Hadronic Parity Violation: A New View Through the Looking Glass

Abstract

Key Words hadronic weak interaction, nuclear parity violation, effective field theory ■ Abstract Studies of the strangeness-changing hadronic weak interaction have produced a number of puzzles that have evaded a complete explanation within the Standard Model. Their origin may lie either in dynamics peculiar to weak interactions involving strange quarks or in more general aspects of the interplay between strong and weak interactions. In principle, studies of the strangeness-conserving hadronic weak interaction using parity-violating hadronic and nuclear observables provide a complementary window to this question. However, progress in this direction has been hampered by the lack of a suitable theoretical framework for interpreting hadronic parity violation measurements in a model-independent way. Recent work involving effective field theory ideas has led to the formulation of such a framework, simultaneously motivating the development of a number of new hadronic parity violation experiments in few-body systems. We review these recent developments and discuss the prospects and opportunities for further experimental and theoretical progress.

Measurement of parity-violatingγ-ray asymmetry in the capture of polarized cold neutrons on protons

“…Both nonleptonic weak kaon decays (which have been known for decades to be greatly amplified in the ∆I = 1/2 channel) and nonleptonic weak decays of hyperons exhibit deviations from the expected relative sizes of weak amplitudes whose dynamical source is still not fully understood [6]. If these unexpected patterns in the isospin dependence of nonleptonic weak amplitudes are confirmed by measurements in the NN and few nucleon systems, it would indicate that this dynamical puzzle operates for all light quarks (rather than just the strange quark) and is therefore a nontrivial QCD dynamical phenomenon of general interest [1]. The weak NN interaction is also one of the few systems thought to be sensitive to quark-quark neutral current effects at low energy since charged currents are suppressed in ∆I = 1 NN processes by V 2 us /V 2 ud ≃ 0.1.…”

“…The HWI in general and the NN weak interaction amplitudes in particular are scientifically interesting for several reasons [1][2][3][4][5]. Because the range for W and Z exchange between quarks is small compared to the nucleon size, HWI are first-order sensitive to quark-quark correlations in hadrons.…”

“…For processes in which the momentum transfers involved are below ≈ 40 MeV a pionless EFT is appropriate, whereas for higher momenta it becomes important to include explicit pion degrees of freedom. One version [1,5,31] termed a "hybrid"EFT treats the weak interaction with nucleon contact interactions parameterized in terms of twelve coupling constants which specify the strength of twelve possible parity violating, CP conserving four-nucleon terms in the Lagrangian, and implements the strong interaction using NN potentials. The low energy limit of the pionful version of this theory possesses six independent parameters: five weak S-P transition amplitudes (λ 0,1,2 S , λ T , and ρ T )) which reduce to the five parameters in the pionless version, and a longer-range termC π 6 from explicit pion exchange (proportional to h 1 π in DDH).…”

Parity violation in radiative neutron capture on the deuteron

“…(see, for example [1][2][3][4] and references therein) that it is rather difficult to describe the available experimental data with the same set of weak nucleon coupling constants using the traditional DDH [5] weak meson exchange potential.…”

“…As a possible solution for this problem, a new approach, based on the effective field theory (EFT), has been introduced to parameterize PV effects in a model independent way (see, papers [1,4,6] and references therein). The main goal of the EFT approach is to describe a large number of PV effects in terms of a small number of constants (LEC), which are the free parameters of the theory.…”