29,610 research outputs found
Two-pion exchange potential and the amplitude
We discuss the two-pion exchange potential which emerges from a box diagram
with one nucleon (the spectator) restricted to its mass shell, and the other
nucleon line replaced by a subtracted, covariant scattering amplitude
which includes , Roper, and isobars, as well as contact terms
and off-shell (non-pole) dressed nucleon terms. The amplitude satisfies
chiral symmetry constraints and fits data below 700 MeV pion
energy. We find that this TPE potential can be well approximated by the
exchange of an effective sigma and delta meson, with parameters close to the
ones used in one-boson-exchange models that fit data below the pion
production threshold.Comment: 9 pages (RevTex) and 7 postscript figures, in one uuencoded gzipped
tar fil
Interacting Fermion Systems from Two Dimensional QCD
We consider two dimensional U(N) QCD on the cylinder with a timelike Wilson
line in an arbitrary representation. We show that the theory is equivalent to N
fermions with internal degrees of freedom which interact among themselves with
a generalized Sutherland-type interaction. By evaluating the expectation value
of the Wilson line in the original theory we explicitly find the spectrum and
degeneracies of these particle systems.Comment: 11 pages, UVA-93-11, CERN-TH-6994/9
Two-dimensional gauge theories of the symmetric group S(n) and branched n-coverings of Riemann surfaces in the large-n limit
Branched n-coverings of Riemann surfaces are described by a 2d lattice gauge
theory of the symmetric group S(n) defined on a cell discretization of the
surface. We study the theory in the large-n limit, and we find a rich phase
diagram with first and second order transition lines. The various phases are
characterized by different connectivity properties of the covering surface. We
point out some interesting connections with the theory of random walks on group
manifolds and with random graph theory.Comment: Talk presented at the "Light-cone physics: particles and strings",
Trento, Italy, September 200
The stability of the spectator, Dirac, and Salpeter equations for mesons
Mesons are made of quark-antiquark pairs held together by the strong force.
The one channel spectator, Dirac, and Salpeter equations can each be used to
model this pairing. We look at cases where the relativistic kernel of these
equations corresponds to a time-like vector exchange, a scalar exchange, or a
linear combination of the two. Since the model used in this paper describes
mesons which cannot decay physically, the equations must describe stable
states. We find that this requirement is not always satisfied, and give a
complete discussion of the conditions under which the various equations give
unphysical, unstable solutions
Enhancement of kinetic energy fluctuations due to expansion
Global equilibrium fragmentation inside a freeze out constraining volume is a
working hypothesis widely used in nuclear fragmentation statistical models. In
the framework of classical Lennard Jones molecular dynamics, we study how the
relaxation of the fixed volume constraint affects the posterior evolution of
microscopic correlations, and how a non-confined fragmentation scenario is
established. A study of the dynamical evolution of the relative kinetic energy
fluctuations was also performed. We found that asymptotic measurements of such
observable can be related to the number of decaying channels available to the
system at fragmentation time.Comment: 6 pages, 4 figure
Emergence of hierarchical networks and polysynchronous behaviour in simple adaptive systems
We describe the dynamics of a simple adaptive network. The network
architecture evolves to a number of disconnected components on which the
dynamics is characterized by the possibility of differently synchronized nodes
within the same network (polysynchronous states). These systems may have
implications for the evolutionary emergence of polysynchrony and hierarchical
networks in physical or biological systems modeled by adaptive networks.Comment: 4 pages, 4 figure
Matrix string states in pure 2d Yang Mills theories
We quantize pure 2d Yang-Mills theory on a torus in the gauge where the field
strength is diagonal. Because of the topological obstructions to a global
smooth diagonalization, we find string-like states in the spectrum similar to
the ones introduced by various authors in Matrix string theory. We write
explicitly the partition function, which generalizes the one already known in
the literature, and we discuss the role of these states in preserving modular
invariance. Some speculations are presented about the interpretation of 2d
Yang-Mills theory as a Matrix string theory.Comment: Latex file of 38 pages plus 6 eps figures. A note and few references
added, figures improve
Direct evaluation of the isotope effect within the framework of density functional theory for superconductors
Within recent developments of density functional theory, its numerical implementation and of the superconducting density functional theory is nowadays possible to predict the superconducting critical temperature, Tc, with sufficient accuracy to anticipate the experimental verification. In this paper we present an analytical derivation of the isotope coefficient within the superconducting density functional theory. We calculate the partial derivative of Tc with respect to atomic masses. We verified the final expression by means of numerical calculations of isotope coefficient in monatomic superconductors (Pb) as well as polyatomic superconductors (CaC6). The results confirm the validity of the analytical derivation with respect to the finite difference methods, with considerable improvement in terms of computational time and calculation accuracy. Once the critical temperature is calculated (at the reference mass(es)), various isotope exponents can be simply obtained in the same run. In addition, we provide the expression of interesting quantities like partial derivatives of the deformation potential, phonon frequencies and eigenvectors with respect to atomic masses, which can be useful for other derivations and applications
The four-fermion interaction in D=2,3,4: a nonperturbative treatment
A new nonperturbative approach is used to investigate the Gross-Neveu model
of four fermion interaction in the space-time dimensions 2, 3 and 4, the number
of inner degrees of freedom being a fixed integer. The spontaneous symmetry
breaking is shown to exist in and the running coupling constant is
calculated. The four dimensional theory seems to be trivial.Comment: a minor correction: one more acknowledgement is added. Latex 2.09
file, 15 pages, no figures, accepted for publication to Int.J.Mod.Phys.
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