Design and Testing of an Apparatus for CP Violation Search in Positronium Decay

The combination of Charge Conjugation and Parity (CP) violation is necessary to explain why there is more matter than anti-matter in the universe. The amount of CP violation so far observed fails to account for the matter excess. We're constructing an apparatus to search for CP violation in the 3-photon decay of ortho-positronium (Ps) in a magnetic field. To detect the 3-photon decay, we will use an array of gamma detectors (LYSO crystals coupled to silicon photomultipliers). To form Ps, a positron source is placed between containers of granular material in the center of the array. Start detectors will be employed to tag the emitted positrons in order to select the longer-lived ortho-Ps. The start detectors are thin plastic scintillators coupled to long light pipes to carry light out the magnetic field to photomultiplier tubes (PMTs). We will present our work on designs and testing of possible start detectors.