Meson Exchange Currents Research Articles

Inclusive electron scattering off 4He

Inclusive electron scattering off 4 He is calculated exactly with a complete treatment of the final state interaction within a simple semirealistic potential model. We discuss results for both the longitudinal and the transverse response functions, at various momentum transfers. A consistent meson exchange current is implemented. Good agreement with available experimental data is found for the longitudinal response function, while some strength is still missing in the transverse response function.

Low-Energy Inelastic Neutrino Reactions onHe4

The inelastic scattering of neutrino off 4He is calculated microscopically at energies typical for core-collapse supernova environment. The calculation is carried out with the Argonne V18 nucleon-nucleon potential and the Urbana IX three-nucleon force. Full final state interaction is included via the Lorentz integral transform method. The contribution of axial meson exchange currents to the cross sections is taken into account from effective field theory of nucleons and pions to order O(Q3).

Neutrino breakup ofA=3nuclei in supernovae

We extend the virial equation of state to include $^{3}\mathrm{H}$ and $^{3}\mathrm{He}$ nuclei, and predict significant mass-three fractions near the neutrinosphere in supernovae. While alpha particles are often more abundant, we demonstrate that energy transfer cross sections for muon and tau neutrinos at low densities are dominated by breakup of the loosely-bound $^{3}\mathrm{H}$ and $^{3}\mathrm{He}$ nuclei. The virial coefficients involving $A=3$ nuclei are calculated directly from the corresponding nucleon-$^{3}\mathrm{H}$ and nucleon-$^{3}\mathrm{He}$ scattering phase shifts. For the neutral-current inelastic cross sections and the energy transfer cross sections, we perform ab initio calculations based on microscopic two- and three-nucleon interactions and meson-exchange currents.

Extraction of the neutron magnetic form factor from quasielastic3He→(e→,e') atQ2=0.1−0.6 (GeV/c)2

We have measured the transverse asymmetry AT' in the quasielastic 3He→(e→,e') process with high precision at Q2 values from 0.1 to 0.6(GeV/c)2. The neutron magnetic form factor GMn was extracted at Q2 values of 0.1 and 0.2(GeV/c)2 using a nonrelativistic Faddeev calculation which includes both final-state interactions (FSI) and meson-exchange currents (MEC). Theoretical uncertainties due to the FSI and MEC effects were constrained with a precision measurement of the spin-dependent asymmetry in the threshold region of 3He→(e→,e'). We also extracted the neutron magnetic form factor GMn at Q2 values of 0.3 to 0.6(GeV/c)2 based on plane wave impulse approximation calculations.3 MoreReceived 9 May 2006DOI:https://doi.org/10.1103/PhysRevC.75.034003©2007 American Physical Society

Differential cross section for neutron-proton bremsstrahlung

The neutron-proton bremsstrahlung process (np{yields}np{gamma}) is known to be sensitive to meson exchange currents in the nucleon-nucleon interaction. The triply differential cross section for this reaction has been measured for the first time at the Los Alamos Neutron Science Center, using an intense, pulsed beam of up to 700-MeV neutrons to bombard a liquid hydrogen target. Scattered neutrons were observed at six angles between 12 deg. and 32 deg., and the recoil protons were observed in coincidence at 12 deg., 20 deg., and 28 deg. on the opposite side of the beam. Measurement of the neutron and proton energies at known angles allows full kinematic reconstruction of each event. The data are compared with predictions of two theoretical calculations, based on relativistic soft-photon and nonrelativistic potential models.

Superscaling in electroweak excitation of nuclei

Superscaling properties of $^{12}\mathrm{C}$, $^{16}\mathrm{O}$, and $^{40}\mathrm{Ca}$ nuclear responses, induced by electron and neutrino scattering, are studied for momentum transfer values between 300 and 700 MeV/$c$. We have defined two indexes to have quantitative estimates of the scaling quality. We have analyzed experimental responses to get the empirical values of the two indexes. We have then investigated the effects of finite dimensions, collective excitations, meson exchange currents, short-range correlations, and final state interactions. These effects strongly modify the relativistic Fermi gas scaling functions, but they conserve the scaling properties. We used the scaling functions to predict electron and neutrino cross sections and we tested their validity by comparing them with the cross sections obtained with a full calculation. For electron scattering we also made a comparison with data. We have calculated the total charge-exchange neutrino cross sections for neutrino energies up to 300 MeV.

Composite-meson model and meson exchange currents

We study the meson exchange currents (MEC) mechanism for the pion double-charge exchange (DCX) reaction in a composite-meson model. The model assumes that the mesons are two-quark systems and can interact with each other only through quark loops. The contributions of the ρ, σ, and f 0 mesons, the four-quark box diagram as well as a contact diagram has been taken into account. It is shown that the contribution of the ρ, σ, and f 0 mesons increases the forward scattering cross-section in an average by 25% and decreases with energy.

