Weak Meson-nucleon Coupling Constants Research Articles

Parity-violating asymmetry in the reactions 6Li $(n,\alpha)$ 3H and 10B $(n,\alpha)^{7}$ Li* $\rightarrow$ 7Li $+$ $\gamma$

We present two experiments measuring the parity-violating asymmetry (P-odd) of tritons emission in the reaction 6Li(n,α)3H and γ-quanta emission in the reaction 10B(n,α)7Li* → 7Li + γ with polarized cold neutrons, aiming at estimation of the weak meson-nucleon coupling constants. These experiments started in the PNPI, Gatchina and continued in the ILL, Grenoble. We describe an integral method to measure P-odd asymmetry and the two experiments. The results of our measurements are: \(\alpha{}_{P-odd}^{6Li}=(-8.8\pm2.1)\cdot10^{-8}\) and \(\alpha{}_{P-odd}^{10B}=(0.8\pm3.9)\cdot10^{-8}\).

FROM HADRONIC PARITY VIOLATION TO PARITY-VIOLATING ELECTRON SCATTERING AND TESTS OF THE STANDARD MODEL

Searches for parity violation in hadronic systems started soon after the evidence for parity violation in β-decay of 60 Co was presented by Madame Chien-Shiung Wu and in π and μ decay by Leon Lederman in 1957. The early searches for parity violation in hadronic systems did not reach the sensitivity required and only after technological advances in later years was parity violation unambiguously established. Within the meson-exchange description of the strong interaction, theory and experiment meet in a set of seven weak meson-nucleon coupling constants. Even today, after almost five decades, the determination of the seven weak meson-nucleon couplings is incomplete. Parity violation in nuclear systems is rather complex due to the intricacies of QCD. More straight forward in terms of interpretation are measurements of the proton-proton parity-violating analyzing power (normalized differences in scattering yields for positive and negative helicity incident beams), for which there exist three precision experiments (at 13.6, at 45, and 221 MeV). To-date, there are better possibilities for theoretical interpretation using effective field theory approaches. The situation with regard to the measurement of the parity-violating analyzing power or asymmetry in polarized electron scattering is quite different. Although the original measurements were intended to determine the electro-weak mixing angle, with the current knowledge of the electro-weak interaction and the great precision with which electro-weak radiative corrections can be calculated, the emphasis has been to study the structure of the nucleon, and in particular the strangeness content of the nucleon. A whole series of experiments (the SAMPLE experiment at MIT-Bates, the G0 experiment and HAPPEX experiments at Jefferson Laboratory (JLab), and the PVA4 experiment at MAMI) have indicated that the strange quark contributions to the charge and magnetization distributions of the nucleon are tiny. These measurements if extrapolated to zero degrees and zero momentum transfer have also provided a factor five improvement in the knowledge of the neutral weak couplings to the quarks. Choosing appropriate kinematics in parity-violating electron-proton scattering permits nucleon structure effects on the measured analyzing power to be precisely controlled. Consequently, a precise measurement of the ‘running’ of sin 2θw or the electro-weak mixing angle has become within reach. The [Formula: see text] experiment at Jefferson Laboratory is to measure this quantity to a precision of about 4%. This will either establish conformity with the Standard Model of quarks and leptons or point to New Physics as the Standard Model must be encompassed in a more general theory required, for instance, by a convergence of the three couplings (strong, electromagnetic, and weak) to a common value at the GUT scale. The upgrade of CEBAF at Jefferson Laboratory to 12 GeV, will allow a new measurement of sin 2θW in parity-violating electron-electron scattering with an improved precision to the current better measurement (the SLAC E158 experiment) of the ‘running’ of sin 2θW away from the Z0 pole. Preliminary design studies of such an experiment show that a precision comparable to the most precise individual measurements at the Z0 pole (to about ±0.00025) can be reached. The result of this experiment will be rather complementary to the [Formula: see text] experiment in terms of sensitivity to New Physics.

