Electroweak Symmetry Breaking and Bottom-Top Yukawa Unification

The condition of unification of gauge couplings in the minimal supersymmetric standard model provides successful predictions for the weak mixing angle as a function of the strong gauge coupling and the supersymmetric threshold scale. In addition, in some scenarios, e.g.\ in the minimal SO(10) model, the tau lepton and the bottom and top quark Yukawa couplings unify at the grand unification scale. The condition of Yukawa unification leads naturally to large values of $\tan\beta$, implying a proper top quark--bottom quark mass hierarchy. In this work, we investigate the feasibility of unification of the Yukawa couplings, in the framework of the minimal supersymmetric standard model with (assumed) universal mass parameters at the unification scale and with radiative breaking of the electroweak symmetry. We show that strong correlations between the parameters $\mu_0$ and $M_{1/2}$ appear within this scheme. These correlations have relevant implications for the sparticle spectrum, which presents several characteristic features. In addition, we show that due to large corrections to the running bottom quark mass induced through the supersymmetry breaking sector of the theory, the predicted top quark mass and $\tan\beta$ values are significantly lower than those previously estimated in the literature.Comment: 33 pages, 10 figures (not included) CERN-TH.7163/9

τ−μ−e\tau - \mu - e Universality in τ\tau Decays and Constraints on the Slepton Masses

The leptonic $\tau$ decays are calculated at the 1-loop level in the Minimal Supersymmetric Standard Model. The deviation from the $\tau - \mu - e$ universality is studied as a function of the supersymmetric parameters and discussed in the context of the expected improvement of the experimental accuracy.Comment: 10 pages, standard LaTeX, report DFPD 94/TH-27,MPI-Ph/94-24,DESY 94-077. 5 figures can be obtained via conventional mail from P. Chankowski upon reques

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

Impact of CP phases on a light sbottom and gluino sector

We study a scenario in which light bottom squarks and light gluinos with masses in the range $2-5.5$ GeV and 12-16 GeV, respectively, can coexist in the MSSM, without being in conflict with flavor-conserving low-energy observables. We find that in such a scenario, the anomalous magnetic moment of a muon could be as large as $10^{-9}$, if the theory conserves CP. However, if the theory violates CP, we conclude that not both, the gluino and bottom squark, can be light at the same time, after the neutron electric dipole moment constraint on Weinberg's 3-gluon operator has been taken into account.Comment: 10 pages, 3 figures, Revtex4, to be published in Phys. Lett.

Constraints on Supersymmetric Models from the Muon Anomalous Magnetic Moment

We study the impact of present and future $(g-2)_\mu$ measurements on supersymmetric models. The corrections to $(g-2)_\mu$ become particularly relevant in the presence of light sleptons, charginos and neutralinos, especially in the large $\tan\beta$ regime. For moderate or large values of $\tan\beta$, it is possible to rule out scenarios in which charginos and sneutrinos are both light, but nevertheless escape detection at the LEP2 collider. Furthermore, models in which supersymmetry breaking is transferred to the observable sector through gauge interactions can be efficiently constrained by the $(g-2)_{\mu}$ measurement.Comment: 15 pages, 4 figures, Late

Electroweak Symmetry Breaking with Non-Universal Scalar Soft Terms and Large tan⁡β\tan\beta Solutions

We discuss radiative electroweak symmetry breaking with non-universal scalar masses at the GUT scale. Large $\tan\beta$ solutions are investigated in detail and it is shown that qualitatively new (as compared to the universal case) solutions exist, with much less correlation between soft terms. We identify two classes of non-universalities which give solutions with $A_o\simeq B_o\simeq M_{1/2}\simeq O(M_Z)$, $\mu \simeq O(0.5 m_o)$, $m_o>>M_Z$ and $A_o\simeq B_o\simeq M_{1/2} \simeq\mu\simeq O(M_Z)$, $m_o\ge M_Z$, respectively. In each case, after imposing gauge and Yukawa coupling unification, we discuss the predictions for $m_t$, $m_b$ and the spectrum of supersymmetric particles. One striking consequence is the possibility of light charginos and neutralinos which in the second option can be higgsino-like. Cosmological constraint on the relic abundance of the lightest neutralino is also included.Comment: 14 pages, LaTeX, 6 figures (not included), MPI-PhT/94-4

