Each pion consists of a quark and an antiquark and is therefore a meson. The Skyrmion models the nucleon as a topological soliton in a non-linear SU(2) pion field. This term also allows for lepton number violating pion decays, and for proton decay. Similarly, a neutral pion most often decays into two photons. Yukawa's meson was found in 1947, and came to be known as the pion. In the case of interacting nucleons, the dotted line would be a virtual pion. The interaction that does couples the nucleons to the "gradient" of the pion field. The 1947 discovery of the pion ushered in the modern era of particle physics. Thus they concluded that the pion is a pseudoscalar particle. This is a manifestation of an approximate symmetry, a shift symmetry of the pion field. The and have two modes of two pion decay&colon; or . Notable alumni include Tehoka Nanticoke, Marc-Antoine Pion, Zack Pinney and Nick Wood. The 2S7 "Pion" ("peony") or Malka is a Soviet self-propelled 203mm heavy artillery. The initial system name was S-125A "Pion". One interesting feature of the Pion is the firing alarm.