The 4He(π+,π−) reaction at low energies

Using the CHAOS spectrometer at TRIUMF the total cross sections and outgoing pion momentum distributions for the 4 He( π + , π − ) reaction were measured for π + kinetic energies from 70 to 130 MeV. At energies around 100 MeV the total cross sections show an excess by a factor 3 over conventional model calculations which could be ascribed to a contribution from a hypothetical d ′ dibaryon.