arXiv subjects
Publications and source records attributed to Ulf-G. Meißner.
We show that a recent claim (H.Haberzettl, Phys. Rev. Lett. 85 (2000) 3576) that one cannot extract the nucleon weak axial form factor G_A (t) from charged pion threshold electroproduction is incorrect. Thus previous calculations remain valid and threshold charged pion electroproduction experiments can indeed be used to determine G_A (t), and they should certainly be pursued.
We consider pion-nucleon scattering in heavy-baryon chiral perturbation theory to third order. All electromagnetic corrections appearing to this order are included. We thus have a consistent description of strong and electromagnetic effects, which allows us to isolate the strong part of the interaction in an unambiguous way. We give pion-nucleon phase shifts up to pion laboratory momenta of 100 MeV and find sizeable differences for the S-waves of the elastic channels compared to previous phase shift analyses. The precise description of the scattering process also allows us to address the question of isospin violation in the strong interaction. For the usually employed triangle relation we find an isospin breaking effect of -0.7% in the S-wave, whereas the P-waves show an effect of -1.5% and -4 to -2.5%, respectively, for pion laboratory momenta between 25 and 100 MeV.
We construct the minimal effective chiral pion-nucleon SU(2) Lagrangian at fourth order in the chiral expansion. The Lagrangian contains 118 in principle measurable terms. We develop both the relativistic as well as the heavy baryon formulation of the effective field theory. For the latter, we also work out explicitly all 1/m corrections at fourth order. We display all relevant relations needed to find the linearly independent terms.
We highlight some of the recent results in chiral dynamics for systems with one nucleon/baryon presented at Chiral Dynamics 2000. We outline the most urgent experimental and theoretical challenges to be tackled in the coming years.
We study the strong interaction S-wave Λπphase shift in the region of the Ξmass in the framework of a relativistic chiral unitary approach based on coupled channels. All parameters have been previously determined in a fit to strangeness S= -1 S-wave kaon-nucleon data. We find 0^\circ \le δ_0 \le 1.1^\circ in agreement with previous chiral perturbation theory calculations (or extensions thereof). We also discuss why a recent coupled channel K-matrix calculation gives a result for δ_0 that is negative and much bigger in magnitude. We argue why that value should not be trusted.
We study the S-wave kaon-nucleon interactions for strangeness S=-1 in a novel relativistic chiral unitary approach based on coupled channels. Dispersion relations are used to perform the necessary resummation of the lowest order relativistic chiral Lagrangian. A good description of the data in the K^- p, πΣand πΛchannels is obtained. We show how this method can be systematically extended to higher orders, emphasizing its applicability to any scenario of strong self-interactions where the perturbative series diverges even at low energies. Discussions about the differences to existing approaches employing pseudo-potentials in a regulated Lippmann-Schwinger equation are included. Finally, we describe the resonance content of our meson-baryon amplitudes and discuss its nature.
We analyze the electromagnetic form factors of the ground state baryon octet to fourth order in relativistic baryon chiral perturbation theory. Predictions for the Σ^- charge radius and the Λ-Σ^0 transition moment are found to be in excellent agreement with the available experimental information. Furthermore, the convergence behavior of the hyperon charge radii is shown to be more than satisfactory.
We calculate ordinary and radiative muon capture on the proton in an effective field theory of pions, nucleons and delta isobars, working to third and second order in the small scale expansion respectively. Preceding calculations in chiral effective field theories only employed pion and nucleon degrees of freedom and were not able to reproduce the photon spectrum in the pioneering experiment of radiative muon capture on the proton from TRIUMF. For the past few years it has been speculated that the discrepancy between theory and experiment might be due to Delta(1232) related effects, which are only included via higher order contact interactions in the standard chiral approach. In this report we demonstrate that this speculation does not hold true. We show that contrary to expectations from naive dimensional analysis isobar effects on the photon spectrum and the total rate in radiative muon capture are of the order of a few percent, consistent with earlier findings in a more phenomenological approach. We further demonstrate that both ordinary and radiative muon capture constitute systems with a very well behaved chiral expansion, both in standard chiral perturbation theory and in the small scale expansion, and present some new ideas that might be at the bottom of the still unresolved discrepancy between theory and experiment in radiative muon capture. Finally we comment upon the procedure employed by the TRIUMF group to extract new information from their radiative muon capture experiment on the pseudoscalar form factor of the nucleon. We show that it is inconsistent with the ordinary muon capture data.
