Hadronic parity violation

We review the state of hadronic parity violation and propose a program based on an effective field theory potential which can resolve the present confusion.     

Muon anomaly and dark parity violation

The muon anomalous magnetic moment exhibits a 3.6σ discrepancy between experiment and theory. One explanation requires the existence of a light vector boson, Z_{d} (the dark Z), with mass 10-500?MeV that couples weakly to the electromagnetic current through kinetic mixing. Support for such a solution also comes from astrophysics conjectures regarding the utility of a U(1)_{d} gauge symmetry in the dark matter sector. In that scenario, we show that mass mixing between the Z_{d} and ordinary Z boson introduces a new source of "dark" parity violation, which is potentially observable in atomic and polarized electron scattering experiments. Restrictive bounds on the mixing (m_{Z_{d}}/m_{Z})δ are found from existing atomic parity violation results, δ^{2}<2×10^{-5}. Combined with future planned and proposed polarized electron scattering experiments, a sensitivity of δ^{2}～10^{-6} is expected to be reached, thereby complementing direct searches for the Z_{d} boson.     

Overview of hadronic parity violation

The subject of hadronic parity violation is nearly fifty years old, but a good deal of uncertainty remains, despite many efforts both theoretical and experimental. A brief summary of the field is presented and a plan is proposed for new experimental work which, when combined with a new theoretical tack based on effective field theory, should lead to resolution of the present difficulties.     

Future directions in parity violation

I discuss the prospects for future studies of parity-violating (PV) interactions at low energies and the insights they might provide about open questions in the standard model as well as physics that lies beyond it. I cover four types of parity-violating observables: PV electron scattering; PV hadronic interactions; PV correlations in weak decays and searches for the permanent electric dipole moments of quantum systems.     

On parity violation in α-decay

The Fermi liquid model of α-decay and the shell-model wave functions of Zuker, Buck and McGrory are used to calculate the “regular” and “irregular” (parity non-conserving) α-widths to some J^πT = 2±0 low-lying levels of ¹⁶O. The Pauli corrections in the α-channel wave functions are taken into account.     

Neutral current parity violation and gauge flavours

In a large class of gauge theories the trace of the weak neutral axial charge operator over any complete irreducible gauge multiplet is shown to vanish. This implies a relation involving deep inelastic neutrino-induced neutral current parity-violating cross sections which can be used to test the correctness of the Weinberg-Salam gauge multiplet structure of quarks.     

Theoretical aspects of nuclear parity violation

We make use of the topological chiral soliton model of the nucleon to predict the form factors related to the parity-violating meson-nucleon vertices. We find that these are of monopole type at low q² with cut-offs similar to the equivalent strong meson-nucleon form factors. We furthermore investigate the parity-violating πΔN vertex. We find that while the isoscalar coupling vanishes identically, the isovector parity-violating πΔN vertex has a coupling stant of about 2 × 10⁻⁸ being of comparable magnitude as the equivalent weak pion-nucleon vertex strength. We also discuss the parity-violating ωΔN and ρΔN vertices in the model and furthermore comment on the pv meson ΔΔ vertices. In the appendix we will correct the formula for h_ω⁰ of ref. ⁵), where some terms have been omitted.     

Hadronic parity violation and effective field theory

The history and phenomenology of hadronic parity violation is reviewed and a new model-independent approach based on effective field theory is developed. Possible future developments are discussed.     

Spontaneous parity violation and minimal Higgs models

In this paper we present a model for the spontaneous breaking of parity with two Higgs doublets and two neutral Higgs singlets which are even and odd under -parity. The condition can be satisfied without introducing bidoublets, and it is induced by the breaking of -parity through the vacuum expectation value of the odd Higgs singlet. Examples of left–right symmetric and mirror fermions models in grand unified theories are presented. PACS 12.60.Cn; 14.80.Cp; 12.10.Dm     

Atomic parity violation and the HERA anomaly

We show that the two scenarios able to explain the HERA anomaly — a new leptoquark coupling or a new contact interaction — predict new contributions to atomic parity violation. These corrections are sufficiently large and different that a feasible reduction in the dominant atomic theory uncertainty could give some hint in favour of one of the two scenarios.     

