114 research outputs found
An Elusive Z' Coupled to Beauty
By extending the standard gauge group to SU(3)_c \times SU(2)_L \times U(1)_Y
\times U(1)_X with X charges carried only by the third family we accommodate
the LEP measurement of R_b and predict a potentially measurable discrepancy in
A_{FB}^{b} in e^+e^- scattering and that D^0\bar{D}^0 mixing may be near its
experimental limit. The Z', which explicitly violates the GIM mechanism, can
nevertheless be naturally consistent with FCNC constraints. Direct detection of
the Z' is possible but challenging.Comment: 12 pages, plus 1 Postscript figure, uses revtex, Discussion of FCNC
extende
R_b and New Physics: A Comprehensive Analysis
We survey the implications for new physics of the discrepancy between the LEP
measurement of and its Standard Model prediction. Two broad classes of
models are considered: () those in which new Z\bbar b couplings arise at
tree level, through or -quark mixing with new particles, and ()
those in which new scalars and fermions alter the Z \bbar b vertex at one
loop. We keep our analysis as general as possible in order to systematically
determine what kinds of features can produce corrections to of the right
sign and magnitude. We are able to identify several successful mechanisms,
which include most of those which have been recently been proposed in the
literature, as well as some earlier proposals (\eg\ supersymmetric models). By
seeing how such models appear as special cases of our general treatment we are
able to shed light on the reason for, and the robustness of, their ability to
explain .Comment: 60 pages, 8 figures, plain tex, uses epsf. Final version to appear in
Phys. Rev. D; propgating sign error corrected in eqs. 78, 87, 88, 89, 98, and
107; results unchange
Constraints on the Universal Varying Yukawa Couplings: from SM-like to Fermiophobic
Varying the Standard Model (SM) fermion Yukawa couplings universally by a
generic positive scale factor (), we study the phenomenological fit to
the current available experimental results for the Higgs boson search at hadron
colliders. We point out that the Higgs production cross section and its decay
branching ratio to can be varied oppositely by to make
their product almost invariant. Thus, our scenario and the SM Higgs are
indistinguishable in the inclusive channel. The current
measurements on direct Yukawa coupling strength in the
channel are not precise enough to fix the scale factor . The most
promising is the vector-boson-fusion channel in which the CMS has already
observed possible suppression effect on the Yukawa couplings. Further more, the
global fit of the experimental data can get the optimal value by
introducing a suppression factor on the SM Yukawa couplings.Comment: 16 pages, 12 figures, 5 tables, update analysis is supplemente
Constraints on the parameters of the Left Right Mirror Model
We study some phenomenological constraints on the parameters of a left right
model with mirror fermions (LRMM) that solves the strong CP problem. In
particular, we evaluate the contribution of mirror neutrinos to the invisible Z
decay width (\Gamma_Z^{inv}), and we find that the present experimental value
on \Gamma_Z^{inv}, can be used to place an upper bound on the Z-Z' mixing angle
that is consistent with limits obtained previously from other low-energy
observables. In this model the charged fermions that correspond to the standard
model (SM) mix with its mirror counterparts. This mixing, simultaneously with
the Z-Z' one, leads to modifications of the \Gamma(Z --> f \bar{f}) decay
width. By comparing with LEP data, we obtain bounds on the standard-mirror
lepton mixing angles. We also find that the bottom quark mixing parameters can
be chosen to fit the experimental values of R_b, and the resulting values for
the Z-Z' mixing angle do not agree with previous bounds. However, this
disagreement disappears if one takes the more recent ALEPH data.Comment: 7 pages, 2 figures, REVTe
Spontaneous Breaking of Flavor Symmetry and Naturalness of Nearly Degenerate Neutrino Masses and Bi-maximal Mixing
The gauge model with flavor symmetry and three Higgs triplets is
studied. We show how the intriguing nearly degenerate neutrino mass and
bi-maximal mixing scenario comes out naturally after spontaneous breaking of
the symmetry. The hierarchy between the neutrino mass-squared differences,
which is needed for reconciling both solar and atmospheric neutrino data, is
