411 research outputs found
Towards the realistic fermion masses with a single family in extra dimensions
In a class of multidimensional models, topology of a thick brane provides
three chiral fermionic families with hierarchical masses and mixings in the
effective four-dimensional theory, while the full model contains a single
vector-like generation. We carry out numerical simulations and reproduce all
known Standard Model fermion masses and mixings in one of these models.Comment: 12 pages, 2 figures, uses JHEP3.cls. Some minor corrections are mad
Soft two-meson-exchange nucleon-nucleon potentials. II. One-pair and two-pair diagrams
Two-meson-exchange nucleon-nucleon potentials are derived where either one or
both nucleons contains a pair vertex. Physically, the meson-pair vertices are
meant to describe in an effective way (part of) the effects of heavy-meson
exchange and meson-nucleon resonances. {}From the point of view of ``duality,''
these two kinds of contribution are roughly equivalent. The various
possibilities for meson pairs coupling to the nucleon are inspired by the
chiral-invariant phenomenological Lagrangians that have appeared in the
literature. The coupling constants are fixed using the linear model.
We show that the inclusion of these two-meson exchanges gives a significant
improvement over a potential model including only the standard one-boson
exchanges.Comment: 21 pages RevTeX, 7 postscript figures; revised version as to appear
in Phys. Rev.
One Loop Effects of Non-Standard Triple Gauge Boson Vertices
Low energy effects of generic extensions of the Standard Model can be
comprehensively parametrized in terms of higher dimensional effective
operators. After the success of all the recent precission tests on the Standard
Model, we argue that any sensible description of these extensions at the
Z-scale must be stable under higher order quantum corrections. The imposition
of gauge invariance seems to be the simplest and most
natural way to fulfill this requirement. With this assumption, all the possible
deviations from the standard triple gauge boson vertices can be consistently
parametrized in terms of a finite set of gauge invariant operators. We deal
here with those operators that do not give any tree level effect on present
experimental observables and constrain them by computing their effects at the
one-loop level. We conclude that for a light Higgs boson, the direct
measurement at LEP200 can improve present bounds on these "blind directions",
while for a heavy Higgs it is most unlikely to provide any new information.Comment: 17 pags. 2 figures not included, available on request. Latex. CERN-TH
667
Reality, measurement and locality in Quantum Field Theory
It is currently believed that the local causality of Quantum Field Theory
(QFT) is destroyed by the measurement process. This belief is also based on the
Einstein-Podolsky-Rosen (EPR) paradox and on the so-called Bell's theorem, that
are thought to prove the existence of a mysterious, instantaneous action
between distant measurements. However, I have shown recently that the EPR
argument is removed, in an interpretation-independent way, by taking into
account the fact that the Standard Model of Particle Physics prevents the
production of entangled states with a definite number of particles. This result
is used here to argue in favor of a statistical interpretation of QFT and to
show that it allows for a full reconciliation with locality and causality.
Within such an interpretation, as Ballentine and Jarret pointed out long ago,
Bell's theorem does not demonstrate any nonlocality.Comment: 15 pages. Published versio
Triviality from the Exact Renormalization Group
Using the exact renormalization group, it is shown that no physically
acceptable non-trivial fixed points, with positive anomalous dimension, exist
for (i) O(N) scalar field theory in four or more dimensions, (ii) non-compact,
pure Abelian gauge theory in any dimension. It is then shown, for both theories
in any dimension, that otherwise physically acceptable non-trivial fixed points
with negative anomalous dimension are non-unitary. In addition, a very simple
demonstration is given, directly from the exact renormalization group, that
should a critical fixed point exist for either theory in any dimension, then
the n-point correlation functions exhibit the expected behaviour.Comment: 14 pages, 6 figures; v2: Important addition - negative anomalous
dimensions now treated; v3: error in treatment of negative anomalous
dimension corrected and some clarifications added; v4: published in JHEP;
very minor change
Consistent irrelevant deformations of interacting conformal field theories
I show that under certain conditions it is possible to define consistent
irrelevant deformations of interacting conformal field theories. The
deformations are finite or have a unique running scale ("quasi-finite"). They
are made of an infinite number of lagrangian terms and a finite number of
independent parameters that renormalize coherently. The coefficients of the
irrelevant terms are determined imposing that the beta functions of the
dimensionless combinations of couplings vanish ("quasi-finiteness equations").