Theoretical overview of atomic parity violation

Recent advances in interpreting the most accurate-to-date measurement of atomic parity violation in Cs are reviewed. The inferred nuclear weak charge, Q _W( ^133Cs ) = - 72.65(28)_expt(36)_theor , agrees with the prediction of the standard model at 1σ level. Further improved interpretation is limited by an accuracy of solving the basic correlation problem of the atomic structure. We report on our progress in solving this problem within the relativistic coupled-cluster formalism. We include single, double and triple electronic excitations in the coupled-cluster expansion. Numerical results for energies, electric-dipole matrix elements, and hyperfine-structure constants of Cs are presented.     

Nuclear parity violation from supersymmetric QCD

Supersymmetric QCD will exhibit parity violation if the squark partners of left- and right-handed quarks are not mass degenerate. Assuming that the standard electroweak effects are still the major contributor to nuclear parity violation we extract bounds on squark masses and their differences.     

Amplification of atomic L-R asymmetries by stimulated emission: experimental demonstration of sensitivity enhancement valuable for parity violation measurements

While all Atomic Parity Violation experiments on highly forbidden transitions in a Stark field have used the detection of fluorescence signals, our group is engaged in an experiment on the cesium transition that uses a pump-probe scheme. The role of the probe beam is to detect the 7S state by stimulated emission. The detected Left-Right asymmetry () appears directly on the transmitted probe beam and the technique relies on differential-mode atomic polarimetry. We present here experimental results which illustrate two essential features of this approach. First, is amplified when the optical thickness for the probe beam is increased, hence it is an increasing function of the Stark field. Secondly, the experimental sensitivity to is simultaneously increase d, as demonstrated by our measurements of the signal-to-noise ratio. We emphasize also the advantage of choosing a probe transition that involves a “dark” state: the amplification is preserved at high levels of the probe intensity because saturation effects are greatly reduced. Received: 8 october 1997 / Accepted: 21 November 1997     

Space-time parity violation and magnetoelectric interactions in antiferromagnets

The properties of antiferromagnetic materials with violated space-time parity are considered. Particular attention is given to the bismuth ferrite BiFeO₃ ferroelectric magnet. This material is distinguished from other antiferromagnets in that the inversion center is absent in its crystal and magnetic structures. This circumstance gives rise to diversified and unusual properties, namely, to the appearance of a spatially modulated spin structure and to the unique possibility of the linear magnetoelectric effect coexisting with a weak ferromagnetic moment. The magnetic-induced phase transitions accompanied by the suppression of the modulated spin structure and appearance of a number of new and unusual effects are considered. These are the linear magnetoelectric effect and the appearance of a toroidal moment and a weak ferromagnetic moment of the magnetoelectric nature.     

Fermions and parity violation in dimensional reduction schemes

Dimensional reduction, previously applied to Yang-Mills theories, is extended to gauge theories with spinor fields. It is shown that a fairly realistic model in Minkowski space can be obtained from the simplest initial lagrangian, defined in a space-time with extra, compact dimensions. Left-right asymmetry in the fermion sector in four dimensions is possible, and its occurrence is related to a non-vanishing Atiyah-Singer index.     

Observation of a parity violation in cesium

We have measured a parity violation in the 6S–7S transition of Cs in an electric field. Our result is Im $\text{E}{}_1{}^{\mathrm{<mtext>pv</mtext>}}\text{β}\text{= -1.34 ± 0.22}$$\text{(}\text{rms statistical deviation}\text{) ± ～0.11 (}\text{systematic uncertainty}\text{)}\text{mV}\text{cm;}\text{E}{}_1{}^{\mathrm{<mtext>pv</mtext>}}$ is the parity violating electric dipole amplitude, ß is the vector polarizability. This result is consistent with the Weinberg-Salam prediction.     

Search for parity violation in neutron-proton capture

A measurement of the asymmetry in radiative neutron-proton capture has been performed using polarized cold neutrons on a parahydrogen target. The result, A_γ = (0.6±2.1) × 10^?7, is in agreement with values predicted by various theories of the weak interaction.