Weak Nucleon-nucleon Interaction Research Articles

Measurement of the parity-odd angular distribution of γ rays from polarized neutron capture on Cl35

We report a measurement of two energy-weighted γ cascade angular distributions from polarized slow neutron capture on the Cl35 nucleus, one parity-odd correlation proportional to sn⃗·kγ⃗ and one parity-even correlation proportional to sn⃗·kn⃗×kγ⃗. A parity-violating asymmetry can appear in this reaction due to the weak nucleon-nucleon interaction, which mixes opposite parity S- and P-wave levels in the excited compound Cl36 nucleus formed upon slow neutron capture. If parity-violating (PV) and parity-conserving (PC) terms both exist, the measured differential cross section can be related to them via dσdΩ∝1+Aγ,PVcosθ+Aγ,PCsinθ. The PV and PC asymmetries for energy-weighted γ cascade angular distributions for polarized slow neutron capture on Cl35 averaged over the neutron energies from 2.27–9.53 meV were measured to be Aγ,PV=(−23.9±0.7)×10−6 and Aγ,PC=(0.1±0.7)×10−6. These results are consistent with previous experimental results. Systematic errors were quantified and shown to be small compared to the statistical error. These asymmetries in the angular distributions of the γ rays emitted from the capture of polarized neutrons in Cl35 were used to verify the operation and data analysis procedures for the NPDGamma experiment, which measured the parity-odd asymmetry in the angular distribution of γs from polarized slow neutron capture on protons.5 MoreReceived 1 October 2021Accepted 17 May 2022DOI:https://doi.org/10.1103/PhysRevC.106.015504©2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasNuclear spin & parityNuclear tests of fundamental interactionsPolarization phenomenaRadiative captureProperties20 ≤ A ≤ 38TechniquesParticle sources & targetsNuclear Physics

First Observation of P-odd γ Asymmetry in Polarized Neutron Capture on Hydrogen.

We report the first observation of the parity-violating gamma-ray asymmetry A_{γ}^{np} in neutron-proton capture using polarized cold neutrons incident on a liquid parahydrogen target at the Spallation Neutron Source at Oak Ridge National Laboratory. A_{γ}^{np} isolates the ΔI=1, ^{3}S_{1}→^{3}P_{1} component of the weak nucleon-nucleon interaction, which is dominated by pion exchange and can be directly related to a single coupling constant in either the DDH meson exchange model or pionless effective field theory. We measured A_{γ}^{np}=[-3.0±1.4(stat)±0.2(syst)]×10^{-8}, which implies a DDH weak πNN coupling of h_{π}^{1}=[2.6±1.2(stat)±0.2(syst)]×10^{-7} and a pionless EFT constant of C^{^{3}S_{1}→^{3}P_{1}}/C_{0}=[-7.4±3.5(stat)±0.5(syst)]×10^{-11} MeV^{-1}. We describe the experiment, data analysis, systematic uncertainties, and implications of the result.

Photon Asymmetries in $${nd\rightarrow}$$ n d → 3 Hγ Using EFT( $${/\!\!\!\pi}$$ / π ) Approach

The parity-violating Lagrangian of the weak nucleon-nucleon ($NN$) interaction in the pionless effective field theory (EFT($//!/!/!/pi$)) approach contains five independent unknown low-energy coupling constants (LECs). The photon asymmetry with respect to neutron polarization in $np\rightarrow$ $d\gamma$ $A^{np}_\gamma$, the circular polarization of outgoing photon in $np\rightarrow$ $d\gamma$ $P^{np}_\gamma$, the neutron spin rotation in hydrogen $\frac{1}{\rho}\frac{d\phi^{np}}{dl}$, the neutron spin rotation in deuterium $\frac{1}{\rho}\frac{d\phi^{nd}}{dl}$ and the circular polarization of $\gamma$-emission in $nd\rightarrow$ $^3H\gamma$ $P^{nd}_\gamma$ are the parity-violating observables which have been recently calculated in terms of parity-violating LECs in the EFT($//!/!/!/pi$) framework. We obtain the LECs by matching the parity-violating observables to the Desplanques, Donoghue, and Holstein (DDH) best value estimates. Then, we evaluate photon asymmetry with respect to the neutron polarization $a^{nd}_\gamma$ and the photon asymmetry in relation to deuteron polarization $A^{nd}_\gamma$ in $nd\rightarrow$ $^3H\gamma$ process. We finally compare our EFT($//!/!/!/pi$) photon asymmetries results with the experimental values and the previous calculations based on the DDH model.

Measurement of the weak nucleon-nucleon interaction by polarized cold neutron capture on protons

The NPDGamma Experiment at the Spallation Neutron Source at Oak Ridge National Laboratory is measuring the parity-odd correlation between the neutron spin and the direction of the emitted photon in the capture of polarized cold neutrons on protons. A parity violating asymmetry from this process is directly related to the strength of the hadronic weak interaction between nucleons. The experiment was run first with heavier nuclear targets to check systematic e ects, false asymmetries, and backgrounds. Since early 2012 the experiment has been collecting data with a 16-liter liquid parahydrogen target. Data taking will continue through 2013 until statistics for a 10 8 asymmetry mea- surement are expected. The experiment performance will be discussed as well as the status of the asymmetry measurements.

