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     

The \gamma \gamma \to \gamma \gamma process in the standard and SUSY models at high energies

We study the helicity amplitudes of the process at high energy, which in the standard and SUSY models first arise at the one-loop order. In the standard model (SM), the diagrams involve , charged-quark, and lepton loops, while in SUSY, we also have contributions from chargino-, charged-sfermion, and Higgs-loop diagrams. The SUSY contributions are most important in the region above the threshold for producing the supersymmetric partners; there, they interfere most effectively with the primarily imaginary SM amplitudes. Simple expressions for the relevant one-loop functions are given which provide a direct overview of the behaviour of the helicity amplitudes in the whole parameter space at high energies. The various characteristics of a large set of observables are studied in detail. Received: 3 February 1999 / Pulished online: 30 June 1999     

The \gamma \gamma \to Z Z process and the search for virtual SUSY effects at a \gamma \gamma Collider

We study the helicity amplitudes of the process in the Standard Model at high energy. These amplitudes receive contributions from the W and charged quark and lepton loops, analogous to those encountered in the cases studied before. But also receives contributions from the Higgs s-channel poles involving the effective Higgs- vertex. At energies , the amplitudes in all three processes are mainly helicity-conserving and almost purely imaginary; which renders them a very useful tool in searching for New Physics. As an example, a SUSY case is studied, and the signatures due to the virtual effects induced by a chargino-, charged slepton- or a lightest stop-loop in , are explored. These signatures, combined with the analogous ones in and , should help identifying the nature of possible New Physics particles. Received: 3 September 1999 / Published online: 3 February 2000     

The b \to X_s \gamma rate and Higgs boson limits in the constrained minimal supersymmetric model

New NLO calculations have become available using resummed radiative corrections. Using these calculations we perform a global fit of the supergravity inspired constrained minimal supersymmetric model. We find that the resummed calculations show similar constraints as the LO calculations, namely that only with a relatively heavy supersymmetric mass spectrum of (1 TeV) the b– Yukawa unification and the rate can coexist in the large scenario. The resummed calculations are found to reduce the renormalization scale uncertainty considerably. The low scenario is excluded by the present Higgs limits from LEP II. The constraint from the Higgs limit in the plane is severe, if the trilinear coupling at the GUT scale is fixed to zero, but is considerably reduced for . The relatively heavy SUSY spectrum required by corresponds to a Higgs mass of GeV in the CMSSM. Received: 14 February 2001 / Revised version: 22 March 2001 / Published online: 29 June 2001     

The \gamma \gamma \to \gamma Z process at high energies and the search for virtual SUSY effects

We study the helicity amplitudes and the observables of the process at high energy. As in the case of the process studied before, the relevant diagrams in the standard model (SM) involve W, charged-quark, and lepton loops, while in SUSY we also have contributions from charginos and charged-sfermion or Higgs-loop diagrams. Above 250 GeV, the dominant SM amplitudes are themselves dominated by the W loop, and as for , they are helicity conserving and almost purely imaginary. We discuss the complementary information provided by for the identification of possible nonstandard effects. This process, together with , should provide very useful information on the nature of possible new physics particles, above the threshold of their direct production. Received: 26 April 1999 / Published online: 8 September 1999     

K +→π + vv andK L 0 →μ + μ − decays within the Minimal Supersymmetric Standard Model

We present a detailed calculation of the contributions of charginos, scalar quarks, and charged Higgs boson to theK ⁺→π ⁺ vv andK _L ⁰ →μ ⁺ μ ⁻ decays. We include mixings: that of charginos and that of the scalar partners of the left and right handed top quark. We find that the box contribution to the amplitudes is much smaller than the penguin contribution, which can be ∼10% of the Standard Model contribution, even for relatively large SUSY masses. The charged Higgs contribution can be as large as 25% of the SM contribution in the first decay and as much as 40% of the SM contribution in the second decay.     

The coupling of the lightest SUSY Higgs boson to two photons in the decoupling regime

We analyze the contribution of the SUSY particles to the coupling of the lightest Higgs boson to two photons in supersymmetric theories. We discuss to what extent these contributions can be large enough to allow for a discrimination between the lightest SUSY and the standard Higgs particles in the decoupling limit where all other Higgs bosons are very heavy and no supersymmetric particle has been discovered at future colliders. We find that only chargino and top squark loops can generate a sizeable difference between the standard and the SUSY Higgs-photon couplings. For masses above 250 GeV, the effect of chargino loops on the two-photon width is however smaller than ≈ 10% in the entire SUSY parameter space. Top squarks heavier than 250 GeV can induce deviations larger than 10% only if their couplings to the Higgs boson are large. Since top squark contributions can be sizeable, we derive the two-loop QCD correction to squark loops and show that they are well under control.     