We discuss charge symmetry and charge independence breaking in a chiral effective field theory approach for few-nucleon systems based on a modified Weinberg power counting. We construct a two-nucleon potential with bound and scattering states generated by means of a properly regularized Lippmann-Schwinger equation. We systematically introduce strong isospin-violating and electromagnetic operators in the theory. We use standard procedures to treat the Coulomb potential between two protons in momentum space. We present results for phase shifts in the proton-proton, the neutron-proton and the neutron-neutron systems. We discuss the various contributions to charge dependence and charge symmetry breaking observed in the nucleon-nucleon scattering lengths.
We have studied the invariant mass distributions of the ππand K \bar{K} systems for invariant masses up to 1.2 GeV from the J/Ψ\to ϕππ(K\bar{K}) decays. The approach exploits the connection between these processes and the ππand K\bar{K} strange and non-strange scalar form factors by considering the ϕmeson as a spectator. The calculated scalar form factors are then matched with the ones from next-to-leading order chiral perturbation theory, including the calculation of the the K\bar{K} scalar form factors. Final state interactions in the J/Ψ\to ϕππ(K\bar{K}) processes are taken into account as rescattering effects in the system of the two pseudoscalar mesons. A very good agreement with the experimental data from DM2 and MARK-III is achieved. Furthermore, making use of SU(3) symmetry, the S-wave contribution to the π^+π^- event distribution in the J/Ψ\to ωπ^+π^- reaction is also given and the data up to energies of about 0.7 GeV are reproduced. These decays of the J/Ψto a vector and a pair of pseudoscalars turn out to be very sensitive to OZI violating physics which we parametrize in terms of a direct OZI violation parameter and the chiral perturbation theory low energy constants L_4^r and L_6^r. These constants all come out very different from zero, lending further credit to the statement that the OZI rule is not operative in the scalar 0^{++} channel.
We investigate isospin breaking in low-energy pion-nucleon scattering in the framework of chiral perturbation theory. This work extends the systematic analysis of [1] to the energy range above threshold. Various relations, which identically vanish in the limit of isospin symmetry, are used to quantify isospin breaking effects. We study the energy dependence of the S- and P-wave projections of these ratios and find dramatic effects in the S-waves of those two relations which are given in terms of isoscalar quantities only. This effect drops rather quickly with growing center-of-mass energy.
I summarize the theoretical predictions for the spin-dependent nucleon polarizabilities based on chiral effective field theory approaches.
Recently developed chiral nucleon-nucleon (NN) forces at next-to-leading order (NLO) that describe NN phase shifts up to about 100 MeV fairly well have been applied to 3N and 4N systems. Faddeev-Yakubovsky equations have been solved rigorously. The chiral NLO forces depend on a momentum cut-off Λlying between 540-600 MeV/c. The resulting 3N and 4N binding energies are in the same range as found using standard NN potentials. In additon, low-energy 3N scattering observables are very well reproduced like for standard NN forces. Surprisingly, the long standing A_y-puzzle is resolved at NLO. The cut-off dependence of the scattering observables is rather mild.
I review the consequences of the chiral symmetry of QCD for the structure and dynamics of the low-lying baryons, with particular emphasis on the nucleon.
We analyze the electromagnetic form factors of the nucleon to fourth order in relativistic baryon chiral perturbation theory. We employ the recently proposed infrared regularization scheme and show that the convergence of the chiral expansion is improved as compared to the heavy fermion approach. We also discuss the inclusion of vector mesons and obtain an accurate description of all four nucleon form factors for momentum transfer squared up to Q^2 \simeq 0.4 GeV^2.
We analyze elastic-pion nucleon scattering to third order in the so-called small scale expansion. It is based on an effective Lagrangian including pions, nucleons and deltas as active degrees of freedom and counting external momenta, the pion mass and the nucleon-delta mass splitting as small parameters. The fermion fields are considered as very heavy. We present results for phase shifts, threshold parameters and the sigma term. We discuss the convergence of the approach. A detailed comparison with results obtained in heavy baryon chiral perturbation theory to third and fourth order is also given.
We analyse elastic-pion nucleon scattering to fourth order in heavy baryon chiral perturbation theory, extending the third order study published in Nucl. Phys. A640 (1998) 199. We use various partial wave analyses to pin down the low-energy constants from data in the physical region. The S-wave scattering lengths are consistent with recent determinations from pionic hydrogen and deuterium. We find an improved description of the P-waves. We also discuss the pion-nucleon sigma term and problems related to the prediction of the subthreshold parameters.
Threshold neutral pion electroproduction on the deuteron is studied in the framework of baryon chiral perturbation theory at next-to-leading order in the chiral expansion. To this order in small momenta, the amplitude is finite and a sum of two- and three-body interactions with no undetermined parameters. We calculate the S-wave multipoles for threshold production and the deuteron S-wave cross section as a function of the photon virtuality. We also discuss the sensitivity to the elementary neutron amplitudes.