193 research outputs found
Fun with Higgsless Theories
Motivated by recent works on ``Higgsless theories,'' I discuss an
gauge theory with arbitrary bifundamental
(or custodial SU(2) preserving) symmetry breaking between the gauge subgroups
and with ordinary matter transforming only under the U(1) and . When
the couplings, , of the other SU(2)s are very large, this reproduces the
standard model at the tree level. I calculate the and masses and other
electroweak parameters in a perturbative expansion in , and give
physical interpretations of the results in a mechanical analog built out of
masses and springs. In the mechanical analog, it is clear that even for
arbitrary patterns of symmetry breaking, it is not possible (in the
perturbative regime) to raise the Higgs mass by a large factor while keeping
the parameter small.Comment: 20 pages, 5 figures, LaTeX - another referenc
WW Scattering in Walking Technicolor
We analyze the WW scattering in scenarios of dynamical electroweak symmetry
breaking of walking technicolor type. We show that in these theories there are
regions of the parameters space allowed by the electroweak precision data, in
which unitarity violation is delayed at tree level up to around 3-4 TeV without
the inclusion of any sub-TeV resonances.Comment: 2 Columns RevTeX, 10 pages, 9 Figure
Constraints on Little Higgs with Fully-Radiative Electroweak Symmetry Breaking
In a recent paper, we introduced a new Little Higgs model, which contains the
gauge structure , embedded in an approximate global
symmetry. After breaking to the standard model, , this produces two heavy bosons and two heavy
bosons, along with a single Standard Model-like Higgs scalar.
The unique feature of the model was that it was possible to obtain electroweak
symmetry breaking and a light Higgs mass entirely from perturbative loop
contributions to the Higgs effective potential. In this paper we consider the
electroweak constraints on this model, including tree and loop contributions to
the universal oblique and non-oblique parameters, tree-level corrections to the
vertex, and tree and loop level corrections to . The most
significant corrections are positive tree-level corrections to and
negative fermion-loop corrections to , which require that the scale
for the global symmetry breaking be TeV, depending on the top-quark
mixing parameter and the extra gauge couplings. In addition, the loop
corrections to contain a divergence that must be absorbed into the
coefficient of a new operator in the theory. The finite part of this
correction, however, is negligible.Comment: 28 pages, 16 figures, RevTeX forma
Chiral-logarithmic Corrections to the S and T Parameters in Higgsless Models
Recently, Higgsless models have proven to be viable alternatives to the
Standard Model (SM) and supersymmetric models in describing the breaking of the
electroweak symmetry. Whether extra-dimensional in nature or their
deconstructed counterparts, the physical spectrum of these models typically
consists of ``towers'' of massive vector gauge bosons which carry the same
quantum numbers as the SM W and Z. In this paper, we calculate the one-loop,
chiral-logarithmic corrections to the S and T parameters from the lightest
(i.e. SM) and the next-to-lightest gauge bosons using a novel application of
the Pinch Technique. We perform our calculation using generic Feynman rules
with generic couplings such that our results can be applied to various models.
To demonstrate how to use our results, we calculate the leading
chiral-logarithmic corrections to the S and T parameters in the deconstructed
three site Higgsless model. As we point out, however, our results are not
exclusive to Higgsless models and may, in fact, be used to calculate the
one-loop corrections from additional gauge bosons in models with fundamental
(or composite) Higgs bosons.Comment: 45 pages, 15 figures, added references, analysis of three site model
expanded to include delocalized fermion
Direct Search Implications for a Custodially-Embedded Composite Top
We assess current experimental constraints on the bi-doublet + singlet model
of top compositeness previously proposed in the literature. This model extends
the standard model's spectrum by adding a custodially-embedded vector-like
electroweak bi-doublet of quarks and a vector-like electroweak singlet quark.