Chargino and neutralino decays revisited

We perform a comprehensive analysis of the decays of charginos and neutralinos in the minimal supersymmetric standard model where the neutralino is assumed to be the lightest supersymmetric particle. We focus, in particular, on the three-body decays of the next-to-lightest neutralino and the lightest chargino into the lightest neutralino and fermion–antifermion pairs and include vector boson, Higgs boson and sfermion exchange diagrams, where in the latter contribution the full mixing in the third generation is included. The radiative corrections to the heavy fermion and SUSY particle masses will also be taken into account. We present complete analytical formulae for the Dalitz densities and the integrated partial decay widths in the massless fermion case, as well as the expressions of the differential decay widths including the masses of the final fermions and the polarization of the decaying charginos and neutralinos. We then discuss these decay modes, in particular in scenarios where the parameter is large and in models without universal gaugino masses at the grand unification scale, where some new decay channels, such as decays into gluinos and pairs, open up. Received: 12 April 2001 / Published online: 8 June 2001     

Supersymmetric penguin contributions to the decay b\rightarrow s\gamma with non-universal squarks masses

We give explicit expressions for the amplitudes associated with the supersymmetric (SUSY) contributions to the process in the context of SUSY extensions of the standard model (SM) with non-universal soft SUSY breaking terms. From experimental data we deduce limits on the squark mass insertions obtained from different contributions (gluinos, neutralinos and charginos). Received: 20 April 2001 / Revised version: 14 December 2001 / Published online: 5 April 2002     

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     

Process Z{\rightarrow}h(A)+\gamma in the 2HDM and the experimental constraints from LEP

The one-loop branching ratios for the process are calculated in the general two Higgs doublet model (Model II), taking into account existing constraints on the model parameters. For Higgs boson masses below 50 GeV and of the fraction of such Z decays are at the level of , but can be significantly stronger for very low or high , where the dependence of these results on other model parameters like and the mass of the charged Higgs boson is found to be of little importance. The results are compared to the LEP measurements, which are sensitive to branching ratios of of the order for masses GeV, but approach for low masses. By relating the expectation to the experimental limits, constraints on the parameter space of the 2HDM are derived. Received: 28 November 1998 / Published online: 27 April 1999     

Exceptional Point of the Neutral Heavy Higgs System H2–H3

We study the singularity of the surface that represents the masses of the isolated doublet of heavy, neutral Higgs bosons, H ₂–H ₃, in a toy model based on the MSSM with CP violation, in parameter space. These two heavy, neutral Higgs bosons are coherent and, for large values of the masses, nearly degenerate. In this scenario, mixing between the mass eigenstates of the H ₂–H ₃ system could be very large and exact degeneracy is possible. As function of the Lagrangian parameters, the physical mass of the doublet has an algebraic branch point of rank one at the exceptional point where the two masses are equal. The real and imaginary parts of the masses in the doublet have branch cuts that start at the same branch point but extend in opposite directions in parameter space. Associated with this branch point, the propagator of the mixing doublet of neutral heavy Higgs bosons has a double pole in the complex s-plane of the energy squared. We computed the mass surface of the isolated doublet of H ₂–H ₃ bosons as function of the Lagrangian parameters in the neighbourhood of the exceptional point in a toy model of the system H ₂–H ₃. We also computed the trajectories of the poles of the transition matrix for values of the Lagrangian parameters close to the exceptional point and explained the characteristic change of identity seen in these trajectories in the s-plane as a manifestation of the topology of the two-sheeted mass surfaces in the space of Lagrangian parameters.     

Upper bounds on superpartner masses from upper bounds on the Higgs boson mass

The LHC is putting bounds on the Higgs boson mass. In this Letter we use those bounds to constrain the minimal supersymmetric standard model (MSSM) parameter space using the fact that, in supersymmetry, the Higgs mass is a function of the masses of sparticles, and therefore an upper bound on the Higgs mass translates into an upper bound for the masses for superpartners. We show that, although current bounds do not constrain the MSSM parameter space from above, once the Higgs mass bound improves big regions of this parameter space will be excluded, putting upper bounds on supersymmetry (SUSY) masses. On the other hand, for the case of split-SUSY we show that, for moderate or large tanβ, the present bounds on the Higgs mass imply that the common mass for scalars cannot be greater than 10(11) GeV. We show how these bounds will evolve as LHC continues to improve the limits on the Higgs mass.     

Search for Higgs bosons in SUSY cascades in CMS and dark matter with non-universal gaugino masses

In grand-unified theories (GUT), non-universal boundary conditions for the gaugino masses may arise at the unification scale and may affect the observability of the neutral MSSM Higgs bosons (h/H/A) at the LHC. The implications of such non-universal gaugino masses are investigated for Higgs boson production in the SUSY cascade decay chain , , , produced in pp interactions. In the singlet representation with universal gaugino masses only the light Higgs boson can be produced in this cascade with the parameter region of interest for us, while with non-universal gaugino masses heavy neutral MSSM Higgs boson production may dominate. The allowed parameter space in the light of the WMAP constraints on the cold dark-matter relic density is investigated in the above scenarios for gaugino mass parameters. We also demonstrate that combination of representations can give the required amount of dark matter in any point of the parameter space. In the non-universal case we show that heavy Higgs bosons can be detected in the cascade studied in parameter regions with the WMAP preferred neutralino relic density.     

