Radiative Electroweak Symmetry Breaking Research Articles

Effects of the SO(10) D-term on Yukawa unification and unstable minima of the supersymmetric scalar potential

We study the effects of SO(10) D-terms on the allowed parameter space (APS) in models with t–b–τ and b–τ Yukawa unification. The former is allowed only for moderate values of the D-term, if very precise (⩽5%) unification is required. Next we constrain the parameter space by looking for different dangerous directions where the scalar potential may be unbounded from below (UFB1 and UFB3). The common trilinear coupling A0 plays a significant role in constraining the APS. For very precise t–b–τ Yukawa unification, −m16 ⪅ A0 ⪅ m16 can be probed at the LHC, where m16 is the common soft breaking mass for the sfermions. Moreover, an interesting mass hierarchy with very heavy sfermions but light gauginos, which is strongly disfavoured in models without D-terms, becomes fairly common in the presence of the D-terms. The APS exhibits interesting characteristics if m16 is not the same as the soft breaking mass m10 for the Higgs sector. In b–τ unification models with D-terms, the APS consistent with Yukawa unification and radiative electroweak symmetry breaking, increases as the UFB1 constraint becomes weaker. However, for A0 ⪅ 0, a stronger UFB3 condition still puts, for a given m16, a stringent upper bound on the common gaugino mass (m1/2) and a lower bound on m16 for a given m1/2. The effects of sign of μ on Yukawa unification and UFB constraints are also discussed.

Radiative electroweak symmetry breaking from a quasi-localized top quark

We consider 5D supersymmetric SU(3)× SU(2)× U(1) theories compactified at the TeV scale on S 1/ Z 2 with supersymmetry broken by boundary conditions. Localizing the top quark at a boundary of a fifth dimension by a bulk mass term M t , reduces the strength of radiative electroweak symmetry breaking. For M t R≈1–2, the natural value for the top and bottom squark masses are raised to 500–1200 GeV, and all other superpartners may have masses of the compactification scale, which has a natural range of 1/ R≃1.5–3.5 TeV. The superpartner masses depend only on 1/ R, and are precisely correlated amongst themselves and with the mass of the Higgs boson, which is lighter than 130 GeV.

Lepton flavour violation in the constrained MSSM with natural neutrino mass hierarchy

We present predictions for μ→ eγ and τ→ μγ in the CMSSM in which there is a natural hierarchy of neutrino masses resulting from the sequential dominance of three right-handed neutrinos. We perform a global analysis of this model in the ( m 0, M 1/2) plane, assuming radiative electroweak symmetry breaking, and including all observed laboratory data. We confirm that a large (small) τ→ μγ rate results from the dominant right-handed neutrino being the heaviest (lightest) one. We show that the μ→ eγ rate may determine the order of the subdominant neutrino Yukawa couplings in the flavour basis, but may also be sensitive to effects beyond the leading log and leading mass insertion approximations. We also show that the μ→ eγ rate is independent of θ 13, but measurement of this angle may determine a ratio of subdominant Yukawa couplings.

Two-loop corrections to radiative electroweak symmetry breaking in the MSSM

We study the O(α tα s+α t 2) two-loop corrections to the minimization conditions of the MSSM effective potential, providing compact analytical formulae for the Higgs tadpoles. We connect these results with the renormalization group running of the MSSM parameters from the grand unification scale down to the weak scale, and discuss the corrections to the Higgs mixing parameter μ and to the running CP-odd Higgs mass m A in various scenarios of gravity-mediated SUSY breaking. We find that the O(α tα s) and O(α t 2) contributions partially cancel each other in the minimization conditions. In comparison with the full one-loop corrections, the O(α tα s+α t 2) two-loop corrections significantly weaken the dependence of the parameters μ and m A on the renormalization scale at which the effective potential is minimized. The residual two-loop and higher-order corrections to μ and m A are estimated to be at most 1% in the considered scenarios.

Re-examination of electroweak symmetry breaking in supersymmetry and implications for light superpartners

We examine arguments that could avoid light superpartners as an implication of supersymmetric radiative electroweak symmetry breaking. We argue that, from the point of view of string theory and standard approaches to generating the μ term, cancellations among parameters are not a generic feature. While the coefficients relating MZ to parameters in the soft supersymmetry breaking Lagrangian can be made smaller, these same mechanisms lead to lighter superpartner masses at the electroweak scale. Consequently we strengthen the implication that gluinos, neutralinos, and charginos are light and likely to be produced at the Fermilab Tevatron and a linear collider.

