3 research outputs found
Resolving the A_{FB}^b puzzle in an extra dimensional model with an extended gauge structure
It is notorious that, contrary to all other precision electroweak data, the
forward-backward asymmetry for b quarks measured in Z decays at LEP1
is nearly three standard deviations away from the predicted value in the
Standard Model; significant deviations also occur in measurements of the
asymmetry off the Z pole. We show that these discrepancies can be resolved in a
variant of the Randall-Sundrum extra-dimensional model in which the gauge
structure is extended to to allow for
relatively light Kaluza-Klein excitations of the gauge bosons. In this
scenario, the fermions are localized differently along the extra dimension, in
order to generate the fermion mass hierarchies, so that the electroweak
interactions for the heavy third generation fermions are naturally different
from the light fermion ones. We show that the mixing between the Z boson with
the Kaluza-Klein excitations allows to explain the anomaly without
affecting (and even improving) the agreement of the other precision
observables, including the partial decay width, with experimental
data. Some implications of this scenario for the ILC are summarized.Comment: 23 pages, 5 figure
The precision electroweak data in warped extra-dimension models
The Randall-Sundrum scenario with Standard Model fields in the bulk and a
custodial symmetry is considered. We determine the several minimal quark
representations allowing to address the anomalies in the forward-backward
b-quark asymmetry A^b_FB, while reproducing the bottom and top masses via wave
function overlaps. The calculated corrections of the Zbb coupling include the
combined effects of mixings with both Kaluza-Klein excitations of gauge bosons
and new b'-like states. It is shown that the mechanism, in which the
left-handed doublet of third generation quarks results from a mixing on the UV
boundary of introduced fields Q_1L and Q_2L, is necessary for phenomenological
reasons. Within the obtained models, both the global fit of R_b with A^b_FB [at
the various center of mass energies] and the fit of last precision electroweak
data in the light fermion sector can simultaneously be improved significantly
with respect to the pure Standard Model case, for M_KK = 3,4,5 TeV (first KK
gauge boson) and a best-fit Higgs mass m_h > 115 GeV i.e. compatible with the
LEP2 direct limit. The quantitative analysis of the oblique parameters S,T,U
even shows that heavy Higgs mass values up to ~500 GeV may still give rise to
an acceptable quality of the electroweak data fit, in contrast with the
Standard Model. The set of obtained constraints on the parameter space, derived
partly from precision electroweak data, is complementary of a future direct
exploration of this parameter space at the LHC. In particular, we find that
custodians, like b' modes, can be as light as ~1200 GeV i.e. a mass lying
possibly in the potential reach of LHC.Comment: 24 pages, 8 figures. Added references, corrected typos and Higgs mass
dependence discussion complete