeV and the LHC is emphasized. Received: 18 September 2000 / Revised version: 22 December 2000 / Published online: 15 March 2001     

Decoupling of supersymmetric particles

The possibility of a heavy supersymmetric spectrum at the Minimal Supersymmetric Standard Model is considered and the decoupling from the low energy electroweak scale is analyzed in detail. The formal proof of decoupling of supersymmetric particles from low energy physics is stated in terms of the effective action for the particles of the Standard Model that results by integrating out all the sparticles in the limit where their masses are larger than the electroweak scale. The computation of the effective action for the standard electroweak gauge bosons , Z and is performed by integrating out all the squarks, sleptons, charginos and neutralinos to one-loop. The Higgs sector is not considered in this paper. The large sparticle masses limit is also analyzed in detail. Explicit analytical formulae for the two-point functions of the electroweak gauge bosons to be valid in that limit are presented. Finally, the decoupling of sparticles in the S, T and U parameters is studied analitically. A discussion on how the decoupling takes place in terms of both the physical sparticle masses and the non-physical mass parameters as the -parameter and the soft-breaking parameters is included. Received: 27 March 1998 / Published online: 5 October 1998     

Interference of Higgs boson resonances in $\mu^ + \mu^- \to\tilde{\chi}^0_i\tilde{\chi}^0_j$ with longitudinal beam polarization

We study the interference of resonant Higgs boson exchange in neutralino production in m⁺ m^-\mu^ + \mu^- annihilation with longitudinally polarized beams. We use the energy distribution of the decay lepton in the process [(c)\tilde]⁰_j ? l^± [(l)\tilde]^-±\tilde{\chi}^0_j \to \ell^{\pm} \tilde{\ell}^\mp to determine the polarization of the neutralinos. In the CP-conserving minimal supersymmetric standard model a non-vanishing asymmetry in the lepton energy spectrum is caused by the interference of Higgs boson exchange channels with different CP-eigenvalues. The contribution of this interference is large if the heavy neutral bosons H and A are nearly degenerate. We show that the asymmetry can be used to determine the couplings of the neutral Higgs bosons to the neutralinos. In particular, the asymmetry allows one to determine the relative phase of the couplings. We find large asymmetries and cross sections for a set of reference scenarios with nearly degenerate neutral Higgs bosons.     

Dark Supersymmetry

We propose a model of Dark Supersymmetry, where a supersymmetric dark sector is coupled to the classically scale invariant non-supersymmetric Standard Model through the Higgs portal. The dark sector contains a mass scale that is protected against radiative corrections by supersymmetry, and the portal coupling mediates this scale to the Standard Model, resulting in a vacuum expectation value for the Higgs field and the usual electroweak symmetry breaking mechanism. The supersymmetric dark sector contains dark matter candidates, and we show that the observed dark matter abundance is generated for a natural choice of parameters, while avoiding the current experimental bounds on direct detection. Future experiments can probe this scenario if the dark sector mass scale is not too high.     

Particle spectrum in the modified nonminimal supersymmetric standard model in the strong Yukawa coupling regime

A theoretical analysis of solutions of renormalization group equations in the minimal supersymmetric standard model, which lead to a quasi-fixed point has shown that the mass of the lightest Higgs boson in these models does not exceed 94 ± 5 GeV. This implies that a considerable part of the parameter space in the minimal supersymmetric model is in fact eliminated by existing LEPII experimental data. In the nonminimal supersymmetric standard model the upper bound on the mass of the lightest Higgs boson reaches its maximum in the strong Yukawa coupling regime when the Yukawa constants are substantially greater than the gauge constants on the grand unification scale. In the present paper the particle spectrum is studied using the simplest modification of the nonminimal supersymmetric standard model which gives a self-consistent solution in this region of parameter space. This model can give m _h ～ 125 GeV even for comparatively low values of β ≥ 1.9. The spectrum of Higgs bosons and neutralinos is analyzed using the method of diagonalizing mass matrices proposed earlier. In this model the mass of the lightest Higgs boson does not exceed 130.5 ± 3.5 GeV.     

SUSY spectrum and the Higgs mass in the BLMSSM

The predictions for the mass of the light CP-even Higgs are investigated in the context of a simple extension of the Minimal Supersymmetric Standard Model where the baryon and lepton numbers are local gauge symmetries. This theory predicts the existence of light charged and neutral leptons which give extra contributions to the Higgs mass at the one-loop level. We show the possibility to satisfy the LEP2 bound and achieve a Higgs mass around 125 GeV in a supersymmetric spectrum with light sfermions and small left–right mixing in the stop sector. We make a brief discussion of the unique leptonic signals at the Large Hadron Collider. This theory predicts baryon number violation at the low scale and one could avoid the current LHC bounds on the supersymmetric mass spectrum.     

Yukawa coupling quantum corrections to the self-couplings of the lightest MSSM Higgs boson

A detailed analysis of the top-quark/squark quantum corrections to the lightest CP-even Higgs boson self-couplings is presented in the MSSM. By considering the leading one-loop Yukawa-coupling contributions of , we discuss the decoupling behavior of these corrections when the top squarks are heavy compared to the electroweak scale. As shown analytically and numerically, the large corrections can almost completely be absorbed into the -boson mass. Our conclusion is that the self-couplings remain similar to the coupling of the SM Higgs boson for the heavy top-squark sector. Received: 15 November 2001 / Published online: 25 January 2002     

<Emphasis Type="Italic">H</Emphasis>/A Higgs mixing in <Emphasis Type="Italic">CP</Emphasis>-noninvariant supersymmetric theories

For large masses, the two heavy neutral Higgs bosons are nearly degenerate in many 2-Higgs doublet models, and particularly in supersymmetric models. In such a scenario the mixing between the states can be very large if the theory is CP-noninvariant. We analyze the formalism describing this configuration, and we point to some interesting experimental consequences.Received: 1 October 2004, Revised: 10 January 2005, Published online: 9 March 2005     

First-order electroweak phase transition powered by additional F-term loop effects in an extended supersymmetric Higgs sector

We investigate the one-loop effect of new charged scalar bosons on the Higgs potential at finite temperatures in the supersymmetric standard model with four Higgs doublet chiral superfields as well as a pair of charged singlet chiral superfields. In this model, the mass of the lightest Higgs boson h is determined only by the D-term in the Higgs potential at the tree-level, while the triple Higgs boson coupling for hhh can receive a significant radiative correction due to nondecoupling one-loop contributions of the additional charged scalar bosons. We find that the same nondecoupling mechanism can also contribute to realize stronger first order electroweak phase transition than that in the minimal supersymmetric standard model, which is definitely required for a successful scenario of electroweak baryogenesis. Therefore, this model can be a new candidate for a model in which the baryon asymmetry of the Universe is explained at the electroweak scale.