Parity-nonconserving observables in thermal neutron capture on a proton

We calculate parity nonconserving observables in the processes where a neutron is captured on a proton at the threshold energy radiating a photon. Various potential models such as Paris, Bonn and Argonne $v18$ are used for the strong interactions, and the meson-exchange description is employed for the weak interactions between hadrons. The photon polarization $P_\gamma$ in the unpolarized neutron capture process and photon asymmetry $A_\gamma$ in the polarized neutron capture process are obtained in terms of the weak meson-nucleon coupling constants. $A_\gamma$ turns out to be basically insensitive to the employed strong interaction models and thus can be uniquely determined in terms of the weak coupling constants, but $P_\gamma$ depends significantly on the strong interaction models.

Parity violation in proton-proton scattering at 221 MeV

pp elastic scattering at 221.3 MeV incident proton energy. This comprehensive paper includes details of the corrections, some of magnitude comparable to Az itself, required to arrive at the final result. The largest correction was for the effects of first moments of transverse polarization. The addition of the result, Az = (0.84 ± 0.29(stat.) ± 0.17(syst.)) × 10 7 , to the ~ pp parity violation experimental data base greatly improves the experimental constraints on the weak meson-nucleon coupling constants h pp and h pp , and also has implications for the interpretation of electron parity violation experiments.

Nuclear spin-isospin correlations, parity violation, and thefπproblem

The strong interaction effects of isospin- and spin-dependent nucleon-nucleon correlations observed in many-body calculations are interpreted in terms of a one-pion exchange mechanism. Including such effects in computations of nuclear parity violating effects leads to enhancements of about 10%. A larger effect arises from the one-boson exchange nature of the parity non-conserving nucleon- nucleon interaction, which depends on both weak and strong meson-nucleon coupling constants. Using values of the latter that are constrained by nucleon-nucleon phase shifts leads to enhancements of parity violation by factors close to two. Thus much of previously noticed discrepancies between weak coupling constants extracted from different experiments can be removed.

Parity violation in proton-proton scattering at 221 MeV.

The parity-violating longitudinal analyzing power, A(z), has been measured in pvectorp elastic scattering at an incident proton energy of 221 MeV. The result obtained is A(z) = [0.84+/-0.29(stat)+/-0.17(syst)]x10(-7). This experiment is unique in that it selects a single parity violating transition amplitude (3P2 - 1D2) and consequently directly constrains the weak meson-nucleon coupling constant h(pp)(rho). When this result is taken together with the existing pvectorp parity violation data, the weak meson-nucleon coupling constants h(pp)(rho) and h(pp)(omega) can, for the first time, both be determined.

Symmetries and symmetry breaking

Several new proton-proton parity violation experiments are presently either being performed or are being prepared for execution in the near future. Similarly, a new measurement of the parity-violating gamma-ray asymmetry in polarized neutron capture on the proton is being developed with a ten-fold improvement over previous measurements. These experiments are intended to provide stringent constraints on the set of seven effective weak meson-nucleon coupling constants. Time-reversal-invariance non-conservation has now been unequivocally demonstrated in a direct measurement at CPLEAR. Tests may also be made of time-reversal-invariance non-conservation in systems other than the kaon system. There exist two classes of time-reversal invariance breaking interactions: P-odd/T-odd and P-even/T-odd interactions. Constraints on the first ones stem from measurements of the electric dipole moment of the neutron, while constraints on the second ones stem from the same and measurements of charge symmetry breaking in neutron-proton elastic scattering and from $K$ semi-leptonic decays. A series of precision experiments, either ongoing or being prepared, will determine the neutral weak current of the proton by measuring the parity-violating normalized asymmetry in electron-proton elastic scattering. A direct comparison between the electromagnetic and neutral weak ground state currents of the nucleon will allow a delineation of the contributions to these currents of the various quark flavours, including quarks which belong exclusively to the nucleon sea. An extension of these precision experiments to very low momentum transfer would permit stringent limits to be placed on physics beyond the standard model.

