The infrared fixed point of the top quark mass and its implications within the MSSM

We analyse the general features of the Higgs and supersymmetric particle spectrum associated with the infrared fixed point solution of the top quark mass in the Minimal Supersymmetric Standard Model. We consider the constraints on the mass parameters, which are derived from the condition of a proper radiative electroweak symmetry breaking in the low and moderate \tan\beta regime. In the case of universal soft supersymmetry breaking parameters at the high energy scale, the radiative SU(2)_L \times U(1)_Y breaking, together with the top quark Yukawa fixed point structure imply that, for any given value of the top quark mass, the Higgs and supersymmetric particle spectrum is fully determined as a function of only two supersymmetry breaking parameters. We show that, for the interesting range of top quark mass values M_t\simeq 175\pm 10 GeV, both a light chargino and a light stop may be present in the spectrum. In addition, for a given top quark mass, the infrared fixed point solution of the top quark Yukawa coupling minimizes the value of the lightest CP-even Higgs mass m_h. The resulting upper bounds on m_h read m_h \leq 90 \;(105) \;(120) GeV for M_t \leq 165 \;(175) \;(185) GeV

Soft supersymmetry breaking parameters and minimal SO(10) unification

The minimal supersymmetric SO(10) model, in which not only the gauge but also the third generation fermion Yukawa couplings are unified, provides a simple and highly predictive theoretical scenario for the understanding of the origin of the low energy gauge interactions and fermion masses. In the framework of the MSSM, with universal soft supersymmetry breaking parameters at the grand unification scale, large values of the universal gaugino mass M_{1/2}\geq 300 GeV are needed in order to induce a proper breakdown of the electroweak symmetry. In addition, in order to obtain acceptable experimental values for both the pole bottom mass and the b\rightarrow s\gamma decay rate, even larger values of the gaugino masses are required. The model is strongly constrained by theoretical and phenomenological requirements and a heavy top quark, with mass M_t\geq 170 GeV, is hard to accomodate within this scheme. However, we show that to accommodate a top quark mass M_t\simeq180 GeV, it is sufficient to relax the condition of universality of the scalar soft SUSY breaking parameters at the grand unification scale. Still, the requirement of a heavy top quark demands a very heavy squark spectrum, unless specific relations between the soft SUSY breaking parameters are fulfilled.The minimal supersymmetric SO(10) model, in which not only the gauge but also the third generation fermion Yukawa couplings are unified, provides a simple and highly predictive theoretical scenario for the understanding of the origin of the low energy gauge interactions and fermion masses. In the framework of the MSSM, with universal soft supersymmetry breaking parameters at the grand unification scale, large values of the universal gaugino mass $M_{1/2}\geq 300$ GeV are needed in order to induce a proper breakdown of the electroweak symmetry. In addition, in order to obtain acceptable experimental values for both the pole bottom mass and the $b\rightarrow s\gamma$ decay rate, even larger values of the gaugino masses are required. The model is strongly constrained by theoretical and phenomenological requirements and a heavy top quark, with mass $M_t\geq 170$ GeV, is hard to accomodate within this scheme. However, we show that to accommodate a top quark mass $M_t\simeq180$ GeV, it is sufficient to relax the condition of universality of the scalar soft SUSY breaking parameters at the grand unification scale. Still, the requirement of a heavy top quark demands a very heavy squark spectrum, unless specific relations between the soft SUSY breaking parameters are fulfilled

