How are K0 and anti-K0 related to K_short?
The question of K0’s in FLUKA is very complicated. You can have “pure states” and superpositions of states, hadronic and leptonic, short and long: and some of these are relevant at production time but not at transport time and vice-versa.
FLUKA considers neutral Kaons in different ways according to their origin: If kaons0 are coming from the weak decay of some resonance, like for instance a phi particle, where the transport eigenstates matter, then they are labelled KAONLONG (I.D.: 12) and KAONSHRT (I.D.: 19). Otherwise, if they are coming directly from the hadronisation chain, or from strong decays of resonances, they are labelled KAONZERO (I.D.: 24) and AKAONZER (I.D.: 25), according to their parton content. These kaons, during particle transport, are then treated as a proper combination of KAONLONG and KAONSHRT in order to have the right decay properties.
From the point of view of hadronic interactions instead, only KAONZERO and AKAONZER have meaning. Therefore, if a KAONLONG has to interact with another hadron it is first decomposed in KAONZERO/AKAONZER.