9,383 research outputs found
On the Experimental Effects of the Off-shell Structure in Anomalous Neutral Triple Gauge Vertices
We discuss differences between on-shell and off-shell treatments in the
search for anomalous neutral triple gauge couplings in
collisions. We find that the usual on-shell framework represents an optimal
starting point, covering all scenarios in which a reasonable experimental
sensitivity is expected. We show that off-shell effects lead to negligible
deviations at the experimental level, provided that and \mathrm{e^+ e^-}\to\mathr
performed in regions where
\mathrm{Z}^*\to\mathrm{f}\bar{\mathrm{f}},\mathrm{f^\prime}\math production is
dominant. For consistency reasons, we advocate the use of a natural extension
of the on-shell definitions, which takes into account the correct off-shell
dependences. Contrary to what has been recently suggested in the literature, we
find that no constraints among neutral triple gauge
couplings can be imposed in a general case.Comment: 15 pages, 6 figures. Final versio
Generalization of the matrix product ansatz for integrable chains
We present a general formulation of the matrix product ansatz for exactly
integrable chains on periodic lattices. This new formulation extends the matrix
product ansatz present on our previous articles (F. C. Alcaraz and M. J. Lazo
J. Phys. A: Math. Gen. 37 (2004) L1-L7 and J. Phys. A: Math. Gen. 37 (2004)
4149-4182.)Comment: 5 pages. to appear in J. Phys. A: Math. Ge
Cerenkov angle and charge reconstruction with the RICH detector of the AMS experiment
The Alpha Magnetic Spectrometer (AMS) experiment to be installed on the
International Space Station (ISS) will be equipped with a proximity focusing
Ring Imaging Cerenkov (RICH) detector, for measurements of particle electric
charge and velocity. In this note, two possible methods for reconstructing the
Cerenkov angle and the electric charge with the RICH, are discussed. A
Likelihood method for the Cerenkov angle reconstruction was applied leading to
a velocity determination for protons with a resolution of around 0.1%. The
existence of a large fraction of background photons which can vary from event
to event, implied a charge reconstruction method based on an overall efficiency
estimation on an event-by-event basis.Comment: Proceedings submitted to RICH 2002 (Pylos-Greece
Exactly solvable interacting vertex models
We introduce and solvev a special family of integrable interacting vertex
models that generalizes the well known six-vertex model. In addition to the
usual nearest-neighbor interactions among the vertices, there exist extra
hard-core interactions among pair of vertices at larger distances.The
associated row-to-row transfer matrices are diagonalized by using the recently
introduced matrix product {\it ansatz}. Similarly as the relation of the
six-vertex model with the XXZ quantum chain, the row-to-row transfer matrices
of these new models are also the generating functions of an infinite set of
commuting conserved charges. Among these charges we identify the integrable
generalization of the XXZ chain that contains hard-core exclusion interactions
among the spins. These quantum chains already appeared in the literature. The
present paper explains their integrability.Comment: 20 pages, 3 figure
The Exact Solution of the Asymmetric Exclusion Problem With Particles of Arbitrary Size: Matrix Product Ansatz
The exact solution of the asymmetric exclusion problem and several of its
generalizations is obtained by a matrix product {\it ansatz}. Due to the
similarity of the master equation and the Schr\"odinger equation at imaginary
times the solution of these problems reduces to the diagonalization of a one
dimensional quantum Hamiltonian. We present initially the solution of the
problem when an arbitrary mixture of molecules, each of then having an
arbitrary size () in units of lattice spacing, diffuses
asymmetrically on the lattice. The solution of the more general problem where
we have | the diffusion of particles belonging to distinct class of
particles (), with hierarchical order, and arbitrary sizes is also
solved. Our matrix product {\it ansatz} asserts that the amplitudes of an
arbitrary eigenfunction of the associated quantum Hamiltonian can be expressed
by a product of matrices. The algebraic properties of the matrices defining the
{\it ansatz} depend on the particular associated Hamiltonian. The absence of
contradictions in the algebraic relations defining the algebra ensures the
exact integrability of the model. In the case of particles distributed in
classes, the associativity of the above algebra implies the Yang-Baxter
relations of the exact integrable model.Comment: 42 pages, 1 figur
Exact solutions of exactly integrable quantum chains by a matrix product ansatz
Most of the exact solutions of quantum one-dimensional Hamiltonians are
obtained thanks to the success of the Bethe ansatz on its several formulations.