Light higgsino detection at LEP1.5

Within the minimal supersymmetric extension of the Standard Model, the best fit to the most recent precision-measurement data requires charginos and neutralinos, with dominant Higgsino components and with masses within the reach of LEP1.5 (\sqrt{s}=140 GeV). In this work, we present a detailed analysis of the neutralino and chargino production processes for the favoured region of parameter space, that is low values of |\mu| and either low or large values of \tan\beta. We find that chargino and neutralino searches can cover the Higgsino region in the (\mu,M_2) plane for values of M_2 \simlt 1~TeV, at the next phases of the LEP collider. We also show that, due mainly to phase-space constraints, the lightest neutralinos should be more easily detectable than charginos in most of the parameter space preferred by precision-measurement data.Within the minimal supersymmetric extension of the Standard Model, the best fit to the most recent precision-measurement data requires charginos and neutralinos, with dominant Higgsino components and with masses within the reach of LEP1.5 ($\sqrt{s}=140$ GeV). In this work, we present a detailed analysis of the neutralino and chargino production processes for the favoured region of parameter space, that is low values of $|\mu|$ and either low or large values of $\tan\beta$. We find that chargino and neutralino searches can cover the Higgsino region in the ($\mu,M_2$) plane for values of M_2 \simlt 1TeV, at the next phases of the LEP collider. We also show that, due mainly to phase-space constraints, the lightest neutralinos should be more easily detectable than charginos in most of the parameter space preferred by precision-measurement data.Within the minimal supersymmetric extension of the Standard Model, the best fit to the most recent precision-measurement data requires charginos and neutralinos, with dominant Higgsino components and with masses within the reach of LEP1.5 ( S =140 GeV ). In this work, we present a detailed analysis of the neutralino and chargino production processes for the favoured region of parameter space, that is low values of |μ| and either low or large values of tan β. We find that chargino and neutralino searches can cover the Higgsino region in the (μ, M 2 ) plane for values of M 2 ≲ 1 TeV, at the next phases of the LEP collider. We also show that, due mainly to phase-space constraints, the lightest neutralinos should be more easily detectable than charginos in most of the parameter space preferred by precision-measurement data

Higgs-Boson Pole Masses in the MSSM with Explicit CP Violation

Extending previous results obtained in the effective-potential approach, we derive analytic expressions for the pole masses of the charged and neutral Higgs bosons in the minimal supersymmetric standard model (MSSM) with explicit CP violation. In such a minimal supersymmetric model, the CP invariance of the Higgs potential is explicitly broken by quantum effects dominated by third-generation squarks at the one-loop level and by one-loop gluino- and higgsino-mediated threshold corrections to the top- and bottom-quark Yukawa couplings at the two-loop level. Field-theoretic issues arising in the description of a two-level Higgs-boson system strongly mixed due to CP violation are addressed. Numerical results for the pole masses of the CP-violating Higgs bosons are presented, and comparisons are made with previous results obtained in the effective-potential approach.Extending previous results obtained in the effective-potential approach, we derive analytic expressions for the pole masses of the charged and neutral Higgs bosons in the minimal supersymmetric standard model (MSSM) with explicit CP violation. In such a minimal supersymmetric model, the CP invariance of the Higgs potential is explicitly broken by quantum effects dominated by third-generation squarks at the one-loop level and by one-loop gluino- and higgsino-mediated threshold corrections to the top- and bottom-quark Yukawa couplings at the two-loop level. Field-theoretic issues arising in the description of a two-level Higgs-boson system strongly mixed due to CP violation are addressed. Numerical results for the pole masses of the CP-violating Higgs bosons are presented, and comparisons are made with previous results obtained in the effective-potential approach

Photon Signatures for Low Energy Supersymmetry Breaking and Broken R-parity

The possible phenomenological consequences of R-parity violating interactions in the framework of low energy supersymmetry breaking are studied. It is pointed out that even very weak R-parity violation would completely overshadow one of the basic signatures of low energy supersymmetry breaking models, that is, the decay of the next to lightest supersymmetric particle into a photon (lepton) and missing energy. Thus, the observation of these decays would put very strong limits on R-parity violating couplings. Vice-versa, if R-parity violation is established experimentally, before a detailed knowledge of the spectrum is obtained, it will be very difficult to distinguish gravity mediated from low energy gauge mediated supersymmetry breaking. Those conclusions are very model independent. We also comment on the possibility of mixing between charged and neutral leptons with charginos and neutralinos, respectively, and its phenomenological consequences for the photon (lepton) signatures, in scenarios where this mixing is generated by the presence of bilinear or trilinear R-parity violating terms in the superpotential.Comment: 12 pages, Late

The Higgs Boson Mass as a Probe of the Minimal Supersymmetric Standard Model

Recently, the LEP collaborations have reported a lower bound on a Standard Model-like Higgs boson of order 89 GeV. We discuss the implications of this bound for the minimal supersymmetric extension of the Standard Model (MSSM). In particular, we show that the lower bound on $\tan\beta$, which can be obtained from the presently allowed Higgs boson mass value, becomes stronger than the one set by the requirement of perturbative consistency of the theory up to scales of order $M_{GUT}$ (associated with the infrared fixed-point solution of the top quark Yukawa coupling) in a large fraction of the allowed parameter space. The potentiality of future LEP2 searches to further probe the MSSM parameter space is also discussed.Comment: 15 pages, 5 figures, LateX, psfi