The expansion in powers of the energy is meaningful for energies much smaller
than an effective Planck mass. Multiple deformations can be considered also. I
study the general conditions to have non-trivial solutions. As an example, I
construct the Pauli deformation of the IR fixed point of massless non-Abelian
Yang-Mills theory with N_c colors and N_f <~ 11N_c/2 flavors and compute the
couplings of the term F^3 and the four-fermion vertices. Another interesting
application is the construction of finite chiral irrelevant deformations of N=2
and N=4 superconformal field theories. The results of this paper suggest that
power-counting non-renormalizable theories might play a role in the description
of fundamental physics.Comment: 23 pages, 5 figures; reference updated - JHE
Monte Carlo and Renormalization Group Effective Potentials in Scalar Field Theories
We study constraint effective potentials for various strongly interacting
theories. Renormalization group (RG) equations for these quantities
are discussed and a heuristic development of a commonly used RG approximation
is presented which stresses the relationships among the loop expansion, the
Schwinger-Dyson method and the renormalization group approach. We extend the
standard RG treatment to account explicitly for finite lattice effects.
Constraint effective potentials are then evaluated using Monte Carlo (MC)
techniques and careful comparisons are made with RG calculations. Explicit
treatment of finite lattice effects is found to be essential in achieving
quantitative agreement with the MC effective potentials. Excellent agreement is
demonstrated for and , O(1) and O(2) cases in both symmetric and
broken phases.Comment: 16 pages, 4 figures appended to end of this fil
Spontaneous Magnetization of the O(3) Ferromagnet at Low Temperatures
We investigate the low-temperature behavior of ferromagnets with a
spontaneously broken symmetry O(3) O(2). The analysis is performed within
the perspective of nonrelativistic effective Lagrangians, where the dynamics of
the system is formulated in terms of Goldstone bosons. Unlike in a
Lorentz-invariant framework (chiral perturbation theory), where loop graphs are
suppressed by two powers of momentum, loops involving ferromagnetic spin waves
are suppressed by three momentum powers. The leading coefficients of the
low-temperature expansion for the partition function are calculated up to order
. In agreement with Dyson's pioneering microscopic analysis of the
cubic ferromagnet, we find that, in the spontaneous magnetization, the
magnon-magnon interaction starts manifesting itself only at order . The
striking difference with respect to the low-temperature properties of the O(3)
antiferromagnet is discussed from a unified point of view, relying on the
effective Lagrangian technique.Comment: 23 pages, 4 figure
Rare Kaon Decays in the -Expansion
We study the unknown coupling constants that appear at order in the
Chiral Perturbation Theory analysis of ,
and decays. To that
end, we compute the chiral realization of the Hamiltonian
in the framework of the -expansion of the low-energy action. The
phenomenological implications are also discussed.Comment: 18 pages, LaTeX, CPT-92/P.279
Finite Temperature Pion Scattering to one-loop in Chiral Perturbation Theory
We present the pion-pion elastic scattering amplitude at finite temperature
to one-loop in Chiral Perturbation Theory. The thermal scattering amplitude
properly defined allows to generalize the perturbative unitarity relation to
the case. Our result provides a model independent prediction of an
enhanced pion-pion low-energy phase shift with the temperature and it has
physical applications within the context of Relativistic Heavy Ion Collisions.Comment: 11 pages, 2 figures. Some references and clarifying comments added
and new figures included. Final version to appear in Physics Letters
- …