8 research outputs found
Resonance states below pion-nucleon threshold and their consequences for nuclear systems
Regular sequences of narrow peaks have been observed in the missing mass
spectra in the reactions pp --> p pi^+ X and pd --> ppX_1 below pion-production
threshold. They are interpreted in the literature as manifestations of
supernarrow light dibaryons, or nucleon resonances, or light pions forming
resonance states with the nucleon in its ground state. We discuss how existence
of such exotic states would affect properties of nuclear systems. We show that
the neutron star structure is drastically changed in all three cases. We find
that in the presence of dibaryons or nucleon resonances the maximal possible
mass of a neutron star would be smaller than the observational limit. Presence
of light pions does not contradict the observed neutron star masses. Light
pions allow for the existence of extended nuclear objects of arbitrary size,
bound by strong and electromagnetic forces.Comment: preprint ECT*-02-18, 6 pages, 3 figure
Search for NN-decoupled dibaryons using the process below the pion production threshold
The energy spectrum for high energy -rays ( MeV)
from the process emitted at in the laboratory
frame has been measured at an energy below the pion production threshold,
namely, at 216 MeV. The resulting photon energy spectrum extracted from
coincidence events consists of a narrow peak at a photon energy
of about 24 MeV and a relatively broad peak in the energy range of (50 - 70)
MeV. The statistical significances for the narrow and broad peaks are
5.3 and 3.5, respectively. This behavior of the photon energy
spectrum is interpreted as a signature of the exotic dibaryon resonance
with a mass of about 1956 MeV which is assumed to be formed in the
radiative process followed by its electromagnetic
decay via the mode. The experimental spectrum is
compared with those obtained by means of Monte Carlo simulations.Comment: 14 pages, LaTex, 6 eps-figures, accepted for publication in
Phys.Rev.
Chiral effective field theories of the strong interactions
Effective field theories of the strong interactions based on the approximate
chiral symmetry of QCD provide a model-independent approach to low-energy
hadron physics. We give a brief introduction to mesonic and baryonic chiral
perturbation theory and discuss a number of applications. We also consider the
effective field theory including vector and axial-vector mesons.Comment: 22 pages, 9 figures, proceedings of "Many-Body Structure of Strongly
Interacting Systems", Mainz, Germany, Feb. 23-25 201
Couplings of light I=0 scalar mesons to simple operators in the complex plane
The flavour and glue structure of the light scalar mesons in QCD are probed
by studying the couplings of the I=0 mesons and to the
operators , and to two photons. The Roy dispersive
representation for the amplitude is used to determine the
pole positions as well as the residues in the complex plane. On the real axis,
is constrained to solve the Roy equation together with elastic
unitarity up to the K\Kbar threshold leading to an improved description of
the . The problem of using a two-particle threshold as a matching
point is discussed. A simple relation is established between the coupling of a
scalar meson to an operator and the value of the related pion form-factor
computed at the resonance pole. Pion scalar form-factors as well as two-photon
partial-wave amplitudes are expressed as coupled-channel Omn\`es dispersive
representations. Subtraction constants are constrained by chiral symmetry and
experimental data. Comparison of our results for the couplings with
earlier determinations of the analogous couplings of the lightest I=1 and
scalar mesons are compatible with an assignment of the ,
, , into a nonet. Concerning the gluonic operator
we find a significant coupling to both the and the
.Comment: 31 pages, 5 figure
MO analysis of the high statistics Belle results on with chiral constraints
We reconsider Muskhelishvili-Omn\`es (MO) dispersive representations of
photon-photon scattering to two pions, motivated by the very high statistics
results recently released by the Belle collaboration for charged as well as
neutral pion pairs and also by recent progress in the determination of the
low-energy scattering amplitude. Applicability of this formalism is
extended beyond 1 GeV by taking into account inelasticity due to . A
modified MO representation is derived which has the advantage that all
polynomial ambiguities are collected into the subtraction constants and have
simple relations to pion polarizabilities. It is obtained by treating
differently the exactly known QED Born term and the other components of the
left-hand cut. These components are approximated by a sum over resonances. All
resonances up to spin two and masses up to GeV are included. The
tensor contributions to the left-hand cut are found to be numerically
important. We perform fits to the data imposing chiral constraints, in
particular, using a model independent sum rule result on the chiral
coupling . Such theoretical constraints are necessary because the
experimental errors are dominantly systematic. Results on further
couplings and pion dipole and quadrupole polarizabilities are then derived from
the fit. The relevance of the new data for distinguishing between two possible
scenarios of isospin breaking in the region is discussed.Comment: 44 pages, 12 figure