213 research outputs found
Towards a high precision calculation for the pion-nucleus scattering lengths
We calculate the leading isospin conserving few-nucleon contributions to pion
scattering on H, He, and He. We demonstrate that the strong
contributions to the pion-nucleus scattering lengths can be controlled
theoretically to an accuracy of a few percent for isoscalar nuclei and of 10%
for isovector nuclei. In particular, we find the -He scattering length
to be where the uncertainties are
due to ambiguities in the -N scattering lengths and few-nucleon effects,
respectively. To establish this accuracy we need to identify a suitable power
counting for pion-nucleus scattering. For this purpose we study the dependence
of the two-nucleon contributions to the scattering length on the binding energy
of H. Furthermore, we investigate the relative size of the leading two-,
three-, and four-nucleon contributions. For the numerical evaluation of the
pertinent integrals, aMonte Carlo method suitable for momentum space is
devised. Our results show that in general the power counting suggested by
Weinberg is capable to properly predict the relative importance of -nucleon
operators, however, it fails to capture the relative strength of - and
-nucleon operators, where we find a suppression by a factor of 5
compared to the predicted factor of 50. The relevance for the extraction of the
isoscalar -N scattering length from pionic H and He is discussed.
As a side result, we show that beyond the calculation of the -H
scattering length is already beyond the range of applicability of heavy pion
effective field theory.Comment: 24 pages, 14 figures, 10 table
Interplay of quark and meson degrees of freedom in a near-threshold resonance
We investigate the interplay of quark and meson degrees of freedom in a
physical state representing a near-threshold resonance for the case of a single
continuum channel. We demonstrate that such a near-threshold resonance may
possess quite peculiar properties if both quark and meson dynamics generate
weakly coupled near-threshold poles in the S-matrix. In particular, the
scattering t-matrix may possess zeros in this case. We also discuss possible
implications for production reactions as well as studies within lattice QCD.Comment: LaTeX2e, 11 pages, minor typo corrections, to appear in Eur.Phys.J.
Chiral corrections to the isovector double scattering term for the pion-deuteron scattering length
The empirical value of the real part of the pion-deuteron scattering length
can be well understood in terms of the dominant isovector -double
scattering contribution. We calculate in chiral perturbation theory all
one-pion loop corrections to this double scattering term which in the case of
-scattering close the gap between the current-algebra prediction and the
empirical value of the isovector threshold T-matrix . In addition
to closing this gap there is in the -system a loop-induced off-shell
correction for the exchanged virtual pion. Its coordinate space representation
reveals that it is equivalent to -exchange in the deuteron. We evaluate
the chirally corrected double scattering term and the off-shell contribution
with various realistic deuteron wave functions. We find that the off-shell
correction contributes at most -8% and that the isovector double scattering
term explains at least 90% of the empirical value of the real part of the -scattering length.Comment: 4 pages, 2 figures, to be published in The Physical Review
Phenomenology of pp->pp eta reaction close to threshold
The recent high statistics measurement of the pp -> pp eta reaction at an
excess energy Q=15.5 MeV has been analysed by means of partial wave
decomposition of the cross section. Guided by the dominance of the final state
1S0 pp interaction (FSI), we keep only terms involving the FSI enhancement
factor. The measured p-p and p-eta effective mass spectra can be well
reproduced by lifting the standard on-shell approximation in the enhancement
factor and by allowing for a linear energy dependence in the leading 3P0->1S0,s
partial wave amplitude. Higher partial waves seem to play only a marginal role
Production of -mesons in pp and pn reactions
We investigate the cross section for the reaction near
threshold and at medium energies. An effective Lagrangian approach with
one-pion exchange is applied to analyze different contributions to the cross
section for different isospin channels. The Reggeon exchange mechanism is also
considered. The results are used to calculate the contribution of the
meson to the cross sections and invariant mass distributions of the
reactions and . It is found that the
experimental observation of mesons in the reaction is much more promising than the observation of mesons in the
reaction .Comment: 26 pages, including 11 eps figures, to be bublished in J. Phys.
Threshold neutral pion photoproduction off the tri-nucleon to O(q^4)
We calculate electromagnetic neutral pion production off tri-nucleon bound
states (3H, 3He) at threshold in chiral nuclear effective field theory to
fourth order in the standard heavy baryon counting. We show that the fourth
order two-nucleon corrections to the S-wave multipoles at threshold are very
small. This implies that a precise measurement of the S-wave cross section for
neutral pion production off 3He allows for a stringent test of the chiral
perturbation theory prediction for the S-wave electric multipole E_{0+}^{pi0
n}.Comment: 17 pages, 5 figures, title changed, final version to appear in EPJA.
arXiv admin note: substantial text overlap with arXiv:1103.340
Charge Symmetry Violation Effects in Pion Scattering off the Deuteron
We discuss the theoretical and experimental situations for charge symmetry
violation (CSV) effects in the elastic scattering of pi+ and pi- on deuterium
(D) and 3He/3H. Accurate comparison of data for both types of targets provides
evidence for the presence of CSV effects. While there are indications of a CSV
effect in deuterium, it is much more pronounced in the case of 3He/3H. We
provide a description of the CSV effect on the deuteron in terms of single- and
double- scattering amplitudes. The Delta-mass splitting is taken into account.
Theoretical predictions are compared with existing experimental data for pi-d
scattering; a future article will speak to the pi-three nucleon case.Comment: 16 pages of RevTeX, 7 postscript figure
Study of the , , Decays with an SND Detector on a VEPP-2M Collider
The process was studied in the energy range
MeV with an SND detector on a VEPP-2M collider. The
decay branching ratios , , and
were measured.Comment: 5 pages, 4 figure
Chiral perturbation theory calculation for pn -> dpipi at threshold
We investigate the reaction pn -> dpipi in the framework of Chiral
Perturbation Theory. For the first time a complete calculation of the leading
order contributions is presented. We identify various diagrams that are of
equal importance as compared to those recognized in earlier works. The diagrams
at leading order behave as expected by the power counting. Also for the first
time the nucleon-nucleon interaction in the initial, intermediate and final
state is included consistently and found to be very important. This study
provides a theoretical basis for a controlled evaluation of the non-resonant
contributions in two-pion production reactions in nucleon-nucleon collisions.Comment: 24 pages, 3 figures, 3 table
Dynamical coupled-channel approaches on a momentum lattice
Dynamical coupled-channel approaches are a widely used tool in hadronic
physics that allow to analyze different reactions and partial waves in a
consistent way. In such approaches the basic interactions are derived within an
effective Lagrangian framework and the resulting pseudo-potentials are then
unitarized in a coupled-channel scattering equation. We propose a scheme that
allows for a solution of the arising integral equation in discretized momentum
space for periodic as well as twisted boundary conditions. This permits to
study finite size effects as they appear in lattice QCD simulations. The new
formalism, at this stage with a restriction to S-waves, is applied to
coupled-channel models for the sigma(600), f0(980), and a0(980) mesons, and
also for the Lambda(1405) baryon. Lattice spectra are predicted.Comment: 7 pages, 4 figure
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