Thomas R. Hemmert

Complete One-Loop Analysis of the Nucleon's Spin Polarizabilities

We present a complete one-loop analysis of the four nucleon spin polarizabilities in the framework of heavy baryon chiral-perturbation theory. The first nonvanishing contributions to the isovector and first corrections to the isoscalar spin polarizabilities are calculated. No unknown parameters enter these predictions. We compare our results to various dispersive analyses. We also discuss the convergence of the chiral expansion and the role of the delta isobar.

The Delta nucleon transition form-factors in chiral perturbation theory

The three complex form factors entering the ∆ → Nγ∗ vertex are calculated to O(ε3) in the framework of a chiral effective theory with explicit ∆(1232) degrees of freedom included. Corrections to leading contributions entering the expansion in inverse powers of the chiral symmetry breaking scale Λχ are identified. It is shown that the low q 2 behavior of the form factors is governed by πN , π∆ loop effects. Furthermore, the role of presently unknown low energy constants that affect the values of the transition-multipole ratios EMR and CMR is elucidated. PACS numbers: Typeset using REVTEX ∗e-mail: ggellas@atlas.cc.uoa.gr †email: th.hemmert@fz-juelich.de ‡e-mail: gpoulis@rtm.iasa.uoa.gr 1

STRANGE MAGNETISM IN THE NUCLEON

Abstract Using heavy baryon chiral perturbation theory to one loop, we derive an analytic and parameter-free expression for the momentum dependence of the strange magnetic form factor of the nucleon G M ( s ) ( Q 2 ) and its corresponding radius. This should be considered as a lower bound. We also derive a model-independent relation between the isoscalar magnetic and the strange magnetic form factors of the nucleon based on chiral symmetry and SU(3) only. This gives an upper bound on the strange magnetic form factor. We use these limites to derive bounds on the strange magnetic moment of the proton from the recent measurement of G M (s) (Q 2 =0.1 GeV 2 ) by the SAMPLE collaboration. We further stress the relevance of this result for the on-going and future experimental programs at various electron machines.

BARYON FORM FACTORS

Abstract We calculate the form factors of the baryon octet in the framework of heavy baryon chiral perturbation theory. The calculated charge radius of the Σ− is in agreement with recent measurements from CERN and Fermilab. We show that kaon loop effects can play a significant role in the neutron electric form factor. Furthermore, we derive generalized Caldi–Pagels relations between various charge radii which are free of chiral loop effects.

Strange chiral nucleon form factors

We investigate the strange electric and magnetic form factors of the nucleon in the framework of heavy baryon chiral perturbation theory to third order in the chiral expansion. All counterterms can be fixed from data. In particular, the two unknown singlet couplings can be deduced from the parity-violating electron-scattering experiments performed by the SAMPLE and the HAPPEX Collaborations. Within the given uncertainties, our analysis leads to a small and positive electric strangeness radius,

Ordinary and radiative muon capture on the proton and the pseudoscalar form factor of the nucleon

〈{r}_{E,s}^{2}〉=(0.05\ifmmode\pm\else\textpm\fi{}0.09) {\mathrm{fm}}^{2}.

Charged radiative pion capture on the nucleon in heavy baryon chiral perturbation theory

We also deduce the consequences for the upcoming MAMI A4 experiment.

Generalized polarizabilities of the nucleon in chiral effective theories

Abstract 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 Δ (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 the 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.

Gellas, Hemmert, and Meissner reply to Comment on `Complete one-loop analysis of the nucleon's spin polarizabilities'

The differential cross sections and s-wave and p-wave multipoles for

NN and NDelta Form Factors viewed from ChPT

\pi^- p \to \gamma n