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.     

Heavy charged Higgs boson production at next generation e^\pm\gamma colliders

We assess the potential of future electron-positron linear colliders operating in the mode in detecting charged Higgs bosons with mass around and larger than the top quark mass, using Compton back-scattered photons from laser light. We compare the pair production mode, , to a variety of channels involving only one charged Higgs scalar in the final state, such as the tree-level processes ( and ) and ( and ) as well as the loop-induced channel . We show that, when the charged Higgs boson mass is smaller than or comparable to half the collider energy, , single production cross sections are of the same size as the pair production rate, whereas, for charged Higgs boson masses larger than , all processes are heavily suppressed. In general, production cross sections of charged Higgs bosons via scatterings are smaller than those induced at an collider and the latter represents a better option to produce and analyse such particles. Received: 29 August 2001 / Published online: 23 November 2001     

The decayH 2 0 →gg within the Minimal Supersymmetric Standard Model

I present a detailed SUSY QCD calculation of the decay rate of the lightest Higgs bosonH ₂ ⁰ into two gluons, where all quarks and scalar quarks are taken within the relevant loop diagrams. I include the mixing of all the scalar partners of the left and right handed quarks and show that their contribution is more than several tens of per cent compared to the quark contribution in the MSSM for some SUSY parameter space. Furthermore the MSSM fermionic contribution is enhanced by several factors for large tanβ and large Higgs masses. As a result, the two gluon decay rate ofH ₂ ⁰ is much larger than the two gluon decay rate of an equal mass standard model Higgs boson. I further compare the decay mode ofH ₂ ⁰ →gg to the similar decay modes ofH ₂ ⁰ →cc andH ₂ ⁰ →bb including one loop QCD corrections and show that in some casesΓ(H ₂ ⁰ →gg) is even higher thanΓ(H ₂ ⁰ →cc), although still much smaller thanΓ(H ₂ ⁰ →bb).     

The Higgs mass beyond the CMSSM

The recent discovery of a new boson at the LHC, which resembles a SM-like Higgs boson with m _h =125 GeV, is starting to provide strong guidelines into SUSY model building. For instance, the identification of such a state with the lightest CP-even Higgs boson of the MSSM (h ⁰), requires large values of tanβ and/or heavy sfermions. One outcome of this result is the possibility to solve the SUSY flavor and CP problems by decoupling, which points towards some realization of Split-inspired SUSY scenarios, in which scalars are much heavier than gauginos and higgsinos. However, we argue here that the remaining Higgs bosons of the MSSM (H ⁰, A ⁰, H ^±) do not have to be as heavy as the sfermions, and having them with masses near the EW scale does not pose any conflict with current MSSM constraints. We discuss then some SUSY scenarios with heavy sfermions, from a bottom-up approach, which contain the full Higgs sector, as well as a possible dark matter candidate, with masses near the EW scale, and identify distinctive signals from these scenarios that could be searched at the LHC.     

Search for gauge mediated SUSY breaking topologies at \sqrt{{\mathrm s}} \sim 189 GeV

Searches for topologies characteristic of Gauge Mediated SUSY Breaking models (GMSB) are performed by analysing 173.6 of data collected at = 188.6 GeV with the ALEPH detector. These topologies include acoplanar photons, non-pointing single photon, acoplanar leptons, large impact parameter leptons, detached slepton decay vertices, heavy stable charged sleptons and four leptons plus missing energy final states. No evidence for these new phenomena is observed and limits on production cross sections and sparticle masses are derived. A scan of a minimal GMSB parameter space is performed and model dependent lower limits of about 45 GeV/ on the next-to-lightest supersymmetric particle (NLSP) mass and of about 9 TeV on the mass scale parameter are derived, independently of the NLSP lifetime. Received: 30 November 1999 / Published online: 14 April 2000     

Masses and couplings of the lightest Higgs bosons in (M + 1)SSM

We study the upper limits on the mass of the lightest and second lightest CP even Higgs bosons in the (M + 1)SSM, the MSSM extended by a gauge singlet. The dominant two loop contributions to the effective potential are included, which reduce the Higgs masses by GeV. Since the coupling R of the lightest Higgs scalar to gauge bosons can be small, we study in detail the relations between the masses and couplings of both lightest scalars. We present upper bounds on the mass of a ”strongly” coupled Higgs (R < 1/2) as a function of lower experimental limits on the mass of a ”weakly” coupled Higgs (R < 1/2). With the help of these results, the whole parameter space of the model can be covered by Higgs boson searches. Received: 7 September 1999 / Published online: 12 July 2002