Neutrino Masses and Mixing, Quark-lepton Symmetry and Strong Right-handed Neutrino Hierarchy

Assuming the same form of all mass matrices as motivated by quark-lepton symmetry, we discuss conditions under which bi-large mixing in the lepton sector can be obtained with a minimal amount of fine tuning requirements for possible models. We assume hierarchical mass matrices, dominated by the 3-3 element, with off-diagonal elements much smaller than the larger neighboring diagonal element. Characteristic features of this scenario are strong hierarchy in masses of right-handed neutrinos, and comparable contributions of both lighter right-handed neutrinos to the resulting left-handed neutrino Majorana mass matrix. Due to obvious quark lepton symmetry, this approach can be embedded into grand unified theories. The mass of the lightest neutrino does not depend on details of a model in the leading order. The right-handed neutrino scale can be identified with the GUT scale in which case the mass of the lightest neutrino is given as (m_{top}^2/M_{GUT}) |U_{\tau 1}|^2.Comment: 7 page

Escaping the Large Fine Tuning and Little Hierarchy Problems in the Next to Minimal Supersymmetric Model and h-> aa Decays

We demonstrate that the NMSSM can have small fine tuning and modest light stop mass while still evading all experimental constraints. For small tan(beta) [large tan(beta)], the relevant scenarios are such that there is always (often) a SM-like Higgs boson that decays to two lighter -- possibly much lighter -- pseudoscalar Higgses.Comment: 4 pages, 4 figures, revised to conform with PRL published versio

Many Light Higgs Bosons in the NMSSM

The next-to-minimal supersymmetric model with a light doublet-like CP-odd Higgs boson and small $\tan \beta$ can satisfy all experimental limits on Higgs bosons even with light superpartners. In these scenarios, the two lightest CP-even Higgs bosons, \hi and \hii, and the charged Higgs boson, \hp, can all be light enough to be produced at LEP and yet have decays that have not been looked for or are poorly constrained by existing collider experiments. The channel \hi\to \ai\ai (where \ai is the lightest CP-odd boson and has mass below $2m_b$) with \ai\to \tau^+\tau^- or $2j$ is still awaiting LEP constraints for \mhi>86\gev or 82\gev, respectively. LEP data may also contain \epem\to \hii\ai events where \hii\to Z\ai is the dominant decay, a channel that was never examined. Decays of the charged Higgs bosons are often dominated by H^\pm \to W^{\pm (\star)} \ai with \ai \to gg,c \bar c, \tau^+ \tau^-. This is a channel that has so far been ignored in the search for t\to \hp b decays at the Tevatron. A specialized analysis might reveal a signal. The light \ai might be within the reach of $B$ factories via \Upsilon\to \gamma \ai decays. We study typical mass ranges and branching ratios of Higgs bosons in this scenario and compare these scenarios where the \ai has a large doublet component to the more general scenarios with arbitrary singlet component for the \ai.Comment: 15 pages, 47 figure

A Comparison of Mixed-Higgs Scenarios In the NMSSM and the MSSM

We study scenarios in the minimal and next-to minimal supersymmetric models in which the lightest CP-even Higgs boson can have mass below the 114 GeV standard model LEP limit by virtue of reduced ZZ coupling due to substantial mixing among the Higgs bosons. We pay particular attention to the size of corrections from superpartners needed for these scenarios to be viable and point to boundary conditions at large scales which lead to these scenarios while at the same time keeping electroweak fine tuning modest in size. We find that naturalness of electroweak symmetry breaking in the mixed-Higgs scenarios of both models points to the same region of soft supersymmetry breaking terms, namely those leading to large mixing in the stop sector at the electroweak scale, especially if we also require that the lightest CP-even Higgs explains the Higgs-like LEP events at about 98 GeV.Comment: 14 pages, 19 figure

The NMSSM Close to the R-symmetry Limit and Naturalness in h→aah \to aa Decays for m_a<2\mb

Dominant decay of a SM-like Higgs boson into particles beyond those contained in the minimal supersymmetric standard model has been identified as a natural scenario to avoid fine tuning in electroweak symmetry breaking while satisfying all LEP limits. In the simplest such extension, the next-to-minimal supersymmetric model, the lightest CP-even Higgs boson can decay into two pseudoscalars. In the scenario with least fine tuning the lightest CP-even Higgs boson has mass of order 100 GeV. In order to escape LEP limits it must decay to a pair of the lightest CP-odd Higgs bosons with Br(h -> aa)>.7 and m_a \tau^+ \tau^- or light quarks and gluons). The mass of the lightest CP-odd Higgs boson is controlled by the soft-trilinear couplings, A_\lambda(m_Z) and A_\kappa(m_Z). We identify the region of parameter space where this situation occurs and discuss how natural this scenario is. It turns out that in order to achieve m_a < 2 m_b with A_\lambda(m_Z), A_\kappa(m_Z) of order the typical radiative corrections, the required tuning of trilinear couplings need not be larger than 5-10 %. Further, the necessity for this tuning can be eliminated in specific SUSY breaking scenarios. Quite interestingly, Br(h -> aa) is typically above 70 % in this region of parameter space and thus an appropriately large value requires no additional tuning.Comment: 33 pages, 25 figure

