2,830 research outputs found
About heavy neutrinos: Lepton-flavor violation in decays of charged leptons
The fundamental description of nature, beyond the Standard Model (SM), may
include heavy neutrinos that mix and thus allow processes in which lepton
flavor is not preserved. We investigate the impact of charged currents that
couple heavy gauge bosons to heavy neutrinos and SM leptons on
lepton-flavor-violating decays of SM leptons into three charged leptons, with
no final-state neutrinos. We implement our expressions for the leading
contributions to , which
hold for either Dirac or Majorana neutrinos, to the trilepton decay ,
of the muon, and so determine sets of masses of heavy neutrinos and the heavy
gauge boson, within GeVs to few TeVs, that are consistent with the upper bounds
provided by the SINDRUM Collaboration. We find, however, that constraints
dictated by the upper bound on , from the MEG
Collaboration, are more stringent. We utilize such parameters to find that the
contributions to tau decays are , well below bounds from
factories. The mixing of heavy and SM charged bosons is also investigated.
We find that current experimental data from MEG and SINDRUM would allow mixing
angles as large as , for a relatively light new charged boson, but
the expected sensitivity of the Mu3e experiment would be capable of setting an
upper bound on this angle as small as if the mass of this boson
is within the range of few TeVs.Comment: Accepted for publication in Journal of Physics G, 26 pages, 12
figures, 4 tables, several new elements were added for this version of the
manuscript, the whole discussion was improved as wel
as a test for the existence of a light meson
We work out predictions of the Linear Sigma Model for the cross section. We consider the sigma width, which is introduced
in a consistent way with chiral Ward identities. The results of Chiral
Perturbation Theory are recovered in the limit. A fit to
existing experimental results is consistent with a light and broad
meson.Comment: 7 pages, 3 figures. The sigma width is introduced in a consistent way
with chiral symmetry. Results for the cross section are affected by the
corresponding change
Implications of Lorentz violation on Higgs-mediated lepton flavor violation
The lepton flavor violating decay of the Higgs boson is studied
within two qualitatively different extensions of the Yukawa sector: one
renormalizable and the other nonrenormalizable; both incorporating Lorentz
violation in a model-independent fashion. These extensions are characterized by
Yukawa-like matrices, the former by a constant Lorentz 2-tensor , whereas the latter by a constant Lorentz vector . It is
found that the experimental constraints on the decays
severely restrict lepton flavor violating Higgs signals in the renormalizable
scenario. In this context, it is found that and
cannot be larger than and ,
respectively. In the nonrenormalizable scenario, transitions mediated by the
Higgs or the gauge boson are induced at tree level, and we find mild
restrictions on lepton flavor violation. Using the experimental limits on the
three-body decays to constraint the vector
, it is found that the branching ratio for the decays is of about , more important, a branching ratio of
is found for the mode. Accordingly, the
decay could be at the reach of future measurements.
The lepton flavor violating decays of the gauge boson were also studied. In
the renormalizable scenario, it was found the undetectable branching ratios
and . In the nonrenormalizable scenario, it was found
that and . Although the latter branching ratio is relatively
large, it still could not be within the range of future measurements.Comment: Updated to essentially match published versio
Decay H^+ -> W^+ gamma in a nonlinear R_xi-gauge
A new evaluation of the charged Higgs boson decay is
presented in the context of the general two-Higgs doublet model. A nonlinear
-gauge which considerably simplifies the calculation is introduced and
simple expressions are obtained for the fermionic and bosonic contributions.