NUCLEON-NUCLEON PARITY VIOLATION EXPERIMENTS

Measurements of parity-violating longitudinal analyzing powers Az (normalized asymmetries) in polarized proton-proton scattering and in polarized neutron capture on the proton (n-p→d - γ) provide a unique window on the interplay between the weak and strong interactions between and within hadrons. Several new proton-proton parity violation experiments are presently either being performed or are being prepared for execution in the near future: at TRIUMF at 221 MeV and 450 MeV and at COSY (Forschungszentrum Jülich) in the multi-GeV range. A new measurement of the parity-violating γ ray asymmetry with a ten-fold improvement in the accuracy over previous measurements is being developed at LANSCE. These experiments are intended to provide stringent constraints on the set of six effective weak meson-nucleon coupling constants, which characterize the weak interaction between hadrons in the energy domain where meson exchange models provide an appropriate description. The 221 MeV p - p experiment is unique in that it selects a single transition amplitude (3P2-1D2) and consequently constrains the weak meson-nucleon coupling constant [Formula: see text]. The n-p→d-γ experiment is mainly sensitive to the weak pion-nucleon coupling constant fπ. Together with the existing p-p parity violation experimental results one may be able to delineate the various weak meson-nucleon coupling constants. The TRIUMF 221 MeV p-p parity violation experiment will be described in some detail. Other parity violation nucleon-nucleon and nucleon-very-light-nucleus experiments are commented on. The anomalous result obtained at 6 GeV/c on a water target requires that a new multi-GeV p-p parity violation experiment be performed.

Weak radiative hyperon decays and vector meson dominance

We study the question whether the phenomenologically successful VMD approach to weak radiative hyperon decays can be made consistent with Hara's theorem and still yield the pattern of asymmetries exhibited by experimental data. It appears that an essential ingredient which governs the pattern of asymmetries is the off-shell behaviour of the input electromagnetic 1/2^- 1/2^+ \gamma couplings. Although this behaviour can be chosen in such a way that the experimentally observed pattern is obtained, and yet Hara's theorem satisfied, at the same time the approach yields a definite prediction for the size of weak meson-nucleon coupling constants. Comparison with experiment reveals then another conflict.

Problems and advantages of measuring parity and time-reversal invariance in nuclei

In the low energy regime parity violation for pure hadronic systems can be expressed essentially by six effective weak meson-nucleon coupling constants, three of which are only known at present. Additional independent experiments have been proposed involving light, heavy and compound nuclei, to determine all six coupling constants unambiguously. Though heavy nuclei offer enhancement factors of up to (10 5), the difficulty to extract relevant weak meson-nucleon coupling constants from experiments involving heavy nuclei increases substantially. In this context the statistical model of compound nuclei provides an additional option for the extraction of these coupling constants. In contrast to parity violation (PV) tests, time-reversal in variance (TRI) information that is extracted from 2-body reactions, results from a comparison of two different observables. As a consequence, the accuracy of these experiments is reduced. In view of the mechanisms that enhance PV, TRI tests have been performed that utilize heavy nuclei too. At COSY a novel, true T-odd, P-even null-experiment has been proposed that has the potential to test TRI to an accuracy of 10 −6 in p - d -scattering. For this test the COSY ring serves as accelerator, ideal forward spectrometer and detector.

Parity violation in proton-proton scattering

Measurements of parity-violating longitudinal analyzing powers (normalized asymmetries) in polarized proton-proton scattering provide a unique window on the interplay between the weak and strong interactions between and within hadrons. Several new proton-proton parity violation experiments are presently either being performed or are being prepared for execution in the near future: at TRIUMF at 221 MeV and 450 MeV and at COSY (Kernforschungsanlage Jülich) at 230 MeV and near 1.3 GeV. These experiments are intended to provide stringent constraints on the set of six effective weak meson-nucleon coupling constants, which characterize the weak interaction between hadrons in the energy domain where meson exchange models provide an appropriate description. The 221 MeV is unique in that it selects a single transition amplitude ( 3P 2 - 1D 2) and consequently constrains the weak meson-nucleon coupling constant h ϱ pp . The TRIUMF 221 MeV proton-proton parity violation experiment is described in some detail. A preliminary result for the longitudinal analyzing power is A z = (1.1 ± 0.4 ± 0.4) × 10 −7. Further proton-proton parity violation experiments are commented on. The anomaly at 6 GeV/c requires that a new multi-GeV proton-proton parity violation experiment be performed.

Nuclear parity violating effects: Anapole moment

We present here the relativistic formulation for the calculation of the nuclear anapole moment in the relativistic mean field (RMF) approach. The RMF results are presented for relevant nuclei. We also report the result of our preliminary calculations for the nuclear anapole moment for 133Cs obtained with different sets of weak meson-nucleon coupling constants.

