40 research outputs found
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 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 ) with \ai\to \tau^+\tau^- or 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 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 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 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 events at
LEP in the 90-105 GeV region of the 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 -jets excess with an {\boldmath } Decay Scenario and Low-Fine-Tuning NMSSM Models
We examine the LEP limits for the '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 coupling that decays
partly via h\to b\anti b+\tauptaum [with \br(h\to b\anti b)\sim 0.08] but
dominantly via [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, , with m_h\lsim
100\gev. In the minimal supersymmetric model, such an decays mainly to
b\anti b and is ruled out by LEP constraints. However, if the MSSM Higgs
sector is expanded so that decays mainly to still lighter Higgs bosons,
e.g. , with , and if , then the LEP
constraints are satisfied. In this letter, we show that in the next-to-minimal
supersymmetric model the above and 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) 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 decays with so that
a\to \tauptaum or jets. The residual 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 . 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 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