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Ulf-G. Meißner

Publications and source records attributed to Ulf-G. Meißner.

At least 631 records · Page 35Linked to original sources
arXiv1996

Chiral corrections to the Kroll-Ruderman theorem

We calculate the one--loop corrections to the Kroll--Ruderman low--energy theorems for charged pion photoproduction in the framework of heavy baryon chiral perturbation theory. We predict the threshold S--wave multipole $E_{0+}$ to be $E_{0+}^{\rm thr}(γp \to π^+ n) = ( 28.2 \pm 0.6 )\cdot 10^{-3}/M_π$ and $E_{0+}^{\rm thr} (γn \toπ^- p) = ( -32.7 \pm 0.6 )\cdot 10^{-3}/M_π$, respectively, for a fixed pion--nucleon coupling constant, $g_{πN} = 13.4$. A comparison to the existing data is also given.

hep-ph↗
arXiv1996

The reaction $πN \to ππN$ at threshold in chiral perturbation theory

In the framework of heavy baryon chiral perturbation theory, we give thIn the framework of heavy baryon chiral perturbation theory, we give the chiral expansion for the $πN \to ππN$ threshold amplitudes $D_1$ and $D_2$ to quadratic order in the pion mass. The theoretical results agree within one standard deviation with the empirical values. We also derive a relation between the two threshold amplitudes of the reaction $πN \to ππN$ and the $ππ$ S--wave scattering lengths, $a_0^0$ and $a_0^2$, respectively, to order ${\cal O}(M_π^2)$. We show that there are uncertainties mostly related to resonance excitation which make an accurate determination of the $ππ$ scattering length $a_0^0$ from the $ππN$ threshold amplitudes at present very difficult. The situation is different in the $ππ$ isospin two final state. Here, the chiral series converges and one finds $a_0^2 = -0.031 \pm 0.007$ consistent with the one--loop chiral perturbation theory prediction.

hep-ph↗
arXiv1996

Kaon loops in pion photoproduction: Facts and fancy

We show that a recent claim of huge kaon loop corrections to the electric dipole amplitude in neutral pion photoprodcution at threshold is incorrect. The difference between the two and three flavor chiral perturbation theory calculation is marginal and thus previous claims of a good understanding of this reaction remain valid.

nucl-th↗
arXiv1996

Improved description of threshold pion electroproduction in chiral perturbation theory

We investigate neutral pion electroproduction off protons in the framework of heavy baryon chiral perturbation theory. The chiral expansion of the S--wave multipoles $E_{0+}$ and $L_{0+}$ is carried out to three orders. There appear several undetermined low--energy constants. Three are taken from a recent study of the new TAPS threshold $π^0$ photoproduction data, one is fixed from the proton Dirac form factor and the novel S--wave constants appearing at orders $q^4$ and $q^5$ are determined from a best fit (constrained by resonance exchange) to the recent NIKHEF and MAMI data at photon momentum transfer squared $k^2 = -0.1 \, \, {\rm GeV}^2$. The inclusion of a particular set of dimension five operators is forced upon the fit by a soft--pion theorem which severely constrains the momentum dependence of the longitudinal S--wave multipole $L_{0+}$ at order $q^4$. We give predictions for lower photon virtualities for the various multipoles and differential cross sections. Further improvements are briefly touched upon.

hep-ph↗
arXiv1995

Chiral symmetry and the reaction $γp \to π^0 p$

We analyze the new threshold data for neutral pion photoproduction off protons in the framework of heavy baryon chiral perturbation theory. We show that large loop corrections are needed to understand the S--wave multipole $E_{0+}$ and that all pertinent low--energy constants can be understood within the framework of resonance exchange saturation. Previous inconsistencies in the description of this reaction in the threshold region are resolved.

hep-ph↗
arXiv1995

Chiral Lagrangians for Baryons coupled to massive Spin-1 Fields

We analyze the effective low--energy field theory of Goldstone bosons and baryons chirally coupled to massive spin--1 fields. We use the electromagnetic baryon form factors to demonstrate the formal equivalence between the vector and the tensor field formulation for the spin--1 fields. We also discuss the origin of the so--called Weinberg term in pion--nucleon scattering and the role of $ρ$--meson exchange. Chirally coupled vector mesons do not give rise to this two--pion nucleon seagull interaction but rather to higher order corrections. Some problems of the formal equivalence arising in higher orders and related to loops are touched upon.

hep-ph↗
arXiv1995

Strange Twists in Neutral Pion Photo/Electro-Production

I review the interesting tale of the electric dipole amplitude in neutral pion photoproduction and the resulting consequences. I also discuss why there is new life related to P--wave multipoles. Electroproduction is briefly touched upon.

hep-ph↗
arXiv1995

Nucleon Form Factors from Dispersion Theory

I review the results of a recent dispersion--theoretical analysis of the nucleon electromagnetic form factors and comment on the strangeness form factors. The need for a better data basis at low, intermediate and large momentum transfer is stressed.