Parity nonconservation in 19F nuclei

The parity nonconserving asymmetry A γ in the decay of polarized 19F ∗(110keV) nuclei has been measured. A value of A γ = − (6.83 ± 2.11) · 10 −5 (total error) was found. Systematic errors are extensively investigated and found to be small. The absolute normalization is given by the 19F ∗ polarization, which is found to be P F = −0.52 ± 0.08 in a separate experiment, using a calibrated Compton polarimeter. The new result A γ ( 19F) is compared to earlier experiments and recent theoretical calculations. From an analysis including 18F and 21Ne results, constraints on the weak meson-nucleon coupling constants ƒ π and h 0 ρ are deduced. Agreement with calculations based on the standard electroweak theory and QCD is found.

Parity Nonconservation in ElasticpαScattering and the Determination of the Weak Meson-Nucleon Coupling Constants

Parity Nonconservation in Elastic<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>p</mml:mi><mml:mi>α</mml:mi></mml:math>Scattering and the Determination of the Weak Meson-Nucleon Coupling Constants

Parity nonconservation in elastic p alpha scattering and the determination of the weak meson-nucleon coupling constants.

Parity nonconservation has been studied in elastic $p\ensuremath{\alpha}$ scattering at 46 MeV with longitudinally polarized protons. The longitudinal analyzing power averaged over the range of laboratory scattering angles of roughly 30\ifmmode^\circ\else\textdegree\fi{} to 60\ifmmode^\circ\else\textdegree\fi{} is found to be ${A}_{z}=(\ensuremath{-}3.3\ifmmode\pm\else\textpm\fi{}0.9)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$. This result and measurements in other light nuclear systems are used to determine weak meson-nucleon coupling constants.

Constraints on Weak Meson-Nucleon Coupling from Parity Nonconservation inF19

The parity-nonconservation asymmetry ${A}_{\ensuremath{\gamma}}$ in the decay of polarized $^{19}\mathrm{F}^{*}$ (110-keV) nuclei has been measured to high precision and is found to be ${A}_{\ensuremath{\gamma}}=\ensuremath{-}(6.8\ifmmode\pm\else\textpm\fi{}1.8)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$. Absolute uncertainties have been largely removed. This new value for ${A}_{\ensuremath{\gamma}}$ together with recent nuclear structure calculations provides severe constraints on the weak meson-nucleon coupling constants, in particular on ${f}_{\ensuremath{\pi}}$ and ${h}_{\ensuremath{\rho}}^{0}$.

Nuclear-Structure Effects on Parity Nonconservation in Light Nuclei

The nucleon-nucleon parity-nonconserving potentials given by Desplangues, Donoghue, and Holstein (DDH) are used to calculate matrix elements between states of opposite parity in $^{10}\mathrm{B}$, $^{16}\mathrm{O}$, $^{18}\mathrm{F}$, $^{19}\mathrm{F}$, $^{20}\mathrm{Ne}$, and $^{21}\mathrm{Ne}$. The sensitivity of the parity-nonconserving matrix elements to various approximations in the microscopic shell-model wave functions is investigated. The final results using the DDH estimates for the weak mesonnucleon coupling constants based on the Weinberg-Salam theory are several times larger than experiment.

Nuclear-Structure Effects on Parity Nonconservation in Light Nuclei

The nucleon-nucleon parity-nonconserving potentials given by Desplangues, Donoghue, and Holstein (DDH) are used to calculate matrix elements between states of opposite parity in $^{10}\mathrm{B}$, $^{16}\mathrm{O}$, $^{18}\mathrm{F}$, $^{19}\mathrm{F}$, $^{20}\mathrm{Ne}$, and $^{21}\mathrm{Ne}$. The sensitivity of the parity-nonconserving matrix elements to various approximations in the microscopic shell-model wave functions is investigated. The final results using the DDH estimates for the weak mesonnucleon coupling constants based on the Weinberg-Salam theory are several times larger than experiment.

Parity-non-conserving nuclear forces

Theoretical and phenomenological approaches to parity-non-conserving nuclear forces are reviewed. Recent developments in the calculation of weak meson-nucleon coupling constants, whose knowledge is necessary to determine theoretically the parity-non-conserving nucleon-nucleon potential, are described. The consistency of different measurements of parity-non-conserving effects is discussed and the information they provide is compared to theoretical predictions.