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Abstract
Using the Gammasphere array of high-purity germanium detectors at the 88" Cyclotron at Lawrence Berkeley National Laboratory, we have made several measurements to test discrete fundamental symmetries in positronium annihilation. We tested charge conjugation symmetry (C) by searching for decays of ortho- and para-positronium to the "wrong" number of photons. We measured the five-photon annihilation mode of ortho-positronium, finding agreement with tree-level QED calculations at order α8. In a second experiment, we searched for decays of polarized ortho-positronium which violate the combined symmetry operation CPT. We improved existing limits on this CPT test by roughly a factor of ten. We are studying the feasibility of an experiment to search for "invisible" decays of ortho-positronium. Invisible decays could be caused by extra dimensions (invoked in string theories). General requirements for an experiment sensitive to decay branching ratios of order 10-9 are discussed.
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