Deuteron tensor polarization componentT20(Q2)as a crucial test for deuteron wave functions

The deuteron tensor polarization component ${T}_{20}({Q}^{2})$ is calculated by the relativistic Hamiltonian dynamics approach. It is shown that in the range of momentum transfers available with current experiments, relativistic effects, meson exchange currents, and the choice of nucleon electromagnetic form factors almost do not influence the value of ${T}_{20}({Q}^{2})$. However, this value depends strongly on the actual form of the deuteron wave function, that is, on the model of the $\mathit{NN}$ interaction in the deuteron. So the existing data for ${T}_{20}({Q}^{2})$ provide a crucial test for deuteron wave functions.

Proton emission induced by polarized photons

The proton emission induced by polarized photons is studied in the energy range above the giant resonance region and below the pion emission threshold. Results for the 12C, 16O and 40Ca nuclei are presented. The sensitivity of various observables to final state interaction, meson exchange currents and short range correlations is analyzed. We found relevant effects due to the virtual excitation of the $\Delta$ resonance.

Measurement of theH2(n,γ)H3reaction cross section between 10 and 550 keV

We have measured for the first time the cross section of the $^{2}\mathrm{H}$$(n,\ensuremath{\gamma})$$^{3}\mathrm{H}$ reaction at an energy relevant to big-bang nucleosynthesis by employing a prompt discrete \ensuremath{\gamma}-ray detection method. The outgoing photons have been detected by means of anti-Compton NaI(Tl) spectrometers with a large signal-to-noise ratio. The resulting cross sections are $2.23\ifmmode\pm\else\textpm\fi{}0.34,1.99\ifmmode\pm\else\textpm\fi{}0.25$, and $3.76\ifmmode\pm\else\textpm\fi{}0.41\ensuremath{\mu}$b at ${E}_{n}=30.5,54.2$, and 531 keV, respectively. At ${E}_{n}=30.5$ keV the cross section differs from the value reported previously by a factor of 2. Based on the present data the reaction rate has been obtained for temperatures in the range ${10}^{7}\ensuremath{-}{10}^{10}$ K. The astrophysical impact of the present result is discussed. The obtained cross sections are compared with a theoretical calculation based on the Faddeev approach, which includes meson exchange currents as well as a three-nucleon force.

Weak axial nuclear heavy meson exchange currents and interactions of solar neutrinos with deuterons

Starting from the axial heavy meson exchange currents, constructed earlier in conjunction with the Bethe–Salpeter equation, we first present the axial ρ-, ω- and a 1 -meson exchange Feynman amplitudes that satisfy the partial conservation of the axial current. Employing these amplitudes, we derive the corresponding weak axial heavy meson exchange currents in the leading order in the 1 / M expansion ( M is the nucleon mass), suitable for the nuclear physics calculations beyond the threshold energies and with wave functions obtained by solving the Schrödinger equation with one-boson exchange potentials. The constructed currents obey the nuclear form of the partial conservation of the axial current. We apply the space component of these currents in calculations of the cross sections for the disintegration of deuterons by low energy (anti)neutrinos. The deuteron and the final state nucleon–nucleon wave functions are derived (i) from a variant of the OBEPQB potential, and (ii) from the Nijmegen 93 and Nijmegen I nucleon–nucleon interaction. The extracted values of the constant L 1 , A , entering the axial exchange currents of the pionless effective field theory, are in a reasonable agreement with its value predicted by the dimensional analysis.

New data for totalHe3(γ,p)D andHe3(γ,pp)n cross sections compared to current theory

A simultaneous measurement of the cross sections of the $^{3}\mathrm{He}$($\ensuremath{\gamma},p$)D and $^{3}\mathrm{He}$($\ensuremath{\gamma},\mathit{pp}$)n reactions has been performed for the first time using monoenergetic pulsed \ensuremath{\gamma}-rays at $\ensuremath{\langle}{E}_{\ensuremath{\gamma}}\ensuremath{\rangle}=$10.2 and 16.0 MeV. Charged fragments from the reactions were detected with an efficiency of 100% using a 4\ensuremath{\pi} time projection chamber containing $^{3}\mathrm{He}$ gas as an active target. The incident \ensuremath{\gamma}-ray flux was measured by a \ensuremath{\gamma}-ray detector. Both the track and energy loss signals of charged fragments were obtained in an off-line analysis and used to clearly identify the reaction channel. Thus, the ($\ensuremath{\gamma},p$) and ($\ensuremath{\gamma},\mathit{pp}$) cross sections have been determined with small uncertainty. A comparison of the new data to current theory based on the AV18+Urbana IX nuclear forces including \ensuremath{\pi}- and \ensuremath{\rho}-like meson exchange currents shows a severe discrepancy at 10.2 MeV, while at 16.0 MeV data and theory agree within about 12%. Three-nucleon force effects are small, but in general shift the theory in the correct direction.

