Self energies of the pion and the Δ isobar from the 3He(e,e′π+)3H reaction

Abstract

In a kinematically complete experiment at the Mainz microtron MAMI, pion angular distributions of the 3 He(e,e'π + ) 3 H reaction have been measured in the excitation Preprint submitted to Elsevier Science 28 April 2018 region of the ∆ 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 ∆-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 ∆m π + = −1.7 + 1.7 − 2.1 MeV/c 2 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 ∆ self energy determined from measurements of π 0 photoproduction from 4 He and heavier nuclei.

Self consistent and covariant propagation of pions, nucleon and isobar resonances in cold nuclear matter

“…There are strong hints from the empirical photon nucleus absorption cross section as well as from pion-nucleus scattering data that the low-lying nucleon and delta resonances do change their properties in nuclear matter substantially already at nuclear saturation density [15,16,17,18,19,20]. Recent data on electroproduction of isobars off helium three [21] are interpreted in terms of a repulsive in-medium mass change and an increased decay width of the isobar-resonance [21]. Since most nucleon and isobar resonances have a substantial decay fraction into one nucleon and one pion, the in-medium resonance structure reflects to a large extent the medium modified propagation properties of pions.…”

Photoabsorption off nuclei with self-consistent vertex corrections

“…The phenomenological spreading potential [1] suggests a small repulsive mass shift of the isobar together with an increase of its width. Also recent data on electroproduction of isobars off helium three appear consistent with the latter interpretation [3]. With the exceptions [5,11,14] model computations of the isobar self energy claim results compatible with a small repulsive mass shift.…”

“…There is empirical evidence from photon nucleus absorption cross sections that the delta resonance changes its properties in nuclear matter substantially already at nuclear saturation density [1,2,3]. The microscopic description of the isobar self energy is a challenge taken up by various groups [4,5,6,7,8,9,10,11,12,13,14,15,16].…”