hep-ph↗
arXiv1995

The Reaction $πN \to ππN$ at Threshold

I summarize the results of the complete one--loop chiral perturbation theory calculation performed recently. It is shown that it allows to accurately pin down the isospin two, S--wave $ππ$ scattering length $a_0^2$. On the other hand, interesting resonance physics makes a precise determination of $a_0^0$ very difficult.

hep-ph↗
arXiv1995

The Strangeness Radius and Magnetic Moment of the Nucleon Revisited

We update Jaffe's estimate of the strange isoscalar radius and magnetic moment of the nucleon. We make use of a recent dispersion--theoretical fit to the nucleon electromagnetic form factors and an improved description of symmetry breaking in the vector nonet. We find $μ_s = -0.24 \pm 0.03$~n.m. and $r_s^2 = 0.21 \pm 0.03$~fm$^2$. The strange formfactor $F_2^s (t)$ follows a dipole with a cut--off mass of 1.46~GeV, $F_2^s (t)= μ_s (1-t/2.14 \, {\rm GeV}^2 )^{-2}$. These numbers should be considered as upper limits on the strange vector current matrix--elements in the nucleon.

hep-ph↗
arXiv1995

Baryon masses and pion-nucleon $σ$-term to second order in the quark masses

We analyze the octet baryon masses and the pion--nucleon $σ$--term in the framework of heavy baryon chiral perturbation theory. In contrast to previous investigations, we include {\it all} terms up-to-and-including quadratic order in the light quark masses. The pertinent low--energy constants are fixed from resonance exchange. This leaves as the only free parameter the baryon mass in the chiral limit, $\krig{m}$. We find $\krig{m} = 749 \pm 125$~MeV together with $σ_{πN} (0) = 48 \pm 10$ ~MeV. We discuss various implications of these results.

hep-ph↗
arXiv1995

Non-perturbative structure of the nucleon

Chiral perturbation theory is the effective field theory of the standard model. In this talk, I discuss some applications of this framework to the pion--nucleon system. These are chiral corrections to the S--wave pion--nucleon scattering lengths, the reaction $πN \to ππN$ at threshold and low--energy theorems in $π^0$ photoproduction.

hep-ph↗
arXiv1995

Dispersion-Theoretical Analysis of the Nucleon Electromagnetic Formfactors

Dispersion relations allow for a coherent description of the nucleon electromagnetic form factors measured over a large range of momentum transfer, $Q^2 \simeq 0 \ldots 35$ GeV$^2$. Including constraints from unitarity and perturbative QCD, we present a novel parametrisation of the absorptive parts of the various isoscalar and isovector nucleon form factors. Using the current world data, we obtain results for the electromagnetic form factors, nucleon radii and meson couplings. We stress the importance of measurements at large momentum transfer to test the predictions of perturbative QCD.

hep-ph↗
arXiv1995

Pion Observables in the Extended NJL Model with Vector and Axial-Vector Mesons

The momentum-space bosonization method of a Nambu and Jona-Lasinio type model with vector and axial-vector mesons is applied to $ππ$ scattering. Unlike the case in earlier published papers, we obtain the $ππ$ scattering amplitude using the linear and nonlinear realizations of chiral symmetry and fully taking into account the momentum dependence of meson vertices. We show the full physical equivalence between these two approaches. The chiral expansion procedure in this model is discussed in detail. Chiral expansions of the quark mass, pion mass and constant $f_π$ are obtained. The low-energy $ππ$ phase shifts are compared to the available data. We also study the scalar form factor of the pion.

hep-ph↗
arXiv1995

Chiral Prediction for the $πN$ Scattering Length $a^-$ to Order ${\cal O}(M_π^4)$

We evaluate the S-wave pion--nucleon scattering length $a^-$ in the framework of heavy baryon chiral perturbation theory up--to--and--including terms of order $M_π^4$. We show that the order $M_π^4$ piece of the isovector amplitude at threshold, $T^-_{\rm thr}$, vanishes exactly. We predict for the isovector scattering length, $0.088 \, M_{π^+}^{-1} \le a^- \le 0.096 \, M_{π^+}^{-1}$.

hep-ph↗
arXiv1995

CHIRAL SYMMETRY CONSTRAINTS ON THE $K^+$ INTERACTION WITH THE NUCLEAR PION CLOUD

The real part of the $K^{+}$ selfenergy for the interaction of the $K^{+}$ with the pion nuclear cloud is evaluated in lowest order of chiral perturbation theory and is found to be exactly zero in symmetric nuclear matter. This removes uncertainties in that quantity found in former phenomenological analyses and is supported by present experimental data on $K^{+}$ nucleus scattering.

nucl-th↗
arXiv1995

On the Interpretation of $πN \to ππN$ Data near Threshold

Near threshold pion production experiments have been recently carried out and used to extract S--wave $ππ$ scattering lengths. We emphasize here that at present these processes are related only at the tree level (and its first correction) in chiral perturbation theory. Higher order corrections (including loops) must be evaluated before rigorous claims concerning S--wave $ππ$ scattering lengths can be made.

hep-ph↗