13,750 research outputs found
A gauge-independent approach to resonant transition amplitudes
We present a new gauge-independent approach to resonant transition amplitudes with nonconserved external currents, based on the pinch technique method. In the context of 2\to 2 and 2\to 3 scattering processes, we show explicitly that the analytic results derived respect U(1)_{em} gauge symmetry and do not depend on the choice of the SU(2)_L gauge fixing. Our analytic approach treats, on equal footing, fermionic as well as bosonic contributions to the resummed gauge boson propagators, does not contain any residual space-like threshold terms, shows the correct high-energy unitarity behaviour, admits renormalization, and satisfies a number of other required properties, including the optical theorem. Even though our analysis has mainly focused on the Standard Model gauge bosons, our method can easily be extended to the top quark, and be directly applied to the study of unstable particles present in renormalizable models of new physics
Gauge invariance and unstable particles
A gauge-independent approach to resonant transition amplitudes with nonconserved external currents is presented, which is implemented by the pinch technique. The analytic expressions derived with this method are U(1)_{em} invariant, independent of the choice of the gauge-fixing parameter, and satisfy a number of required theoretical properties, including unitarity. Although special attention is paid to resonant scatterings involving the \gamma WW and ZWW vertices in the minimal Standard Model, our approach can be extended to the top quark or other unstable particles appearing in renormalizable models of new physics
Gauge-Independent Approach to Resonant Dark Matter Annihilation
In spontaneously broken gauge theories, transition amplitudes describing
dark-matter (DM) annihilation processes through a resonance may become highly
inaccurate close to a production threshold, if a Breit-Wigner (BW) ansatz with
a constant width is used. To partially overcome this problem, the BW propagator
needs to be modified by including a momentum dependent decay width. However,
such an approach to resonant transition amplitudes generically suffers from
gauge artefacts that may also give rise to a bad or ambiguous high-energy
behaviour for such amplitudes. We address the two problems of gauge dependence
and high-energy unitarity within a gauge-independent framework of resummation
implemented by the so-called Pinch Technique. We study DM annihilation via
scalar resonances in a gauged U(1) complex-scalar extension of the Standard
Model that features a massive stable gauge field which can play the role of the
DM. We find that the predictions for the DM abundance may vary significantly
from previous studies based on the naive BW ansatz and propose an alternative
simple approximation which leads to the correct DM phenomenology. In
particular, our results do not depend on the gauge-fixing parameter and are
consistent with considerations from high-energy unitarity.Comment: 29 pages, 9 figures, v2: minor typos corrected, matches published
versio
Parity-Violating Excitation of the \Delta(1232): Hadron Structure and New Physics
We consider prospects for studying the parity-violating (PV) electroweak
excitation of the \Delta(1232) resonance with polarized electron scattering.
Given present knowledge of Standard Model parameters, such PV experiments could
allow a determination of the N -> \Delta electroweak helicity amplitudes. We
discuss the experimental feasibility and theoretical interpretability of such a
determination as well as the prospective implications for hadron structure
theory. We also analyze the extent to which a PV N -> \Delta measurement could
constrain various extensions of the Standard Model.Comment: 43 pages, RevTex, 8 PS figures, uses epsf.sty, rotate.sty, version to
appear in Nucl. Phys. A, main points emphasized, some typos correcte
Regge-plus-resonance predictions for kaon photoproduction from the neutron
We present predictions for n(gamma,K+)Sigma- differential cross sections and
photon-beam asymmetries and compare them to recent LEPS data. We adapt a
Regge-plus-resonance (RPR) model developed to describe photoinduced and
electroinduced kaon production off protons. The non-resonant contributions to
the amplitude are modelled in terms of K+(494) and K*+(892) Regge-trajectory
exchange. This amplitude is supplemented with a selection of s-channel
resonance diagrams. The three Regge-model parameters of the n(gamma,K+)Sigma-
amplitude are derived from the ones fitted to proton data through SU(2) isospin
considerations. A fair description of the n(gamma,K+)Sigma- data is realized,
which demonstrates the Regge model's robustness and predictive power.
Conversion of the resonances' couplings from the proton to the neutron is more
challenging, as it requires knowledge of the photocoupling helicity amplitudes.
We illustrate how the uncertainties of the helicity amplitudes propagate and
heavily restrain the predictive power of the RPR and isobar models for kaon
production off neutron targets.Comment: 14 pages, 4 figures; Minor revisions; Published in Physics Letters
The Complex-Mass Scheme and Unitarity in perturbative Quantum Field Theory
We investigate unitarity within the Complex-Mass Scheme, a convenient
universal scheme for perturbative calculations involving unstable particles in
Quantum Field Theory which guarantees exact gauge invariance. Since this scheme
requires to introduce complex masses and complex couplings, the Cutkosky
cutting rules, which express perturbative unitarity in theories of stable
particles, are no longer valid. We derive corresponding rules for scalar
theories with unstable particles based on Veltman's Largest-Time Equation and
prove unitarity in this framework.Comment: 25 pages, PDFLaTeX, PDF figures, improved and slightly extended
version appeared in European Physics Journal
CP Violation and Baryogenesis due to Heavy Majorana Neutrinos
We analyze the scenario of baryogenesis through leptogenesis induced by the
out-of-equilibrium decays of heavy Majorana neutrinos and pay special attention
to CP violation. Extending a recently proposed resummation formalism for
two-fermion mixing to decay amplitudes, we calculate the resonant phenomenon of
CP violation due to the mixing of two nearly degenerate heavy Majorana
neutrinos. Solving numerically the relevant Boltzmann equations, we find that
the isosinglet Majorana mass may range from 1 TeV up to the grand unification
scale, depending on the mechanism of CP violation and/or the flavour structure
of the neutrino mass matrix assumed. Finite temperature effects and possible
constraints from the electric dipole moment of electron and other low-energy
experiments are briefly discussed.Comment: 46 pages, LaTeX, 4 encapsulated figures include
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