Polarization observables in the semiexclusive photoinduced three-body breakup ofHe3

The photon and $^{3}\mathrm{He}$ analyzing powers as well as spin correlation coefficients in the semiexclusive three-body photodisintegration of $^{3}\mathrm{He}$ are investigated for incoming photon laboratory energies ${E}_{\ensuremath{\gamma}}=12,40$, and 120 MeV. The nuclear states are obtained by solving three-body Faddeev equations with the AV18 nucleon-nucleon potential alone or supplemented with the UrbanaIX three-nucleon force. Explicit \ensuremath{\pi}- and \ensuremath{\rho}-meson exchange currents are taken into account, but we also compare to other models of the electromagnetic current. In some kinematical conditions we have found strong effects of the three-nucleon force for the $^{3}\mathrm{He}$ analyzing power and spin correlation coefficients, as well strong sensitivities to the choice of the currents. This set of predictions should be a useful guidance for the planning of measurements. In addition, we compare our results for two-body $^{3}\mathrm{He}$ breakup induced by polarized photons with a few existing data.

CORRECTIONS TO THE NUCLEAR AXIAL VECTOR COUPLING IN A NUCLEAR MEDIUM

There is experimental evidence of an increase in the temporal component of the weak axial vector current in nuclei. Most of this increase is accounted for by meson exchange currents. We consider further corrections from pions and the sigma mean-field. These corrections, however, are found to reduce the coupling, which is already insufficiently large.

Measurement of the asymmetries in 3 $ \overrightarrow{\sf He}$ (¯e, e′p)d and 3 $ \overrightarrow{\sf He}$ (¯e, e′p)np

The electron-target-asymmetries A_parallel and A_perpendicular with target spin parallel and perpendicular to the momentum transfer q were measured for both the two-- and three-body breakup of 3He in the 3He(e,e'p)-reaction. Polarized electrons were scattered off polarized 3He in the quasielastic regime in parallel kinematics with the scattered electron and the knocked-out proton detected using the Three-Spectrometer-Facility at MAMI. The results are compared to Faddeev calculations which take into account Final State Interactions as well as Meson Exchange Currents. The experiment confirms the prediction of a large effect of Final State Interactions in the asymmetry of the three-body breakup and of an almost negligible one for the two-body breakup.

The role of the pion pair term in the theory of the weak axial meson exchange currents

The structure of the weak axial pion exchange current is discussed in various models. It is shown how the interplay of the chiral invariance and the double counting problem restricts uniquely the form of the pion potential term, in the case when the nuclear dynamics is described by the Schroedinger equation with the static nucleon-nucleon potential.

Electron-deuteron scattering in the equal-time formalism: Beyond the impulse approximation

Using a three-dimensional formalism that includes relativistic kinematics, the effects of negative-energy states, approximate boosts of the two-body system, and current conservation, we calculate the electromagnetic form factors of the deuteron up to Q^2 of 4 GeV^2. This is done using a dynamical boost for two-body systems with spin. We first compute form factors in impulse approxmation, but then also add an isoscalar meson-exchange current of pion range that involves the gamma-pi contact operator associated with pseudovector pi-N coupling. We also consider effects of the rho-pi-gamma meson-exchange current. The experimentally measured quantities A, B, and t20 are calculated over the kinematic range probed in recent Jefferson Laboratory experiments. The rho-pi-gamma meson-exchange current provides significant strength in A at large Q^2 and the gamma-pi contact-term exchange current shifts t20, providing good agreement with the JLab data. Relativistic effects and the gamma-pi meson-exchange current do not provide an explanation of the B observable, but the rho-pi-gamma current could help to provide agreement if a nonstandard value is used for the tensor rho-N coupling that enters this contribution.

Magnetic moments of the21+states aroundSn132

Development of neutron-rich radioactive beams at the Holifield Radioactive Ion Beam Facility has stimulated experimental and theoretical activity in heavy Sn and Te isotopes. Recently, the g factor of the first ${2}^{+}$ state in $^{132}\mathrm{Te}$ has been measured. We report here new shell-model calculation of magnetic moments for selected Sn and Te isotopes. The residual interaction is based on the CD-Bonn renormalized $G$ matrix. Single-particle spin and orbital effective g factors are evaluated microscopically including core polarization and meson exchange currents effects.

Different formulations of 3He and 3H photodisintegration

Different momentum space Faddeev-like equations and their solutions for the radiative pd-capture and the three-nucleon photodisintegration of 3He are presented. Applications are based on the AV18 nucleon-nucleon and the Urbana IX three nucleon forces. Meson exchange currents are included using the Siegert theorem. A very good agreement has been found in all cases indicating the reliability of the used numerical methods. Predictions for cross sections and polarization observables in the pd-capture and the complete three nucleon breakup of 3He at different incoming deuteron/photon energies are presented.

Corrections to the nuclear axial vector coupling in a nuclear medium

The temporal component of the weak axial vector current in nuclei is increased due to meson exchange currents. We consider further corrections from pions and the sigma mean-field.