Modular invariance, soft breaking, μ and tanβ in superstring models

We go beyond parameterizations of soft terms in superstring models and investigate the dynamical assumptions that lead to the relative strength of the dilaton vs. the moduli contributions in the soft breaking. Specifically, we discuss in some simple heterotic orbifold models sufficient conditions to achieve dilaton dominance. Assuming self-dual points to be minima we find multiple solutions to the trilinear and bilinear soft parameters A0 and B0. We discuss the constraints on μ and tanβ in superstring models in the context of radiative breaking of the electroweak symmetry. We show that string models prefer a small to a moderate value of tanβ, i.e., tanβ⩽10, and a value much larger than this requires a high degree of fine tuning. Further, we show that for large tanβ the radiative electroweak symmetry breaking constraint leads to a value αstring=gstring2/4π which is typically an order of magnitude smaller than implied by the LEP data and the heterotic superstring relation gstring=kigi, where gi is the gauge coupling constant for the gauge group Gi and ki is the corresponding Kac–Moody level in the class of models considered. This situation can be overcome by another fine-tuned cancellation between the dilaton and the moduli contributions in the soft parameters.

SOFTSUSY: A program for calculating supersymmetric spectra

SOFTSUSY is a program which accurately calculates the spectrum of superparticles in the Minimal Supersymmetric Standard Model (MSSM). The program solves the renormalisation group equations with theoretical constraints on soft supersymmetry breaking terms provided by the user. Weak-scale gauge coupling and fermion mass data (including one-loop finite MSSM corrections) are used as a boundary condition, as well as successful radiative electroweak symmetry breaking. The program can also calculate a measure of fine-tuning. The program structure has been designed to easily generalize to extensions of the MSSM . This article serves as a self-contained guide to prospective users, and indicates the conventions and approximations used. Sample results are compared with similar calculations in the literature.

Electroweak symmetry breaking and large extradimensions

In supersymmetric theories with large extradimensions a finite radiative electroweaksymmetry breaking can be triggered if the top/stopmultiplet propagates in the bulk of the extra dimensions. We haveconstructed a consistent supersymmetric five-dimensional model wherethe extra coordinate is compactified on the orbifold S1/ℤ2,with radius R, and exhibiting as generic features:large tan β≃mt/mb, a well defined pattern of Higgs and supersymmetricmass spectra, sneutrinos as the LSP and perturbative couplings for scalesas large as ~ 40/R. In order to understand thefiniteness of the Higgs mass, which requires thecontribution of KK-modes heavier than the cut-off of thefive-dimensional theory (Λ~Ms, the stringscale), we have introduced a string-motivatedregularization where those heavy KK-modes decoupleexponentially as exp {-M2KK/2Λ2}. The regularization does not spoil the five-dimensionalstructure and the finite result is recovered in the limit Λ→∞. We stress that a top in the bulk should lead toobservable phenomenological and experimental consequences.

Soft SUSY breaking, dilaton domination and intermediate scale string models

We present an analysis of the low-energy implications of an intermediate scale ~10^{11} GeV string theory. We mainly focus on the evolution of the physical parameters under the renormalisation group equations (RGEs) and find several interesting new features that differ from the standard GUT scale or Planck scale scenarios. We give a general discussion of soft supersymmetry breaking terms in type I theories and then investigate the renormalization group running. In the dilaton domination scenario, we present the sparticle spectra, analyzing constraints from charge and colour breaking, fine tuning and radiative electroweak symmetry breaking. We compare with the allowed regions of parameter space when the RGEs start running at the standard GUT or the intermediate scales, and find quite remarkably that the dilaton dominated supersymmetry breaking scenario, which is essentially ruled out from constraints on charge and colour breaking if the fundamental scale is close to the Planck mass, is allowed in a large region of parameter space if the fundamental scale is intermediate.

Yukawa unified supersymmetricSO(10)model: Cosmology, rare decays, and collider searches

It has recently been pointed out that viable sparticle mass spectra can be generated in Yukawa unified SO(10) supersymmetric grand unified models consistent with radiative breaking of electroweak symmetry. Model solutions are obtained only if $\tan\beta \sim 50$, $\mu <0$ and positive $D$-term contributions to scalar masses from SO(10) gauge symmetry breaking are used. In this paper, we attempt to systematize the parameter space regions where solutions are obtained. We go on to calculate the relic density of neutralinos as a function of parameter space. No regions of the parameter space explored were actually cosmologically excluded, and very reasonable relic densities were found in much of parameter space. Direct neutralino detection rates could exceed 1 event/kg/day for a $^{73}$Ge detector, for low values of GUT scale gaugino mass $m_{1/2}$. We also calculate the branching fraction for $b\to s \gamma$ decays, and find that it is beyond the 95% CL experimental limits in much, but not all, of the parameter space regions explored. However, recent claims have been made that NLO effects can reverse the signs of certain amplitudes in the $b\to s\gamma$ calculation, leading to agreement between theory and experiment in Yukawa unified SUSY models. For the Fermilab Tevatron collider, significant regions of parameter space can be explored via $b\bar{b}A$ and $b\bar{b}H$ searches. There also exist some limited regions of parameter space where a trilepton signal can be seen at TeV33. Finally, there exist significant regions of parameter space where direct detection of bottom squark pair production can be made, especially for large negative values of the GUT parameter $A_0$.

