A New Paradigm for Hadronic Parity Nonconservation and Its Experimental Implications

The primary experimental goal of studies of hadronic parity nonconservation (PNC) has long been the isolation of the isovector weak nucleon-nucleon interaction, expected to be dominated by long-range pion exchange and enhanced by the neutral current. In meson-exchange descriptions, this interaction, together with an isoscalar interaction generated by. and. exchange, dominates most observables. Both amplitudes have been used to compare and check the consistency of experiments, yet no evidence for isovector hadronic PNC has been found. We argue that the emphasis on isovector hadronic PNC was misplaced. The large-Nc expansion provides an alternative and theoretically better-motivated simplification of effective field theories (EFTs) of hadronic PNC, separating the five low-energy constants (LECs) into two of leading order (LO) and three of next-to-next-to-leading order ((NLO)-L-2). We show that this large-Nc LEC hierarchy accurately describes all existing data on hadronic PNC and discuss opportunities to further test the predicted large-Nc LEC hierarchy. This formalism-combined with future experiments-could lead to rapid progress in the next 5 years. We discuss the impact of anticipated new results and describe future experiments that can yield more precise values of the LO LECs and help to isolate the (NLO)-L-2 similar to 10% corrections.