The branching ratio is analyzed for several values of the
parameters of the model. Although this decay can have a branching fraction as
large as in a certain region of the parameter space, it is found that
such a region is disfavored by the most recent constraints on ,
of the muon, , and the parameter, along with the
exclusions from direct searches at the CERN LEP collider. The
possibility of detecting this decay at future colliders is discussed.Comment: 12 pages, 9 figure
Trilinear Neutral Gauge Boson Couplings in Effective Theories
We list all the lowest dimension effective operators inducing off-shell
trilinear neutral gauge boson couplings Z-Z-Photon, Z-Photon-Photon, and ZZZ
within the effective Lagrangian approach, both in the linear and nonlinear
realizations of the SU(2)_{L} X U(1)_Y gauge symmetry. In the linear scenario
we find that these couplings can be generated only by dimension eight operators
necessarily including the Higgs boson field, whereas in the nonlinear case they
are induced by dimension six operators. We consider the impact of these
couplings on some precision measurements such as the magnetic and electric
dipole moments of fermions, as well as the Z boson rare decay Z ->
neutrino+antineutrino+ photon. If the underlying new physics is of a decoupling
nature, it is not expected that trilinear neutral gauge boson couplings may
affect considerably any of these observables. On the contrary, it is just in
the nonlinear scenario where these couplings have the more promising prospects
of being perceptible through high precision experiments.Comment: 21 pages, 2 figures, RevTex formatte
The anapole moment in scalar quantum electrodynamics
The anapole moment of a charged scalar particle is studied in a model
independent fashion, using the effective Lagrangian technique, as well as
radiatively within the context of scalar quantum electrodynamics (SQED). It is
shown that this gauge structure is characterized by a non renormalizable
interaction, which is radiatively generated at the one--loop. It is found that
the resulting anapole moment for off-shell particles, though free of
ultraviolet divergences, is gauge dependent and thus it is not a physical
observable. We also study some of its kinematical limits. In particular, it is
shown that its value comes out to be zero when all particles are on--shell.Comment: 4 pages, 1 figur
Effective Lagrangian approach to Higgs-mediated FCNC top quark decays
The flavor changing neutral current (FCNC) transitions t --> q'H and t -->
q'V_i (V_i=\gamma, g, Z) are studied in the context of the effective Lagrangian
approach. We focus on the scenario in which these decays are predominantly
induced by new physics effects arising from the Yukawa sector extended with
dimension-six SU_L(2) X U_Y(1)-invariant operators, which generate the most
general CP-even and CP-odd tq'H vertex at the tree level. For the unknown
coefficients, we assume a slightly modified version of the Cheng-Sher ansatz.
We found that the branching ratio for the Higgs-mediated FCNC t--> q'V_i decays
are enhanced by two or three orders of magnitude with respect to the results
expected in models with extended Higgs sectors, such as the general two-Higgs
doublet model. We discuss the possibilities of detecting this class of decays
at the LHC.Comment: 11 pages, 6 figures, submitted to Phys. Rev.
Testing flavor-changing neutral currents in the rare decays t->cViVj
We discuss the Flavor-Changing Neutral Current (FCNC) decays of the top quark
t -> c Vi Vj (Vi=gamma, Z, g) in the framework of the Standard Model (SM) and
in a two-higgs doublet model (2HDM) with tree-level FCNC couplings. While in
the SM the expected branching ratios are extremelly small, in the 2HDM they may
be sizable, of order 10^(-5) - 10^(-5), and thus accesible at the CERN LHC. We
conclude with the interesting observation that the FCNC decay modes may not be
equally suppressed as their corresponding decays t ->c Vi in this 2HDM.Comment: RevTeX, 2 epsi figures, 10 pgs. Comments and references added.
Submitted to Physical Review
The Standard Model with one universal extra dimension
Effects of universal extra dimensions on Standard Model observables first
arise at the one-loop level. The quantization of this class of theories is
therefore essential in order to perform predictions. A comprehensive study of
the Standard Model defined in a space-time manifold with one universal extra
dimension, compactified on the oribifold , is presented. The fact that
the four-dimensional Kaluza-Klein theory is subject to two types of gauge
transformations is stressed and its quantization under the basis of the BRST
symmetry discussed. A -covariant gauge-fixing
procedure for the Kaluza-Klein excitations is introduced. The connection
between gauge and mass eigenstates fields is established in an exact way. An
exhaustive list of the explicit expressions for all physical couplings induced
by the Yang-Mills, Currents, Higgs, and Yukawa sectors is presented. The
one-loop renormalizability of the standard Green's functions, which implies
that the Standard Model observables do not depend on a cutoff scale, is
stressed.Comment: 51 page
Hidden symmetries induced by a canonical transformation and gauge structure of compactified Yang-Mills theories
Compactified Yang-Mills theories with one universal extra dimension were
found [arXiv:1008.4638] to exhibit two types of gauge invariances: the standard
gauge transformations (SGTs) and the nonstandard gauge transformations (NSGTs).
In the present work we show that these transformations are not exclusive to
compactified scenarios. Introducing a notion of hidden symmetry, based on the
fundamental concept of canonical transformation, we analyse three different
gauge systems, each of which is mapped to a certain effective theory that is
invariant under the so-called SGTs and NSGTs. The systems under discussion are:
(i) four dimensional pure Yang-Mills theory, (ii) four dimensional
Yang-Mills with spontaneous symmetry breaking, and (iii) pure
Yang-Mills theory with one universal compact extra dimension. The canonical
transformation, that induces the notion of hidden symmetry, maps objects with
well defined transformation laws under a gauge group to well defined
objects under a non-trivial subgroup . In the case where
spontaneous symmetry breaking is present, the set of SGTs corresponds to the
group into which the original gauge group is broken into, whereas the NSGTs are
associated to the broken generators and can be used to define the unitary
gauge. For the system (iii), the SGTs coincide with the gauge group , whereas the NSGTs do not form a group; in this system the
'fundamental' theory and the effective one are shown to be classically
equivalent.Comment: 26 pages, title has changed, some comments and clarifications added,
some references adde
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