While either of those states alone would produce a model in tension with
constraints from precision electroweak data, in combination they can produce a
viable model. We show that current precision electroweak data, in the wake of
the Higgs discovery, accommodate the model and we explore the impact of direct
collider searches for the partners of the top quark.Comment: 12 pages, 2 figures (updated figures to show sin(beta) of 0.55 rather
than 0.6, to be more informative to the reader)(second update fixes a figure
format issue in Fig 1f
Global Symmetries and Renormalizability of Lee-Wick Theories
In this paper we discuss the global symmetries and the renormalizibility of
Lee-Wick scalar QED. In particular, in the "auxiliary-field" formalism we
identify softly broken SO(1,1) global symmetries of the theory. We introduce
SO(1,1) invariant gauge-fixing conditions that allow us to show in the
two-field formalism directly that the number of superficially divergent
amplitudes in a LW Abelian gauge theory is finite. To illustrate the
renormalizability of the theory, we explicitly carry out the one-loop
renormalization program in LW scalar QED and demonstrate how the counterterms
required are constrained by the joint conditions of gauge- and
SO(1,1)-invariance. We also compute the one-loop beta-functions in LW scalar
QED and contrast them with those of ordinary scalar QED.Comment: 17 pages, 3 eps figures included. Incorporates suggestions by
referee; title change
The Limits of Custodial Symmetry
We introduce a toy model implementing the proposal of using a custodial
symmetry to protect the Zbb coupling from large corrections. This
"doublet-extended standard model" adds a weak doublet of fermions (including a
heavy partner of the top quark) to the particle content of the standard model
in order to implement an O(4) x U(1)_X = SU(2)_L x SU(2)_R x P_{LR} x U(1)_X
symmetry that protects the Zbb coupling. This symmetry is softly broken to the
gauged SU(2)_L x U(1)_Y electroweak symmetry by a Dirac mass M for the new
doublet; adjusting the value of M allows us to explore the range of
possibilities between the O(4)-symmetric (M to 0) and standard-model-like (M to
infinity) limits. In this simple model, we find that the experimental limits on
the Zbb coupling favor smaller M while the presence of a potentially sizable
negative contribution to T strongly favors large M. A fit to all precision
electroweak data shows that the heavy partner of the top quark must be heavier
than about 3.4 TeV, making it difficult to search for at LHC. This result
demonstrates that electroweak data strongly limits the amount by which the
custodial symmetry of the top-quark mass generating sector can be enhanced
relative to the standard model. Using an effective field theory calculation, we
illustrate how the leading contributions to alpha T, alpha S and the Zbb
coupling in this model arise from an effective operator coupling right-handed
top-quarks to the Z-boson, and how the effects on these observables are
correlated. We contrast this toy model with extra-dimensional models in which
the extended custodial symmetry is invoked to control the size of additional
contributions to alpha T and the Zbb coupling, while leaving the standard model
contributions essentially unchanged.Comment: 19 pages, 11 eps figures. Typos correcte
Custodial Isospin Violation in the Lee-Wick Standard Model
We analyze the tension between naturalness and isospin violation in the
Lee-Wick Standard Model (LW SM), by computing tree-level and fermionic one-loop
contributions to the post-LEP electroweak parameters and the Zbb coupling. The
model is most natural when the LW partners of the gauge bosons and fermions are
light, but small partner masses can lead to large isospin violation. The
post-LEP parameters yield a simple picture in the LW SM: the gauge sector
contributes to Y and W only, with leading contributions arising at tree-level,
while the fermion sector contributes to S-hat and T-hat only, with leading
corrections arising at one loop. Hence, W and Y constrain the masses of the LW
gauge bosons to satisfy M1, M2 > 2.4 TeV at 95% CL. Likewise, experimental
limits on T-hat reveal that the masses of the LW fermions must satisfy Mq, Mt >
1.6 TeV at 95% CL if the Higgs mass is light and tend to exclude the LW SM for
any LW fermion masses if the Higgs mass is heavy. Contributions from the
top-quark sector to the Zbb coupling can be even more stringent, placing a
lower bound of 4 TeV on the LW fermion masses at 95% CL.Comment: 16 pages, 8 embedded eps figure
Recommended from our members
Patterns of custodial isospin violation from a composite top
In this paper we consider the effects of top-quark compositeness on the electroweak parameters T^ and S^ and the ZbLb̄L coupling. We do so by using an effective field theory analysis to identify several promising patterns of mixing between standard-model-like and vector fermions, and then analyzing simple extensions of the standard model that realize those patterns. These models illustrate four ways in which an extended O(4) symmetry, which controls the size of radiative corrections to the observables discussed, may be broken. These models may also be viewed as highly deconstructed versions of five-dimensional gauge theories dual to various strongly interacting composite Higgs theories. We comment on how our results relate to extra-dimensional models previously considered, and we demonstrate that one pattern of O(4) breaking is phenomenologically favored. © 2011 American Physical Society
- …