271 research outputs found
Isospin violation in low-energy pion-nucleon scattering revisited
We calculate isospin breaking in pion-nucleon scattering in the threshold
region in the framework of covariant baryon chiral perturbation theory. All
effects due to quark mass differences as well as real and virtual photons are
consistently included. As an application, we discuss the energy dependence of
the triangle relation that connects elastic scattering on the proton pi+- p -->
pi+- p with the charge exchange reaction pi- p --> pi0 n.Comment: 82 pages, 22 figures, version published in Nucl. Phys.
Generalized parton correlation functions for a spin-1/2 hadron
The fully unintegrated, off-diagonal quark-quark correlator for a spin-1/2
hadron is parameterized in terms of so-called generalized parton correlation
functions. Such objects, in particular, can be considered as mother
distributions of generalized parton distributions on the one hand and
transverse momentum dependent parton distributions on the other. Therefore, our
study provides new, model-independent insights into the recently proposed
nontrivial relations between generalized and transverse momentum dependent
parton distributions. We find that none of these relations can be promoted to a
model-independent status. As a by-product we obtain the first complete
classification of generalized parton distributions beyond leading twist. The
present paper is a natural extension of our previous corresponding analysis for
spin-0 hadrons.Comment: 41 pages, 3 figures; v2: added referenc
Improved dispersive analysis of the scalar form factor of the nucleon
We present a coupled system of integral equations for the pi pi --> Nbar N
and Kbar K --> Nbar N S-waves derived from Roy-Steiner equations for
pion-nucleon scattering. We discuss the solution of the corresponding
two-channel Muskhelishvili-Omnes problem and apply the results to a dispersive
analysis of the scalar form factor of the nucleon fully including Kbar K
intermediate states. In particular, we determine the corrections Delta_sigma
and Delta_D, which are needed for the extraction of the pion-nucleon sigma term
from pi N scattering, and show that the difference Delta_D - Delta_sigma=(-1.8
+/- 0.2) MeV is insensitive to the input pi N parameters.Comment: 6 pages, 3 figures, Proceedings for the 7th International Workshop on
Chiral Dynamics, Newport News, Virginia, USA, August 6-10, 201
Bound state nature of the exotic Z_b states
The assumption that the newly observed charged bottomonia states Z_b(10610)
and Z_b(10650) are of molecular nature is confronted with the measured
invariant mass distributions for the transitions of the Upsilon(5S) to the
final states h_b pi^+ pi^- and h_b(2P) pi^+ pi^-. It is shown that the
assumption that the Z_b(10610) and Z_b(10650) are B\bar B^*+{\rm c.c.} and
B^*\bar B^* bound states, respectively, with very small binding energies is
consistent with the data. The calculation is based on a power counting for
bottom meson loops, which is explicitly given up to two-loop in the framework
of a nonrelativistic effective field theory. We also show that if the Z_b
states are of molecular nature, then the data should not be analyzed by using a
Breit-Wigner parametrization.Comment: 8 pages, 5 figures. Typos corrected, version appeared in Eur. Phys.
J.
High-precision determination of the pion-nucleon -term from Roy-Steiner equations
We present a determination of the pion-nucleon () -term
based on the Cheng-Dashen low-energy theorem (LET), taking
advantage of the recent high-precision data from pionic atoms to pin down the
scattering lengths as well as of constraints from analyticity,
unitarity, and crossing symmetry in the form of Roy-Steiner equations to
perform the extrapolation to the Cheng-Dashen point in a reliable manner. With
isospin-violating corrections included both in the scattering lengths and the
LET, we obtain MeV MeV,
where the first error refers to uncertainties in the amplitude and the
second to the LET. Consequences for the scalar nucleon couplings relevant for
the direct detection of dark matter are discussed.Comment: 6 pages, 1 figure; title changed by journal, version to be published
in PR
Extracting the sigma-term from low-energy pion-nucleon scattering
We present an extraction of the pion-nucleon () scattering lengths
from low-energy scattering, by fitting a representation based on
Roy-Steiner equations to the low-energy data base. We show that the resulting
values confirm the scattering-length determination from pionic atoms, and
discuss the stability of the fit results regarding electromagnetic corrections
and experimental normalization uncertainties in detail. Our results provide
further evidence for a large -term, MeV,
in agreement with, albeit less precise than, the determination from pionic
atoms.Comment: 17 pages, 3 figures; journal versio
Matching pion-nucleon Roy-Steiner equations to chiral perturbation theory
We match the results for the subthreshold parameters of pion-nucleon
scattering obtained from a solution of Roy-Steiner equations to chiral
perturbation theory up to next-to-next-to-next-to-leading order, to extract the
pertinent low-energy constants including a comprehensive analysis of systematic
uncertainties and correlations. We study the convergence of the chiral series
by investigating the chiral expansion of threshold parameters up to the same
order and discuss the role of the \Delta(1232) resonance in this context.
Results for the low-energy constants are also presented in the counting scheme
usually applied in chiral nuclear effective field theory, where they serve as
crucial input to determine the long-range part of the nucleon-nucleon potential
as well as three-nucleon forces.Comment: 6 pages, 4 tables; version to appear in PR
Extracting the sigma-term from low-energy pion-nucleon scattering
We present an extraction of the pion-nucleon () scattering lengths
from low-energy scattering, by fitting a representation based on
Roy-Steiner equations to the low-energy data base. We show that the resulting
values confirm the scattering-length determination from pionic atoms, and
discuss the stability of the fit results regarding electromagnetic corrections
and experimental normalization uncertainties in detail. Our results provide
further evidence for a large -term, MeV,
in agreement with, albeit less precise than, the determination from pionic
atoms.Comment: 17 pages, 3 figures; journal versio
Reordering of Baryon Chiral Perturbation Theory
Reordering of the chiral perturbation series, proposed recently by Becher and
Leutwyler in the framework of SU(2) baryonic ChPT, is applied to the SU(3)
case. This results in improved convergence of the chiral expansion of static
properties of the lowest lying baryon octet, which in most cases is quite
impressive. Finite renormalization of coupling constants and the role it plays
in the interpretation of effective field theories is discussed. Some future
tests of the viability of the scheme are proposed too.Comment: 9 pages, one reference adde
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