A New Paradigm for Hadronic Parity Nonconservation and Its Experimental Implications

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 mesonexchange descriptions this interaction together with an isoscalar interaction generated by ρ and ω 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 18 F, no evidence for isovector hadronic PNC has been found. Here we argue, based on recent large-N c treatments and new global analyses, that the emphasis on isovector hadronic PNC was misplaced. Large-N c 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 N 2 LO. 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-N c LEC hierarchy accurately describes all existing data on hadronic PNC. In particular, the null result found in 18 F reflects its dependence on an N 2 LO observable. We discuss opportunities to further test the predicted large-N c hierarchy of LECs, illustrating the kind of analyses experimentalists can use to better constrain the LO theory and to determine the size of N 2 LO corrections. This formalism -combined with a new wave of experiments that will be performed at the SNS cold neutron beam line and the recent demonstration that lattice QCD can now be applied to PNC NN scattering -could lead to rapid progress in the next five years. We discuss the impact of anticipated new results, including NPDGamma and a lattice QCD calculation of isotensor PNC. We also describe future experiments that can yield more precise values of the LO LECs and help isolate the N 2 LO ∼ 10% corrections.

Parity-violating three-nucleon interactions at low energies and large NC

“…Recently, a large-N C analysis has shown that not all of the LECs are equally significant [12,13]. In the large-N C counting [12][13][14] LECs and this has been shown to be consistent with available experimental data [14]. Note,…”

“…Ref. [14] did not consider recent results from the NPDGamma collaboration [2] as it was unavailable at the time.…”

“…Both A γ [20] and A pp L [21] have been calculated in EFT(/ π). While the asymmetry from pp scattering is sensitive to the two LO LECs in the large-N C counting, A γ is primarily sensitive to the isovector contribution in the 3 S 1 → 3 P 1 channel, which is next-to-next-to-leading-order (N 2 LO) in the large-N C counting [14]. Another possible few-body PV experiment is the asymmetry in the photodisintegration cross-section of deuterium using circularly polarized photons, P γ , in the process d γ → np, which could be carried out at an upgraded High Intensity Gamma Ray Source (HIγS) at the Triangle Universities Nuclear Laboratory [22] (TUNL).…”

ANNI – A pulsed cold neutron beam facility for particle physics at the ESS

“…In spite of the substantially smaller viewed area of the ESS cold moderator, pulsed experiments at the ESS should gain more than one order of magnitude in counting rate compared to the ILL. This led us to propose ANNI [11], a pulsed cold neutron beam facility at the ESS optimised for precision measurements of neutron beta decay, hadronic weak interaction in calculable systems [12], and electromagnetic properties of the neutron.…”

Effective field theory in the study of long range nuclear parity violation on lattice

“…The development of the EFT description of HPV [7][8][9][10][11][12][13][14] signifies a switch to a model-independent framework that features pion-exchanges and contact terms, where a systematic power expansion with respect to a typical small momentum scale p ensures the finiteness of the number of operators needed in any given order. Translation tables, sometimes known informally as the "Rosetta stone" [4,15], are available to connect these many different effective descriptions of the same physics [2,12,16] (where the cutoff dependence is also discussed for the translation). Finally, nuclear model calculations have been carried out to connect the HPV coupling strengths to the experimental observables in nuclear or atomic systems; examples in the ∆I = 1 channel include Refs.…”