These are the four processes that account for the dilepton pairs we measure

pi-pi anihilation

Delta Dalitz Decay

Hadron-Hadron Bremsstrahlung

Production of eta meson

The four processes all produce a virtual photon, then decaying into the electron-positron pair we measure. Later on, what we refer to as mass, momentum...and all the kinematic variables are all respect to the virtual photon--the mother of the dilepton pairs.

Wei Yuan Yang
National Taiwan University
Physics Department