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Abstract: The problem of formulating the baryon chiral perturbation theory (BχPT) in the presence of a light resonance, such as the the lightest nucleon resonance, is discussed. It is shown how to extend the power counting of BχPT to correctly account for the resonant contributions. Recent applications of the resulting chiral effective field theory to the description of pion production reactions in the -resonance region are briefly reviewed. [Copyright &y& Elsevier]

Near 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. We develop the multipole decomposition for pion production off spin-1 particles appropriate for the threshold region. The existing data at photon virtuality k2=−0.1 GeV2 can be described satisfactorily. Furthermore, the prediction for the S-wave multipole Ed at the photon point is in good agreement with the data. [Copyright &y& Elsevier]

In a kinematically complete experiment at the Mainz microtron MAMI, pion angular distributions of the 3He(e,e′π+)3H reaction have been measured in the excitation region of the Δ resonance to determine the longitudinal (L), transverse (T), and the LT interference part of the differential cross section. The data are described only after introducing self-energy modifications of the pion and Δ-isobar propagators. Using Chiral Perturbation Theory (ChPT) to extrapolate the pion self energy as inferred from the measurement on the mass shell, we deduce a reduction of the π+ mass of Δmπ+ =–1.7+1.7–2.1 MeV/c2 in the neutron-rich nuclear medium at a density of 𝜌=0.057+0.085–0.057 fm–3.Our data are consistent with the Δ self energy determined from measurements of π0 photoproduction from 4He and heavier nuclei. [Copyright &y& Elsevier]