1,649 research outputs found

### Asymptotic directional structure of radiation for fields of algebraic type D

The directional behavior of dominant components of algebraically special
spin-s fields near a spacelike, timelike or null conformal infinity is studied.
By extending our previous general investigations we concentrate on fields which
admit a pair of equivalent algebraically special null directions, such as the
Petrov type D gravitational fields or algebraically general electromagnetic
fields. We introduce and discuss a canonical choice of the reference tetrad
near infinity in all possible situations, and we present the corresponding
asymptotic directional structures using the most natural parametrizations.Comment: 20 pages, 6 figure

### Experimental Outlook for the Pentaquark

A critical look is taken at both positive and null evidence for the
$\Theta^+$ pentaquark. Potential problems with experiments will be discussed
and the question of what conclusion can be drawn from both the positive and the
null results is examined. First the question of existence of the $\Theta^+$
pentaquark is considered, followed by a discussion of new experiments that are
either planned or in progress to answer questions about its mass, width and
isospin. Finally, indirect evidence for the parity of the $\Theta^+$ is
examined, and suggestions for experiments to measure its parity directly are
given.Comment: MESON2004 conference proceedings, 10 pages, 1 figur

### Exotic Baryons in Two-Dimensional QCD

Two-dimensional QCD has often been used as a laboratory for studying the full
four-dimensional theory, providing, for example, an explicit realization of
baryons as solitons. We review aspects of conventional baryons in
two-dimensional QCD, including the classical and quantum contributions to their
masses. We then discuss the spectrum of exotic baryons in two-dimensional QCD,
commenting on the solitonic radius inferred from the excitation spectrum as
well as the two-dimensional version of the Goldberger-Treiman relation relating
meson couplings to current matrix elements. Two-dimensional QCD provides strong
overall support to the chiral-soliton picture for the structure of normal and
exotic baryons in four dimensions.Comment: 15 pages latex, no figure

### On Electron-Positron Annihilation into Nucleon-Antinucleon Pairs

We discuss the puzzling experimental results on baryon-antibaryon production
in e+e- annihilation close to the threshold, in particular the fact that
sigma(e+e- --> nbar n) >~ sigma(e+e- --> pbar p). We discuss an interpretation
in terms of a two-step process, via an intermediate coherent isovector state
serving as an intermediary between e+e- and the baryon-antibaryon system. We
provide evidence that the isovector channel dominates both e+e- --> pions and
from Nbar N annihilation at rest, and show that the observed ratio of
sigma(e+e- --> nbar n)/sigma(e+e- --> pbarp p) can be understood quantitatively
in this picture.Comment: 13 pages, 4 figure

### On the stability of quark solitons in QCD

We critically re-examine our earlier derivation of the effective low energy action for QCD in 4 dimensions with chiral fields transforming non-trivially under both color and flavor, using the method of anomaly integration. We find several changes with respect to our previous results, leading to much more compact expressions, and making it easier to compare with results of other approaches to the same problem. With the amended effective action, we find that there are no stable soliton solutions. In the context of the quark soliton program, we interpret this as an indication that the full low-energy effective action must include additional terms, reflecting possible modifications at short distances and/or the non-trivial structure of the gauge fields in the vacuum, such as \neq 0. Such terms are absent in the formalism based on anomaly integration

### Hexagonal Structure of Baby Skyrmion Lattices

We study the zero-temperature crystalline structure of baby Skyrmions by
applying a full-field numerical minimization algorithm to baby Skyrmions placed
inside different parallelogramic unit-cells and imposing periodic boundary
conditions. We find that within this setup, the minimal energy is obtained for
the hexagonal lattice, and that in the resulting configuration the Skyrmion
splits into quarter-Skyrmions. In particular, we find that the energy in the
hexagonal case is lower than the one obtained on the well-studied rectangular
lattice, in which splitting into half-Skyrmions is observed.Comment: RevTeX, 7 pages, 6 figure

### Exotic baryon multiplets at large number of colours

We generalize the usual octet, decuplet and exotic antidecuplet and higher
baryon multiplets to any number of colours Nc. We show that the multiplets fall
into a sequence of bands with O(1/Nc) splittings inside the band and
O(1)splittings between the bands characterized by "exoticness", that is the
number of extra quark-antiquark pairs needed to compose the multiplet. Each
time one adds a pair the baryon mass is increased by the same constant which
can be interpreted as a mass of a quark-antiquark pair. At the same time, we
prove that masses of exotic rotational multiplets are reliably determined at
large Nc from collective quantization of chiral solitons.Comment: 13 p., 5 figs. New section and references adde

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