Neutron Measurements and the Weak Nucleon-Nucleon Interaction.

The weak interaction between nucleons remains one of the most poorly-understood sectors of the Standard Model. A quantitative description of this interaction is needed to understand weak interaction phenomena in atomic, nuclear, and hadronic systems. This paper summarizes briefly what is known about the weak nucleon-nucleon interaction, tries to place this phenomenon in the context of other studies of the weak and strong interactions, and outlines a set of measurements involving low energy neutrons which can lead to significant experimental progress.

Measurement of the Parity-Violating Neutron Spin Rotation in (4) He.

In the meson exchange model of weak nucleon-nucleon (NN) interactions, the exchange of virtual mesons between the nucleons is parameterized by a set of weak meson exchange amplitudes. The strengths of these amplitudes from theoretical calculations are not well known, and experimental measurements of parity-violating (PV) observables in different nuclear systems have not constrained their values. Transversely polarized cold neutrons traveling through liquid helium experience a PV spin rotation due to the weak interaction with an angle proportional to a linear combination of these weak meson exchange amplitudes. A measurement of the PV neutron spin rotation in helium (φ PV ( n ,α)) would provide information about the relative strengths of the weak meson exchange amplitudes, and with the longitudinal analyzing power measurement in the p + α system, allow the first comparison between isospin mirror systems in weak NN interaction. An earlier experiment performed at NIST obtained a result consistent with zero: φ PV ( n ,α) = (8.0 ±14(stat) ±2.2(syst)) ×10(-7) rad / m[1]. We describe a modified apparatus using a superfluid helium target to increase statistics and reduce systematic effects in an effort to reach a sensitivity goal of 10(-7) rad/m.

Effects of core polarization on the nuclear Schiff moment

Schiff moments were calculated for a set of nuclei with full account of core polarization effects. A finite range P and T violating weak nucleon-nucleon interaction has been used in the calculations. While in the absence of core polarization the Schiff moment depends on one combination of the weak interaction constants, in the presence of core polarization the Schiff moment depends on all three constants separately. The dominant contribution comes from isovector, $\Delta T=1$, part of the weak interaction. The effects of core polarization were found to have in general a large effect on the Schiff moments.

Parity violation in nuclear systems

Experimental measurements of Parity Non-Conserving (PNC) asymmetries in simple nuclear systems represent always a key-tool for the study of the weak nucleon-nucleon interaction and consequently an accurate experimental method for the determination of the meson-nucleon weak coupling constants of the underlying theory. Recent theoretical analysis on the deuteron photodisintegration with polarized photons, a few MeV above threshold, have drastically improved previous theoretical estimates. Based on that, the feasibility of measuring the photon asymmetry A γ in the reaction \( \bar \gamma + d \to n + p \) with the 10-MeV CW Linac at the Institute of Accelerating Systems and Applications (IASA) is considered here. A brief review on previous experimental results obtained in the deuteron photodisintegration and in the thermal-neutron radiative capture on protons (inverse reaction) is given. The most important parameters in the design of a nuclear parity experiment are presented and the crucial factors, such as beam intensity, beam polarization and neutron detection techniques with the required high accuracy are outlined.

Schiff moment of the mercury nucleus and the proton dipole moment

The Schiff moment of the {sup 199}Hg nucleus is calculated using finite range P- and T-violating weak nucleon-nucleon interaction. Both the contributions of the P- and T-odd interaction and of internal nucleon electric dipole moments to the Schiff moment of {sup 199}Hg are calculated. The contribution of the proton electric dipole moment is obtained via core-polarization effects treated in the framework of RPA with effective residual interactions. We derive a new upper bound |d{sub p}| < 5.4 x 10{sup -24} e cm for the proton electric dipole moment.

P-and T-violating Schiff moment of the mercury nucleus

The Schiff moment of the $^{199}$Hg nucleus was calculated using finite range P- and T-violating weak nucleon-nucleon interaction. Effects of the core polarization were considered in the framework of RPA with effective residual forces.

Sign of P-odd observables in neutron-nucleus interaction

The signs of P-odd observables in neutron-nucleus interaction are analyzed on the basis of recent experiments. A simple formula describing these signs is derived. The proposed approach makes it possible to deduce information about the isospin structure of weak nucleon-nucleon interaction.

P-odd effects observed in neutron-induced reactions and isospin structure of weak nucleon-nucleon interaction

It is shown that, by introducing a resonance phase for two opposite-parity quasistationary states of the same spin, the observed sign dependence of P-odd effects in neutron-induced reactions can be matched with theoretical predictions. The proposed approach makes it possible to deduce information about the isospin structure of weak nucleon-nucleon interaction.

