225 research outputs found
Evidence for the J^p=1/2^+ narrow state at 1650 MeV in the photoproduction of KLambda
We have investigated the existence of the J^p=1/2^+ narrow resonance
predicted by the chiral soliton model by utilizing the kaon photoproduction
process gamma + p --> K^+ + Lambda. For this purpose we have constructed two
phenomenological models based on our previous effective Lagrangian model, which
are able to describe kaon photoproduction from threshold up to W = 1730 MeV. By
varying the mass (width) of an inserted P_{11} resonance from 1620 to 1730 MeV
(0.1 to 1 MeV and 1 to 10 MeV) a number of fits has been performed in order to
search for the resonance mass. Our result indicates that the most promising
candidate mass (width) of this resonance is 1650 MeV (5 MeV). Although our
calculation does not exclude the possibility of narrow resonances with masses
of 1680, 1700 and 1720 MeV, the mass of 1650 MeV is obtained for all
phenomenological models used in this investigation. Variations of the resonance
width and branching ratio are found to have a mild effect on the
chi^2. The possibility that the obtained result originates from other resonance
states is also discussed.Comment: 27 pages, 19 figures, submitted to Phys. Rev.
The weak electroweak phase transition
We present a detailed analysis of the phase transition in the standard model
at finite temperature. Using an improved perturbation theory, where plasma
masses are determined from a set of one-loop gap equations, we evaluate the
effective potential in next-to-leading order, i.e.,
including terms cubic in the gauge coupling , the scalar self-coupling
and the top-quark Yukawa coupling . The gap equations
yield a non-vanishing magnetic plasma mass for the gauge bosons, originating
from the non-abelian self-interactions. We discuss in detail size and origin of
higher order effects and conclude that the phase transition is weakly
first-order up to Higgs masses of about , above which our calculation
is no longer self-consistent. For larger Higgs masses even an approximation
containing all contributions to is not sufficient, at least a
full calculation to order is needed. These results turn out to be rather
insensitive to the top-quark mass in the range . Using
Langer's theory of metastability we calculate the nucleation rate of critical
droplets and discuss some aspects of the cosmological electroweak phase
transition.Comment: LaTeX, 45 pages, 13 figures [not included, can be sent upon
request],DESY 93-02
Scattering and Resonances in QCD_2
Extending previous works on the spectrum of QCD_2, we now investigate the 2D
analogue of meson-baryon scattering. We use semi-classical methods, perturbing
around classical soliton solutions. We start with the abelian case,
corresponding to one flavor,and find that in this case the effective potential
is reflectionless. We obtain an explicit expression for the forward phase
shift. In the non-abelian case of several flavors, the method yields a
potential which depends on the momentum of the incoming particle. In this case
there is both transmission and reflection. In both cases no resonances appear.
As a byproduct, we derive the general conditions for a 2D quantum field
theoretical action to yield a reflectionless effective potential when one
expands in small fluctuations about the classical solution.Comment: Figs. 2 and 3 and the accompanying text amended, following sign
correction in the first term in the potential V in eq. (58). Qualitative
conclusions unchange
Méthodologie pour choisir les pratiques de gestion optimale et analyse économique : Une étude de cas au Québec, Canada
Resonances, and mechanisms of Theta-production
After explaining necessity of exotic hadrons, we discuss mechanisms which
could determine production of the exotic Theta-baryon. A possible important
role of resonances (producing the Theta in real or virtual decays) is
emphasized for various processes. Several experimental directions for studies
of such resonances, and the Theta itself, are suggested. We briefly discuss
also recent negative results on the Theta-baryon.Comment: 6 page
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
The Casimir energy of skyrmions in the 2+1-dimensional O(3)-model
One-loop quantum corrections to the classical vortices in 2+1 dimensional
O(3)-models are evaluated. Skyrme and Zeeman potential terms are used to
stabilize the size of topological solitons. Contributions from zero modes,
bound-states and scattering phase-shifts are calculated for vortices with
winding index n=1 and n=2. For both cases the S-matrix shows a pronounced
series of resonances for magnon-vortex scattering in analogy to the
well-established baryon resonances in hadron physics, while vortices with n>2
are already classically unstable against decay. The quantum corrections
destabilize the classically bound n=2 configuration. Approximate independence
of the results with respect to changes in the renormalization scale is
demonstrated.Comment: 24 pages LaTeX, 14 figure
Strangeness, charm and bottom in a chiral quark-meson model
In this paper we investigate an SU(3) extension of the chiral quark-meson
model. The spectra of baryons with strangeness, charm and bottom are considered
within a "rigid oscillator" version of this model. The similarity between the
quark part of the Lagrangian in the model and the Wess-Zumino term in the
Skyrme model is noted. The binding energies of baryonic systems with baryon
number B=2 and 3 possessing strangeness or heavy flavor are estimated. The
results obtained are in good qualitative agreement with those obtained
previously in the topological soliton (Skyrme) model.Comment: 12 pages, no figures. Journal ref: submitted to Nucl.Phys.
A possible explanation why the Theta+ is seen in some experiments and not in others
To understand the whole set of positive and null data on the
Theta+(1530)-production, we suggest the hypothesis that multiquark hadrons are
mainly generated from many-quark states, which emerge either as short-term
hadron fluctuations, or as hadron remnants in hard processes. This approach
allows us to describe both non-observation of the Theta+ in current null
experiments and peculiar features of its production in positive experiments.
Further, we are able to propose new experiments that might be decisive for the
problem of the Theta+ existence. Distributions of the Theta+ in such
experiments can give important information both on higher Fock components of
conventional hadrons and about structure and hadronization properties of hadron
remnants produced in hard processes. We also explain that description of
multiquark hadrons may require a modified form of the constituent quark model,
with quark masses and couplings being intermediate between their values for the
familiar constituent quarks and the current ones.Comment: 18 pages. Some changes in the text; experimental suggestions
collected in a special subsection, references added and refreshe
Flavored exotic multibaryons and hypernuclei in topological soliton models
The energies of baryon states with positive strangeness, or anti-charm
(-beauty) are estimated in chiral soliton approach, in the "rigid oscillator"
version of the bound state soliton model proposed by Klebanov and Westerberg.
Positive strangeness states can appear as relatively narrow nuclear levels
(Theta-hypernuclei), the states with heavy anti-flavors can be bound with
respect to strong interactions in the original Skyrme variant of the model (SK4
variant). The binding energies of anti-flavored states are estimated also in
the variant of the model with 6-th order term in chiral derivatives in the
lagrangian as solitons stabilizer (SK6 variant). The latter variant is less
attractive, and nuclear states with anti-charm or anti-beauty can be unstable
relative to strong interactions. The chances to get bound hypernuclei with
heavy antiflavors are greater within "nuclear variant" of the model with
rescaled model parameter (Skyrme constant e or e' decreased by ~30%) which is
expected to be valid for baryon numbers greater than B ~10. The rational map
approximation is used to describe multiskyrmions with baryon number up to ~30
and to calculate the quantities necessary for their quantization (moments of
inertia, sigma-term, etc.).Comment: 24 pages, 7 table
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