A Test of Cp Symmetry in Positronium

The aim of this CP symmetry test in positronium is to measure the CP violation amplitude parameter CCP . This is derived from the measurement of the asymmetry in an angular distribution of the photons from the decay of the ortho-positronium in a magnetic field. The Standard Model prediction for CCP is a value of the order of 10 . Thus the observation of a larger CCP value would be signal of physics beyond the Standard Model. A previous measurement has found CCP consistent with zero, with an uncertainty of ∼ 10. We have investigated the possibility of using the existing ETHZINRM-IN2P3 BGO crystal detector, set-up for positronium physics studies, to improve the sensitivity on the CCP measurement. Preliminary calculations indicate that, using such an apparatus, with some modification, in a magnetic field of 4 kGauss, CCP could be measured with an uncertainty in the range between ∼ 10 and ∼ 10, depending mainly on the uncertainty in the asymmetry measurement and the angular resolution of the photon detectors. If CCP is less than ∼ 10 , the experimental technique outlined here appears to be inadequate to observe a CP violating effect and new techniques or different observables must be exploited for better sensitivity.