naturally resulted from an approximate permutation symmetry. The model can also
lead to interesting phenomena on lepton-flavor violations via the
gauge interactions.Comment: 13 pages, latex, no figures, the version appearing in SCIENCE IN
CHINA (Series A), Vol.35 No.9 (2000
Resonant origin for density fluctuations deep within the Sun: helioseismology and magneto-gravity waves
We analyze helioseismic waves near the solar equator in the presence of
magnetic fields deep within the solar radiative zone. We find that reasonable
magnetic fields can significantly alter the shapes of the wave profiles for
helioseismic g-modes. They can do so because the existence of density gradients
allows g-modes to resonantly excite Alfven waves, causing mode energy to be
funnelled along magnetic field lines, away from the solar equatorial plane. The
resulting wave forms show comparatively sharp spikes in the density profile at
radii where these resonances take place. We estimate how big these waves might
be in the Sun, and perform a first search for observable consequences. We find
the density excursions at the resonances to be too narrow to be ruled out by
present-day analyses of p-wave helioseismic spectra, even if their amplitudes
were to be larger than a few percent. (In contrast it has been shown in
(Burgess et al. 2002) that such density excursions could affect solar neutrino
fluxes in an important way.) Because solar p-waves are not strongly influenced
by radiative-zone magnetic fields, standard analyses of helioseismic data
should not be significantly altered. The influence of the magnetic field on the
g-mode frequency spectrum could be used to probe sufficiently large
radiative-zone magnetic fields should solar g-modes ever be definitively
observed. Our results would have stronger implications if overstable solar
g-modes should prove to have very large amplitudes, as has sometimes been
argued.Comment: 18 pages, 6 figures; misprints correcte
Signature for heavy Majorana neutrinos in hadronic collisions
The production and decay of new possible heavy Majorana neutrinos are
analyzed in hadronic collisions. New bounds on the mixing of these particles
with standard neutrinos are estimated according to a fundamental representation
suggested by grand unified models. A clear signature for these Majorana
neutrinos is given by same-sign dileptons plus a charged weak vector boson in
the final state. We discuss the experimental possibilities for the future Large
Hadron Collider (LHC) at CERN.Comment: Latex2e(epsfig), 12 pages, 8 figures, to appear Physical Review
Constraint of the magnetic moment of the top quark
We derive a bound on the magnetic dipole moment of the top quark in the
context of the effective Lagrangian approach by using the values of the ratio
, and . We found
that the oblique corrections are more sensible than the vertex ones for this
moment.Comment: 6 pages, 3 figures, RevTe
Discovering a Light Higgs Boson with Light
We evaluate the prospects for detecting a non-standard light Higgs boson with
a significant branching ratio to two photons, in Run II of the Fermilab
Tevatron. We derive the reach for several channels: inclusive,
jet and jets. We present the expected Run II limits on
the branching ratio of as a function of the Higgs mass, for
the case of ``bosonic'', as well as ``topcolor'' Higgs bosons.Comment: 11 pages, LaTeX, 7 figures, 4 tables, uses aipproc2.sty, contributed
to the Physics at Run II Workshop, analysis redone with optimized cuts and
improved background estimate, references adde
Is Vtb=1 ?
The strongest constraint on Vtb presently comes from the 3 x 3 unitarity of
the CKM matrix, which fixes Vtb to be very close to one. If the unitarity is
relaxed, current information from top production at Tevatron still leaves open
the possibility that Vtb is sizably smaller than one. In minimal extensions of
the standard model with extra heavy quarks, the unitarity constraints are much
weaker and the EW precision parameters entail the strongest bounds on Vtb. We
discuss the experimental perspectives of discovering and identifying such new
physics models at the Tevatron and the LHC, through a precise measurement of
Vtb from the single top cross sections and by the study of processes where the
extra heavy quarks are produced.Comment: 19 pages, 8 figure
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