According to this ansatz the amplitudes of the eigenfunctions of the
Hamiltonian are given by a sum of permutations of appropriate plane waves. In
this paper, alternatively, we present a matrix product ansatz that asserts that
those amplitudes are given in terms of a matrix product. The eigenvalue
equation for the Hamiltonian define the algebraic properties of the matrices
defining the amplitudes. The existence of a consistent algebra imply the exact
integrability of the model. The matrix product ansatz we propose allow an
unified and simple formulation of several exact integrable Hamiltonians. In
order to introduce and illustrate this ansatz we present the exact solutions of
several quantum chains with one and two global conservation laws and periodic
boundaries such as the XXZ chain, spin-1 Fateev-Zamolodchikov model,
Izergin-Korepin model, Sutherland model, t-J model, Hubbard model, etc.
Formulation of the matrix product ansatz for quantum chains with open ends is
also possible. As an illustration we present the exact solution of an extended
XXZ chain with -magnetic fields at the surface and arbitrary hard-core
exclusion among the spins.Comment: 57 pages, no figure
Track fitting in slightly inhomogeneous magnetic fields
A fitting method to reconstruct the momentum and direction of charged
particles in slightly inhomogeneous magnetic fields is presented in detail. For
magnetic fields of the order of 1 T and inhomogeneity gradients as large as 1
T/m the typical momentum bias due to the proposed approximations is of the
order of few MeV, to be compared with scattering components of the order of 20
MeV or even larger. This method is currently being employed in the
reconstruction programs of the AMS experiment.Comment: 12 pages, Nulc. Instr. Meth. A accepte
Critical Behaviour of Mixed Heisenberg Chains
The critical behaviour of anisotropic Heisenberg models with two kinds of
antiferromagnetically exchange-coupled centers are studied numerically by using
finite-size calculations and conformal invariance. These models exhibit the
interesting property of ferrimagnetism instead of antiferromagnetism. Most of
our results are centered in the mixed Heisenberg chain where we have at even
(odd) sites a spin-S (S') SU(2) operator interacting with a XXZ like
interaction (anisotropy ). Our results indicate universal properties
for all these chains. The whole phase, , where the models change
from ferromagnetic to ferrimagnetic behaviour is
critical. Along this phase the critical fluctuations are ruled by a c=1
conformal field theory of Gaussian type. The conformal dimensions and critical
exponents, along this phase, are calculated by studying these models with
several boundary conditions.Comment: 21 pages, standard LaTex, to appear in J.Phys.A:Math.Ge
Anomalous Gauge Boson Couplings in the e^+ e^- -> ZZ Process
We discuss experimental aspects related to the process and to the search for anomalous ZZV couplings
(V) at LEP2 and future colliders. We
present two possible approaches for a realistic study of the reaction and
discuss the differences between them. We find that the optimal method to study
double Z resonant production and to quantify the presence of anomalous
couplings requires the use of a complete four-fermion final-state calculation.Comment: 28 pages, 12 figures, final version for Phys. Rev.
Exactly Solvable Interacting Spin-Ice Vertex Model
A special family of solvable five-vertex model is introduced on a square
lattice. In addition to the usual nearest neighbor interactions, the vertices
defining the model also interact alongone of the diagonals of the lattice. Such
family of models includes in a special limit the standard six-vertex model. The
exact solution of these models gives the first application of the matrix
product ansatz introduced recently and applied successfully in the solution of
quantum chains. The phase diagram and the free energy of the models are
calculated in the thermodynamic limit. The models exhibit massless phases and
our analyticaland numerical analysis indicate that such phases are governed by
a conformal field theory with central charge and continuosly varying
critical exponents.Comment: 14 pages, 11 figure
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