Two-body decays of the lightest stop in minimal supergravity with and without R-parity

We study the decays of the lightest top squark in supergravity models with and without R-parity. Using the simplest model with an effective explicit bilinear breaking of R-parity and radiative electroweak symmetry breaking we show that, below the threshold for decays into charginos $\tilde t_1\to c\chi^+_1$, the lightest stop decays mainly into third generation fermions, $\tilde t_1\to b\tau$ instead of the R-parity conserving mode $\tilde t_1\to c\chi^0_1$, even for tiny tau--neutrino mass values. Moreover we show that, even above the threshold for decays into charginos, the decay $\tilde t_1\to b\tau$ may be dominant. We study the role played by the universality of the boundary conditions on the soft supersymmetry breaking terms. This new decay mode $\tilde t_1\to b\tau$ as well as the cascades originated by the conventional $\tilde t_1\to c\chi^0_1$ decay followed by the R-parity violating neutralino decays can provide new signatures for stop production at LEP and the Tevatron.

Naturalness reach of the large hadron collider in minimal supergravity

We re-analyse the prospects of discovering supersymmetry at the LHC, in order to re-express coverage in terms of a fine-tuning parameter and to extend the analysis to scalar masses (m_0) above 2 TeV. We use minimal supergravity (mSUGRA) unification assumptions for the SUSY breaking parameters. The discovery reach at high m_0 is of renewed interest because this region has recently been found to have a focus point, leading to relatively low fine-tuning, and because it remains uncertain how much of the region can be ruled out due to lack of radiative electroweak symmetry breaking. The best fine tuning reach is found in a mono-leptonic channel, where for mu>0, A_0=0 and tan beta=10 (within the focus point region), and a top mass of 174 GeV, all points in mSUGRA with m_0 < 4000 GeV, with a fine tuning measure up to 210 (500) are covered by the search, where the definition of fine-tuning excludes (includes) the contribution from the top Yukawa coupling. Even for arbitrarily high m_0, mSUGRA can be discovered through gaugino events, provided the gaugino mass parameter M_1/2 < 460 GeV. In this region, the mono-leptonic channel still provides the best reach.

Yukawa unification, b→ sγ and bino–stau coannihilation

The minimal supersymmetric standard model with universal boundary conditions and “asymptotic” Yukawa unification is considered. The full one-loop effective potential for radiative electroweak symmetry breaking as well as the one-loop corrections to the charged Higgs boson, b-quark and τ-lepton masses are included. The CP-even Higgs boson masses are corrected to two-loops. The relic abundance of the lightest supersymmetric particle (bino) is calculated by including its coannihilations with the next-to-lightest supersymmetric particle (lightest stau) consistently with Yukawa unification. The branching ratio of b→ sγ is evaluated by incorporating all the applicable next-to-leading order QCD corrections. The bino-stau coannihilations reduce the bino relic abundance below the upper bound from cold dark matter considerations in a sizable fraction of the parameter space allowed by b→ sγ for μ>0. Thus, the μ>0 case, which also predicts an acceptable b-quark mass, is perfectly compatible with data.

Superparticle mass spectra fromSO(10)grand unified models with Yukawa coupling unification

We examine the spectrum of superparticles obtained from the minimal $\mathrm{SO}(10)$ grand unified model, where it is assumed the gauge symmetry breaking yields the minimal supersymmetric standard model (MSSM) as the effective theory at ${M}_{\mathrm{GUT}}\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{16}$ GeV. In this model, unification of Yukawa couplings implies a value of $\mathrm{tan}\ensuremath{\beta}\ensuremath{\sim}45--55.$ At such high values of $\mathrm{tan}\ensuremath{\beta},$ assuming universality of scalar masses, the usual mechanism of radiative electroweak symmetry breaking breaks down. We show that a set of weak scale sparticle masses consistent with radiative electroweak symmetry breaking can be generated by imposing non-universal GUT scale scalar masses consistent with universality within $\mathrm{SO}(10)$ plus extra D-term contributions associated with the reduction in rank of the gauge symmetry group when $\mathrm{SO}(10)$ spontaneously breaks to $\mathrm{SU}(3)\ifmmode\times\else\texttimes\fi{}\mathrm{SU}(2)\ifmmode\times\else\texttimes\fi{}U(1).$ We comment upon the consequences of the sparticle mass spectrum for collider searches for supersymmetry. One implication of $\mathrm{SO}(10)$ unification is that the light bottom squark can be by far the lightest of the squarks. This motivates a dedicated search for bottom squark pair production at $p\overline{p}$ and ${e}^{+}{e}^{\ensuremath{-}}$ colliders.

