Prof. Počanić is studying basic symmetries and conservation laws manifest at low and intermediate energies, with the aim of obtaining new stringent constraints on the dynamics of the two fundamental short-range interactions: the electroweak and the strong. This work requires precise measurements of the elementary decay and scattering processes involving some of the simplest particles available in nature: mesons, leptons, and nucleons.
The main project in this program is a precise measurement of the pion beta decay rate, at the Paul Scherrer Institute in Switzerland. This decay, π +→π0e+ν, provides the theoretically cleanest means of studying the weak coupling between the up and down quarks. Very precise knowledge of this coupling, at the level of a few tenths of one percent, constrains the unitarity of the quark mixing matrix, and thus tests the various hypotheses of the existence of additional, very heavy “elementary” particles, so far undetected in existing accelerator laboratories. Present evidence, based on the study of certain nuclear beta decays and of the neutron beta decay, is inconsistent, leading to contradictory conclusions regarding the u-d coupling strength. This important issue is being clarified by the new pion-beta decay measurement.