New Vector Boson Near the Z-pole and the Puzzle in Precision Electroweak Data

We show that a Z' with suppressed couplings to the electron compared to the Z-boson, with couplings to the b-quark, and with a mass close to the mass of the Z-boson, provides an excellent fit to forward-backward asymmetry of the b-quark and R_b measured on the Z-pole and $\pm 2$ GeV off the Z-pole, and to A_e obtained from the measurement of left-right asymmetry for hadronic final states. It also leads to a significant improvement in the total hadronic cross section on the Z-pole and R_b measured at energies above the Z-pole. In addition, with a proper mass, it can explain the excess of $Zb\bar b$ events at LEP in the 90-105 GeV region of the $b\bar b$ invariant mass.Comment: 10 pages, 1 figur

Z' near the Z-pole

We present a fit to precision electroweak data in the standard model extended by an additional vector boson, Z', with suppressed couplings to the electron compared to the Z boson, with couplings to the b-quark, and with mass close to the mass of the Z boson. This scenario provides an excellent fit to forward-backward asymmetry of the b-quark measured on the Z-pole and \pm 2 GeV off the Z-pole, and to lepton asymmetry, A_e, obtained from the measurement of left-right asymmetry for hadronic final states, and thus it removes the tension in the determination of the weak mixing angle from these two measurements. It also leads to a significant improvement in the total hadronic cross section on the Z-pole and R_b measured at energies above the Z-pole. We explore in detail properties of the Z' needed to explain the data and present a model for Z' with required couplings. The model preserves standard model Yukawa couplings, it is anomaly free and can be embedded into grand unified theories. It allows a choice of parameters that does not generate any flavor violating couplings of the Z' to standard model fermions. Out of standard model couplings, it only negligibly modifies the left-handed bottom quark coupling to the Z boson and the 3rd column of the CKM matrix. Modifications of standard model couplings in the charged lepton sector are also negligible. It predicts an additional down type quark, D, with mass in a few hundred GeV range, and an extra lepton doublet, L, possibly much heavier than the D quark. We discuss signatures of the Z' at the Large Hadron Collider and calculate the Z'b production cross section which is the dominant production mechanism for the Z'.Comment: 26 pages, 18 figures, minor modifications, one fig. added, results unchange

Consistency of LEP Z+bZ+b-jets excess with an {\boldmath h→aah\to aa} Decay Scenario and Low-Fine-Tuning NMSSM Models

We examine the LEP limits for the $Zh\to Z+b$'s final state and find that the excess of observed events for \mh\sim 100\gev correlates well with there being an \mh\sim 100\gev Higgs boson with SM-like $ZZh$ coupling that decays partly via h\to b\anti b+\tauptaum [with \br(h\to b\anti b)\sim 0.08] but dominantly via $h\to aa$ [with \br(h\to aa)\sim 0.9], where \ma<2\mb so that a\to \tauptaum (or light quarks and gluons) decays are dominant. Scenarios of precisely this type arise in the Next-to-Minimal Supersymmetric Model for parameter choices yielding the lowest possible fine-tuning.Comment: 4 pages, 1 figure, 1 table, substantial modifications to reflect full LHWG analysis result

Probing NMSSM Scenarios with Minimal Fine-Tuning by Searching for Decays of the Upsilon to a Light CP-Odd Higgs Boson

Completely natural electroweak symmetry breaking is easily achieved in supersymmetric models if there is a SM-like Higgs boson, $h$, with m_h\lsim 100\gev. In the minimal supersymmetric model, such an $h$ decays mainly to b\anti b and is ruled out by LEP constraints. However, if the MSSM Higgs sector is expanded so that $h$ decays mainly to still lighter Higgs bosons, e.g. $h\to aa$, with $BR(h\to aa)>0.7$, and if $m_a<2m_b$, then the LEP constraints are satisfied. In this letter, we show that in the next-to-minimal supersymmetric model the above $h$ and $a$ properties (for the lightest CP-even and CP-odd Higgs bosons, respectively) imply a lower bound on BR(\Upsilon\to \gam a) that dedicated runs at present (and future) $B$ factories can explore.Comment: 4 pages, 2 figure

The NMSSM Solution to the Fine-Tuning Problem, Precision Electroweak Constraints and the Largest LEP Higgs Event Excess

We present an extended study of how the Next to Minimal Supersymmetric Model easily avoids fine-tuning in electroweak symmetry breaking for a SM-like light Higgs with mass in the vicinity of 100\gev, as beautifully consistent with precision electroweak data, while escaping LEP constraints due to the dominance of $h\to aa$ decays with $m_a<2m_b$ so that a\to \tauptaum or jets. The residual $\sim 10$ branching ratio for h\to b\anti b explains perfectly the well-known LEP excess at \mh\sim 100\gev. Details of model parameter correlations and requirements are discussed as a function $\tan(\beta)$. Comparisons of fine-tuning in the NMSSM to that in the MSSM are presented. We also discuss fine-tuning associated with scenarios in which the $a$ is essentially pure singlet, has mass m_a>30\gev, and decays primarily to \gam\gam leading to an h\to aa\to 4\gam Higgs signal.Comment: 26 pages, 37 figures, published version with minor text and reference improvement