124 research outputs found
Universal Limits on Massless High-Spin Particles
We present a model-independent argument showing that massless particles
interacting with gravity in a Minkowski background space can have at most spin
two. This result is proven by extending a famous theorem due to Weinberg and
Witten to theories that do not possess a gauge-invariant stress-energy tensor.Comment: 21 pages. To appear in PRD; two additional reference
On a gauge-invariant interaction of spin-\fth resonances
We show that the gauge-invariant coupling suggested by Pascalutsa removes
non-pole terms from a spin-\fth propagator only for a specific choice of free
parameter. For the general case the problem can be solved by including higher
order derivatives of spin-\fth fields or by modifying the original coupling.
In the latter case the obtained Lagrangian depends on one free parameter
pointing to the freedom in choosing an 'off-shell' content of the theory.
However, the physical observables must not be affected by the 'off-shell'
contributions and should not depend on the free parameter of the Lagrangian
Symmetry Nonrestoration at High Temperature in Little Higgs Models
A detailed study of the high temperature dynamics of the scalar sector of
Little Higgs scenarios, proposed to stabilize the electroweak scale, shows that
the electroweak gauge symmetry remains broken even at temperatures much larger
than the electroweak scale. Although we give explicit results for a particular
modification of the Littlest Higgs model, we expect that the main features are
generic. As a spin-off, we introduce a novel way of dealing with scalar
fluctuations in nonlinear sigma models, which might be of interest for
phenomenological applications.Comment: 23 pages, LaTeX, 4 figure
The Off-Shell Nucleon-Nucleon Amplitude: Why it is Unmeasurable in Nucleon-Nucleon Bremsstrahlung
Nucleon-nucleon bremsstrahlung has long been considered a way of getting
information about the off-shell nucleon-nucleon amplitude which would allow one
to distinguish among nucleon-nucleon potentials based on their off-shell
properties. There have been many calculations and many experiments devoted to
this aim. We show here, in contrast to this standard view, that such off-shell
amplitudes are not measurable as a matter of principle. This follows formally
from the invariance of the S-matrix under transformations of the fields. This
result is discussed here and illustrated via two simple models, one applying to
spin zero, and one to spin one half, processes. The latter model is very
closely related to phenomenological models which have been used to study
off-shell effects at electromagnetic vertices.Comment: 6 pages, Latex, uses FBSsuppl.cls - Invited plenary talk at the Asia
Pacific Conference on Few Body Problems in Physics, Noda/Kashiwa, Japan,
August, 1999 - To be published in Few Body Systems Supp
Quantization of U_q[so(2n+1)] with deformed para-Fermi operators
The observation that n pairs of para-Fermi (pF) operators generate the
universal enveloping algebra of the orthogonal Lie algebra so(2n+1) is used in
order to define deformed pF operators. It is shown that these operators are an
alternative to the Chevalley generators. On this background Uq[so(2n+1)] and
its "Cartan-Weyl" generators are written down entirely in terms of deformed pB
operators.Comment: plain TeX, Preprint INRNE-TH-93/7, 6
The equivalence theorem and the Bethe-Salpeter equation
We solve the Bethe-Salpeter equation for two-particle scattering in a
field-theoretical model using two lagrangians related by a field
transformation. The kernel of the equation consists of the sum of all
tree-level diagrams for each lagrangian. The solutions differ even if all four
external particles are put on the mass shell, which implies that observables
calculated by solving the Bethe-Salpeter equation depend on the representation
of the theory. We point out that this violation of the equivalence theorem has
a simple explanation and should be expected for any Bethe-Salpeter equation
with a tree-level kernel. Implications for dynamical models of hadronic
interactions are discussed.Comment: 10 pages, 4 figures, using REVTeX. Fig. 2 corrected, results
unchanged, to be published in Phys. Lett.
Renormalizability of Nonrenormalizable Field Theories
We give a simple and elegant proof of the Equivalence Theorem, stating that
two field theories related by nonlinear field transformations have the same S
matrix. We are thus able to identify a subclass of nonrenormalizable field
theories which are actually physically equivalent to renormalizable ones. Our
strategy is to show by means of the BRS formalism that the "nonrenormalizable"
part of such fake nonrenormalizable theories, is a kind of gauge fixing, being
confined in the cohomologically trivial sector of the theory.Comment: 3 pages, revtex, no figure
Field diffeomorphisms and the algebraic structure of perturbative expansions
We consider field diffeomorphisms in the context of real scalar field
theories. Starting from free field theories we apply non-linear field
diffeomorphisms to the fields and study the perturbative expansion for the
transformed theories. We find that tree level amplitudes for the transformed
fields must satisfy BCFW type recursion relations for the S-matrix to remain
trivial. For the massless field theory these relations continue to hold in loop
computations. In the massive field theory the situation is more subtle. A
necessary condition for the Feynman rules to respect the maximal ideal and
co-ideal defined by the core Hopf algebra of the transformed theory is that
upon renormalization all massive tadpole integrals (defined as all integrals
independent of the kinematics of external momenta) are mapped to zero.Comment: 8 pages, 2 figure
Pairing of Parafermions of Order 2: Seniority Model
As generalizations of the fermion seniority model, four multi-mode
Hamiltonians are considered to investigate some of the consequences of the
pairing of parafermions of order two. 2-particle and 4-particle states are
explicitly constructed for H_A = - G A^+ A with A^+}= 1/2 Sum c_{m}^+ c_{-m}^+
and the distinct H_C = - G C^+ C with C^+}= 1/2 Sum c_{-m}^+ c_{m}^+, and for
the time-reversal invariant H_(-)= -G (A^+ - C^+)(A-C) and H_(+) = -G
(A^+dagger + C^+)(A+C), which has no analogue in the fermion case. The spectra
and degeneracies are compared with those of the usual fermion seniority model.Comment: 18 pages, no figures, no macro
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