Search

We present a preliminary study of the effect of a three-nucleon force (3NF) in p-H(3) and n-He(3) scattering at low energies. The used 3NF is derived from effective field theory at next-to-next-to-leading order. The four-nucleon scattering observables are calculated using the Kohn variational principle and the hyperspherical harmonics technique and the results are compared with available experimental data. We have found that the effect of introducing this type of 3NF is tiny, and sometimes worsens the agreement with the experimental data.

We present a detailed study of the effect of different three-nucleon interactions in p-3He elastic scattering at low energies. In particular, two interactions have been considered: one derived from effective field theory at next-to-next-to-leading order and one derived from a more phenomenological point of view—the so-called Illinois model. The four-nucleon scattering observables are calculated by using the Kohn variational principle and the hyperspherical harmonics technique, and the results are compared with available experimental data. We have found that the inclusion of both interactions improves the agreement with the experimental data, in particular, for the proton vector analyzing power.

The longitudinal asymmetry A_z(n-3He) in the 3He(n, p)3H reaction is an observable directly related to parity-violating (PV) components in the nucleon-nucleon (NN) interaction. Here we study A_z(n-3He) using a PV NN potential derived from an effective field theory framework at next-to-next-to-leading order, including one- and two-pion exchanges, contact interactions and relativistic corrections. This potential depends on six low-energy constants. Some of these parameters are constrained to reproduce the existing accurate measurements of the p-p longitudinal asymmetry. Using these constraints we compute A_z(n-3He) and study the sensitivity of such an observable to the unconstrained parameters.

The muon-capture reactions 2H(μ-,νμ)nn and 3He(μ-,νμ)3H are studied with conventional or chiral realistic potentials and consistent weak currents. The initial and final A=2 and A=3 nuclear wave functions are obtained from the Argonne v18 or chiral next-to-next-to-next-to leading order (N3LO) two-nucleon potential, in combination with, respectively, the Urbana IX or chiral next-to-next-to leading order (N2LO) three-nucleon potential in the case of A=3. The weak current consists of polar- and axial-vector components. The former are related to the isovector piece of the electromagnetic current via the conserved-vector-current hypothesis. These and the axial currents are derived either in a meson-exchange or in a chiral effective field theory (χEFT) framework. There is one parameter (either the N-to-Δ axial coupling constant in the meson-exchange model, or the strength of a contact term in the χEFT model) that is fixed by reproducing the Gamow-Teller matrix element in tritium β decay. The model dependence relative to the adopted interactions and currents (and cutoff sensitivity in the χEFT currents) is weak, resulting in total rates of 392.0±2.3 s-1 for A=2, and 1484±13 s-1 for A=3, where the spread accounts for this model dependence.

The electromagnetic charge operator in a two-nucleon system is derived in chiral effective field theory (χEFT) up to order eQ [or next-to-next-to-next-to-next-to-leading order (N4LO)], where Q denotes the low-momentum scale and e is the electric charge. The specific form of the N3LO and N4LO corrections from, respectively, one-pion-exchange and two-pion-exchange depends on the off-the-energy-shell prescriptions adopted for the nonstatic terms in the corresponding potentials. We show that different prescriptions lead to unitarily equivalent potentials and accompanying charge operators. Thus, provided a consistent set is adopted, predictions for physical observables will remain unaffected by the nonuniqueness associated with these off-the-energy-shell effects.

The longitudinal asymmetry induced by parity-violating (PV) components in the nucleon-nucleon potential is studied in the charge-exchange reaction He-3(n-polarized, p)H-3 at vanishing incident neutron energies. An expression for the PV observable is derived in terms of T-matrix elements for transitions from the L-2S+1(J) = S-1(0) and S-3(1) states in the incoming n-He-3 channel to states with J = 0 and 1 in the outgoing p-H-3 channel. The T-matrix elements involving PV transitions are obtained in first-order perturbation theory in the hadronic weak-interaction potential, while those connecting states of the same parity are derived from solutions of the strong-interaction Hamiltonian with the hyperspherical-harmonics method. The coupled-channel nature of the scattering problem is fully accounted for. Results are obtained corresponding to realistic or chiral two-and three-nucleon strong-interaction potentials in combination with either the DDH or pionless EFT model for the weak-interaction potential. The asymmetries, predicted with PV pion and vector-meson coupling constants corresponding (essentially) to the DDH "best values" set, range from -9.44 to -2.48 in units of 10(-8), depending on the input strong-interaction Hamiltonian. This large model dependence is a consequence of cancellations between long-range (pion) and short-range (vector-meson) contributions and is of course sensitive to the assumed values for the PV coupling constants.

