# A New Paradigm for Hadronic Parity Nonconservation and its Experimental Implications

## TL;DR

This paper proposes a new theoretical framework based on large-Nc QCD for understanding hadronic parity nonconservation, challenging previous focus on isovector interactions, and discusses experimental implications for better constraining the theory.

## Contribution

It introduces a large-Nc based hierarchy of low-energy constants in effective field theories of hadronic PNC, shifting the focus from isovector to isoscalar and isotensor interactions.

## Key findings

- Large-Nc hierarchy accurately describes existing hadronic PNC data.
- The new scheme predicts dominant isoscalar and isotensor contributions.
- Experimental tests can further validate the large-Nc LEC hierarchy.

## Abstract

For decades the primary experimental goal in studies of hadronic parity nonconservation (PNC) has 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 rho and omega exchange dominate most observables. Consequently these two amplitudes have been used to compare and check the consistency of the field's experiments. Yet to date, despite sensitive searches like that performed with 18F, no evidence for isovector hadronic PNC has been found. Here we argue, based on recent large-Nc treatments and new global analyses, that the emphasis on isovector hadronic PNC was misplaced. Large-Nc 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 others that are NNLO. This scheme pivots the isospin coordinates we have traditionally used, placing one dominant axis in the isoscalar plane, and a second along the isotensor direction. We show that this large-Nc LEC hierarchy accurately describes all existing data on hadronic PNC, and we discuss opportunities to further test the predicted large-Nc hierarchy of LECs, illustrating the kind of analyses experimentalists can use to better constrain the LO theory and to determine the size of NNLO corrections.