The τ\tau neutrino as a Majorana particle

A Majorana mass term for the $\tau$ neutrino would induce neutrino - antineutrino mixing and thereby a process which violates fermion number by two units. We study the possibility of distinguishing between a massive Majorana and a Dirac $\tau$ neutrino, by measuring fermion number violating processes in a deep inelastic scattering experiment $\nu p \rightarrow \tau X$. We show that, if the neutrino beam is obtained from the decay of high energetic pions, the probability of obtaining "wrong sign" $\tau$ leptons is suppressed by a factor ${\cal{O}}(m_{\nu_{\tau}}^2 \theta^2/m_{\mu}^2)$ instead of the naively expected suppression factor $\theta^2 m_{\nu_{\tau}}^2/E_{\nu}^2$, where $E_{\nu}$ is the $\tau$ neutrino energy, $m_{\nu_{\tau}}$ and $m_{\mu}$ are the $\tau$-neutrino and muon masses, respectively, and $\theta$ is the $\nu_{\mu}$ - $\nu_{\tau}$ mixing angle. If $m_{\nu_{\tau}}$ is of the order of 10 MeV and $\theta$ is of the order of $0.01 - 0.04$ (the present bounds are ($m_{\nu_{\tau}} < 35 MeV, \theta < 0.04$) the next round of experiments may be able to distinguish between Majorana and Dirac $\tau$-neutrinos.Comment: 14 pages, 4 figures (not included), MPI-Ph/93-4

Constraints on B and Higgs Physics in Minimal Low Energy Supersymmetric Models

We study the implications of minimal flavor violating low energy supersymmetry scenarios for the search of new physics in the B and Higgs sectors at the Tevatron collider and the LHC. We show that the already stringent Tevatron bound on the decay rate B_s -> mu+ mu- sets strong constraints on the possibility of generating large corrections to the mass difference Delta M_s of the B_s eigenstates. We also show that the B_s -> mu+ mu- bound together with the constraint on the branching ratio of the rare decay b -> s gamma has strong implications for the search of light, non-standard Higgs bosons at hadron colliders. In doing this, we demonstrate that the former expressions derived for the analysis of the double penguin contributions in the Kaon sector need to be corrected by additional terms for a realistic analysis of these effects. We also study a specific non-minimal flavor violating scenario, where there are flavor changing gluino-squark-quark interactions, governed by the CKM matrix elements, and show that the B and Higgs physics constraints are similar to the ones in the minimal flavor violating case. Finally we show that, in scenarios like electroweak baryogenesis which have light stops and charginos, there may be enhanced effects on the B and K mixing parameters, without any significant effect on the rate of B_s -> mu+ mu-.Comment: 40 pages, 14 figures; added references and note about recent measurement

Brane Effects on Extra Dimensional Scenarios: A Tale of Two Gravitons

We analyze the propagation of a scalar field in multidimensional theories which include kinetic corrections in the brane, as a prototype for gravitational interactions in a four dimensional brane located in a (nearly) flat extra dimensional bulk. We regularize the theory by introducing an infrared cutoff given by the size of the extra dimensions and a physical ultraviolet cutoff of the order of the fundamental Planck scale in the higher dimensional theory. We show that, contrary to recent suggestions, the radius of the extra dimensions cannot be arbitrarily large. Moreover, for finite radii, the gravitational effects localized on the brane can substantially alter the phenomenology of collider and/or table-top gravitational experiments. This phenomenology is dictated by the presence of a massless graviton, with standard couplings to the matter fields, and a massive graviton which couples to matter in a much stronger way. While graviton KK modes lighter than the massive graviton couple to matter in a standard way, the couplings to matter of the heavier KK modes are strongly suppressed.Comment: 21 pages, latex2e, axodraw.sty, 2 figure

Light Higgsino Detection at LEP1.5

Within the minimal supersymmetric extension of the Standard Model, the best fit to the most recent precision-measurement data requires charginos and neutralinos, with dominant Higgsino components and with masses within the reach of LEP1.5 ($\sqrt{s}=140$ GeV). In this work, we present a detailed analysis of the neutralino and chargino production processes for the favoured region of parameter space, that is low values of $|\mu|$ and either low or large values of $\tan\beta$. We find that chargino and neutralino searches can cover the Higgsino region in the ($\mu,M_2$) plane for values of M_2 \simlt 1 TeV, at the next phases of the LEP collider. We also show that, due mainly to phase-space constraints, the lightest neutralinos should be more easily detectable than charginos in most of the parameter space preferred by precision-measurement data.Comment: 16 pages, 8 Figures, LateX. Figures now include initial state radiation effects on the cross sections