Application of P-odd effects for improvement of the energy resolution of Mössbauer spectra

Using Mn4119mSn-compound as a model we showed the possibility of lines' identification improvement of ferromagnets' Mossbauer spectra. The methods is based on application of P-odd effects of a weak nucleon-nucleon interaction.

Parity violation in threshold neutron-proton scattering

Parity violation in weak neutron-proton scattering is studied at threshold where neutron spin rotation might occur. The authors use the weak nucleon-nucleon interaction suggested by Desplanques, Donoghue and Holstein (1980) together with the Paris or Reid-soft-core forms of the strong interaction. Attention is focused on the contribution of various spin-isospin operators in the weak interaction as well as on the effect of distortion by S- and P-wave strong amplitudes. The spin rotation angle is found to be -8.84*10-9 rad cm-1 (using the Paris potential). In addition to the neutron spin rotation, there should also be an asymmetry in the total cross section (between positive- and negative-helicity states) due to the occurrence of the inelastic capture channel n+p to d+ gamma . Rough estimates of this quantity indicate that it is negligibly small.

Parity mixing in the93TcJ=17/2doublet

The authors demonstrate that the matrix element of the parity-violating potential between the 17/2 and 17/2+ states in 93Tc is expected to be very small (<0.02 eV) because of nuclear physics effects. The small experimental limit of 0.06 eV placed on the matrix element does not indicate an anomaly in the weak nucleon-nucleon interaction.

Predictions of parity-non-conserving effects in nucleon-deuteron scattering

Abstract Parity-non-conserving effects in nucleon-deuteron scattering are calculated in the Faddeev approach for realistic models of the necleon-nucleon strong interaction and various components of the weak nucleon-nucleon interaction. Our results are somewhat larger than expected on the basis of previous estimates. If present calculations of parity-non-conserving effects in nuclei are reliable, they indicate that an asymmetry of about −0.8 × 10 −7 should be measured in the total proton-deuteron cross section for longitudinally polarized protons.

Examination of Parity Nonconservation by Means of Radiative Neutron-Deuteron Capture

We report results for the circular polarization of photons emitted in the reaction $n+d\ensuremath{\rightarrow}{\mathrm{H}}^{3}+\ensuremath{\gamma}$ arising from parity nonconservation in the weak nucleon-nucleon interaction. Polarizations are found to be of the order ${10}^{\ensuremath{-}6}$.

Parity Mixing and Nuclear Structure in the Decays from OrientedGd153,159andTb161

Measurement of the angular distribution of the 363-keV $\ensuremath{\gamma}$ radiation following the decay of $^{159}\mathrm{Gd}$ oriented in Gd${\mathrm{Fe}}_{2}$ at low temperatures has yielded a vanishing value of the parity-non-conserving forward-backward asymmetry, indicating in this transition no large effect resulting from the parity-violating weak nucleon-nucleon interaction. Vanishing asymmetries were observed for other strongly hindered transitions in $^{159}\mathrm{Tb}$ and in $^{161}\mathrm{Dy}$. The 0-90\ifmmode^\circ\else\textdegree\fi{} anisotropy of the 363-keV angular distribution has yielded a value for the magnetic moment of the $^{159}\mathrm{Gd}$ ground state of $\ensuremath{\mu}=(\ensuremath{-}0.44\ifmmode\pm\else\textpm\fi{}0.03){\ensuremath{\mu}}_{N}$. The magnitude of the hyperfine field of Tb in Gd${\mathrm{Fe}}_{2}$ has been estimated to be 370 \ifmmode\pm\else\textpm\fi{} 80 kG, based on observation of the 0-90\ifmmode^\circ\else\textdegree\fi{} anisotropy of $\ensuremath{\gamma}$ transitions in $^{161}\mathrm{Dy}$. The multipole characters of various $\ensuremath{\beta}$ and $\ensuremath{\gamma}$ transitions following the decay of oriented $^{153,159}\mathrm{Gd}$ and $^{161}\mathrm{Tb}$ have been deduced.

Dispersion approach to the investigation of the weak nucleon-nucleon interaction

A dispersion method for calculation of the parity violating NN amplitude is suggested. Under some usual assumptions about the values of the meson-nucleon coupling constant the circular polarization γ-quanta P and its asymmetry α (with respect to neutron spin) in the process n+p → d+ γ is obtained: P = 1.81 × 10 −8, α = 4 × 10 −9.

Paritätsverbotener Alpha-Zerfall in O16

The theory of the parity-nonconserving alpha-decays from levels in O16 is developed. Using the weak nucleon-nucleon interaction of Blin-Stoyle and Herczeg and known O16 wave-functions modified by short-range correlations via Bethe-Goldstone equation, the irregular alpha widths of the states 2−(8.88 MeV) and 3+(11.05 MeV) have been related to known regular widths. The influence by the hard-core of the strong interaction has been studied.