Fermion Masses, Mixing Angles and Supersymmetric SO(10) Unification

We reanalyse the problem of fermion masses in supersymmetric SO(10) grand unified models. In the minimal model, both low energy Higgs doublets belong to the same {\bf{10}} representation of SO(10) implying the unification not only of the gauge but also of the third generation Yukawa couplings. These models predict large values of $\tan\beta \sim 50$. In this paper we study the effects of departing from the minimal conditions in order to see if we can find models with a reduced value of $\tan\beta$. In order to maintain predictability, however, we try to do this with the addition of only one new parameter. We still assume that the fermion masses arise from interactions of the spinor representations with a single ${\bf 10}$ representation, but this ${\bf 10}$ now only contains a part of the two light Higgs doublets. This enables us to introduce one new parameter $\omega=\lambda_b/\lambda_t$. For values of $\omega \ll 1$ we can in principle reduce the value of $\tan\beta$. In fact, $\omega$ is an overall factor which multiplies the down quark and charged lepton Yukawa matrices. Thus the theory is still highly constrained. We show that the first generation quark masses and the CP-violation parameter $\epsilon_K$ yield strong constraints on the phenomenologically allowed models. In the end, we find that large values of $\tan\beta$ are still preferred.Comment: 15 pages, latex, 6 uuencoded figure

Muon anomalous magnetic moment in the standard model with two Higgs doublets

The muon anomalous magnetic moment is investigated in the standard model with two Higgs doublets (S2HDM) motivated from spontaneous CP violation. Thus all the effective Yukawa couplings become complex. As a consequence of the non-zero phase in the couplings, the one loop contribution from the neutral scalar bosons could be positive and negative relying on the CP phases. The interference between one and two loop diagrams can be constructive in a large parameter space of CP-phases. This will result in a significant contribution to muon anomalous magnetic moment even in the flavor conserving process with a heavy neutral scalar boson ($m_h \sim$ 200 GeV) once the effective muon Yukawa coupling is large ($|\xi_\mu|\sim 50$). In general, the one loop contributions from lepton flavor changing scalar interactions become more important. In particular, when all contributions are positive in a reasonable parameter space of CP phases, the recently reported 2.6 sigma experiment vs. theory deviation can be easily explained even for a heavy scalar boson with a relative small Yukawa coupling in the S2HDM.Comment: 8 pages, RevTex file, 5 figures, published version Phys. Rev. D 54 (2001) 11501

Higgs and Supersymmetric Particle Signals at the Infrared Fixed Point of the Top Quark Mass

We study the properties of the Higgs and supersymmetric particle spectrum associated with the infrared fixed point solution of the top quark mass in the MSSM. We concentrate on the possible detection of these particles, analysing the deviations from the Standard Model predictions for the leptonic and hadronic variables measured at LEP and for the decay rate $b\rightarrow s\gamma$. We consider the low and moderate $\tan \beta$ regime, and we study both, the cases of universal and non--universal soft supersymmetry breaking parameters at high energies. In the first case, for any given value of the top quark mass, the Higgs and sparticle spectra are completely determined as a function of two soft supersymmetry breaking parameters. In the case of non--universality, instead, the strong correlations between the sparticle masses are relaxed, allowing a richer structure for the precision data variables. We show, however, that the requirement that the low energy theory proceeds from a grand unified theory with a local symmetry group including $SU(5)$, strongly constrains the set of possible indirect experimental signatures. As a general feature, whenever a significant deviation from the Standard Model value of the precision data variables is predicted, a light sparticle, visible at LEP2, appears in the spectrum.Comment: 42 pages, 16 figures (not included), CERN-TH.7393/9

Viable tt-bb-τ\tau Yukawa Unification in SUSY SO(10)

The supersymmetric SO(10) GUT with $t$--$b$--$\tau$ Yukawa coupling unification has problems with correct electroweak symmetry breaking, experimental constraints (especially $b\rightarrow s\gamma$) and neutralino abundance, if the scalar masses are universal at the GUT scale. We point out that non-universality of the scalar masses at the GUT scale generated both by (1) renormalization group running from the Planck scale to the GUT scale and (2) $D$--term contribution induced by the reduction of the rank of the gauge group, has a desirable pattern to make the model phemenologically viable (in fact the only one which is consistent with experimental and cosmological constraints). At the same time the top quark mass has to be either close to its quasi IR--fixed point value or below $\sim$170 GeV. We also briefly discuss the spectrum of superpartners which is then obtained.Comment: 11 pages LaTeX, 2 PS figures as uuencoded tar-gzipped fil