LargeCPphases and the cancellation mechanism in EDMs in SUSY, string, and brane models

We show that EDMs obey a simple approximate scaling under the transformation $m_0\to \lambda m_0,m_{{1/2}}\to \lambda m_{{1/2}}$ in the large $\mu$ region when $\mu$ itself obeys the same scaling, ie., $\mu \to \lambda \mu$. In the scaling region the knowledge of a single point in the MSSM parameter space where the cancellation in the EDMs occur allows one to generate a trajectory in the $m_0-m_{{1/2}}$ plane where the cancellation mechanism holds and the EDMs are small. We illustrate these results for MSSM with radiative electro-weak symmetry breaking constraints. We also discuss a class of D brane models based on Type IIB superstring compactifications which have non-universal phases in the gaugino mass sector and allow large CP violating phases consistent with the EDM constraints through the cancellation mechanism. The scaling in these D brane models and in a heterotic string model is also discussed.

Calculable sparticle masses with radiatively driven inverted mass hierarchy

Supersymmetric models with an inverted mass hierarchy (IMH: multi-TeV first and second generation matter scalars, and sub-TeV third generation scalars) can ameliorate problems arising from flavor changing neutral currents, CP violating phases and electric dipole moments, while at the same time satisfying conditions on naturalness. It has recently been shown that such an IMH can be generated radiatively, making use of infra-red fixed point properties of renormalization group equations given Yukawa coupling unification and suitable GUT scale boundary conditions on soft SUSY breaking masses. In these models, explicit spectra cannot be obtained due to problems implementing radiative electroweak symmetry breaking (REWSB). We show that use of SO(10) D-term contributions to scalar masses can allow REWSB to occur, while maintaining much of the desired IMH. A somewhat improved IMH is obtained if splittings are applied only to Higgs scalar masses.

Finite supersymmetric grand unified theory reexamined

We analyze the finite supersymmetric SU(5) grand unified model taking into account the threshold corrections at the grand unified theory and supersymmetry breaking scales and the problem of electroweak symmetry breaking which is particularly important in the case of large $\mathrm{tan}\ensuremath{\beta}.$ We find that there are still parameter regions where the low-energy experimental values are consistently reproduced and the Higgs potential parameters actually satisfy both constraints for large $\mathrm{tan}\ensuremath{\beta}$ and radiative electroweak symmetry breaking, provided that a new free parameter is introduced in the boundary condition of the Higgs soft-mass parameter, while it preserves the finiteness requirements.

BOTTOM-UP APPROACH TO B-PARAMETER IN MINIMAL SUPERSYMMETRIC STANDARD MODEL

We study μ and B-parameters in the minimal supersymmetric standard model (MSSM) based on the radiative electroweak symmetry breaking scenario using "bottom-up" approach and show how useful our approach is to select a phenomenologically viable model beyond the MSSM under the assumption that the underlying theory is a string model or a gauge-Yukawa unified gauge model.

Gauge-mediated supersymmetry breaking model with an extra singlet Higgs field

We study in some detail the next-to-minimal supersymmetric standard model with gauge mediation of supersymmetry breaking. We find that it is feasible to spontaneously generate values of the Higgs mass parameters $\mu$ and $B_\mu$ consistent with radiative electroweak symmetry breaking. The model has a phenomenologically viable particle spectrum. Messenger sneutrinos with mass in the range 6 to 25 TeV can serve as cold dark matter. It is also possible to evade the cosmological domain wall problem in this scenario.

Higgsinos in SUSY models with gaugino mass unification

In the MSSM, the assumptions of a common sfermion mass parameter m 0 and a common gaugino mass parameter m 1/2, along with the requirements from radiative electroweak symmetry breaking, lead to relatively large values of the Higgs mixing parameter μ, and consequently to a gaugino-like lightest neutralino χ ̃ 1 0 . Lifting the requirement that the Higgs mass parameters m H D and m H U unify with the sfermion masses is known to allow for smaller μ. We show that a μ parameter sufficiently small to yield a Higgsino-like neutralino χ ̃ 1 0 requires a precise adjustment of the Higgs mass parameter m H U . Consequently a gaugino-type neutralino is still preferred in SUSY models with gaugino mass unification.