Recently, we have derived a two nucleon potential and consistent nuclear electromagnetic currents in chiral effective field theory with pions and nucleons as explicit degrees of freedom The calculation of the currents has been carried out to include (NLO)-L-3 corrections, consisting of two-pion exchange and contact contributions The latter involve unknown low-energy constants (LECs), some of which have been fixed by fitting the up S- and P-wave phase shifts up to 100 MeV lab energies The remaining LECs entering the current operator are determined so as to reproduce the experimental deuteron and trinucleon magnetic moments, as well as the rip cross section This electromagnetic current operator is utilized to study the mid and n(3)He radiative captures at thermal neutron energies Here we discuss our results stressing on the important role played by the LECs in reproducing the experimental data

We report on a study of the nd and n3He radiative captures at thermal neutron energies, using wave functions obtained from either chiral or conventional two- and three-nucleon realistic potentials with the hyperspherical-harmonics method, and electromagnetic currents derived in chiral effective field theory up to one loop. The predicted nd and n3He cross sections are in good agreement with data, but exhibit a significant dependence on the input Hamiltonian. A comparison is also made between these and new results for the nd and n3He cross sections obtained in the conventional framework for both potentials and currents.

It is not possible to reproduce both the three- and four-nucleon binding energies using the available two-nucleon potentials. This is one manifestation of the need to include a three- nucleon force in the corresponding Hamiltonian. In this paper we will analyze the capability of a three- nucleon force model to describe not only the aforementioned binding energies but also some N - d low energy scattering observables.

The application of the hyperspherical harmonic approach to the case of the N −d scattering problem is described. The nuclear Hamiltonian includes local and non-local two- and three-nucleon interactions. Also, Coulomb repulsion is included in the p − d case. Accurate calculation for p − d and n − d cross section and polarization observables at two center-of-mass energies are performed and the theoretical results are compared with the available experimental data. A satisfactory agreement between theory and experiment is found, except for the vector polarization observables. A quantitative estimate of the agreement or disagreement between theory and experiment is obtained performing a chi^2 analysis.

The Kohn variational principle and the hyperspherical harmonics technique are applied to study the n-H-3 elastic scattering at low energies. In this contribution the first results obtained using a non-local realistic interaction derived from the chiral perturbation theory are reported. They are found to be in good agreement with those obtained solving the Faddeev-Yakubovsky equations. The calculated total and differential cross sections are compared with the available experimental data. The effect of including a three-nucleon interaction is also discussed.

The neutron spin rotation induced by parity-violating (PV) components in the nucleon-nucleon potential is studied in n-polarized - d scattering at zero energy. Results are obtained corresponding to the Argonne v(18) two-nucleon and Urbana-IX three-nucleon strong-interaction potentials in combination with either the Desplanques-Donoghue-Holstein or pionless effective field theory models for the weak-interaction potential. We find that this observable is dominated by the contribution of the long-range part of the PV potential associated with pion exchange. Thus its measurement could provide a further constraint, complementary to that coming from measurements of the photon asymmetry in n-polarized - p radiative capture, on the strength of this component of the hadronic weak interaction.

In order to isolate the contribution of the nucleon strange electric form factor to the parity-violating asymmetry measured in He-4(e,e('))He-4 experiments, it is crucial to have a reliable estimate of the magnitude of isospin-symmetry-breaking (ISB) corrections in both the nucleon and He-4. We examine this issue in the present Letter. Isospin admixtures in the nucleon are determined in chiral perturbation theory, while those in He-4 are derived from nuclear interactions, including explicit ISB terms. A careful analysis of the model dependence in the resulting predictions for the nucleon and nuclear ISB contributions to the asymmetry is carried out. We conclude that, at the low momentum transfers of interest in recent measurements reported by the HAPPEX Collaboration at Jefferson Lab, these contributions are of comparable magnitude to those associated with strangeness components in the nucleon electric form factor.

We describe the calculation of the pi-nucleus optical potential to NLO in Chiral Perturbation Theory (ChPT) within a new formulation of the effective theory in the presence of a nuclear background, characterized by a static, non-uniform distribution of the baryon number, describing the finite nucleus. This formulation reduces to the conventional In-medium ChPT in the case of a uniform distribution. In this way we are able to identify unambiguously the nuclear finite size effects and disentangle the S-, P- and D-wave contributions to the optical potential without invoking the local density approximation.

We propose a method for implementing the large-N-c, large-N-f limit of QCD at the effective Lagrangian level. Depending on the value of the ratio N-f/N-c, different patterns of chiral symmetry breaking can arise, leading in particular to different behaviors of the eta ' mass in the combined large-N limit.

We express the two-massless-flavor Gell-Mann-Oakes-Renner ratio in terms of low-energy pi pi observables including the O(p(6)) double chiral logarithms of generalized chiral perturbation theory. Their contribution is sizeable and tends to compensate the one from the single chiral logarithms. However it is not large enough to spoil the convergence of the chiral expansion. As a signal of reduced theoretical uncertainty, we find that the scale dependence from the one-loop single logarithms is almost completely canceled by the one from the two-loop double logarithms.

The possibility of detecting the S-wave of the decays tau -> 3 pi + nu(tau) in the threshold region is explored, with emphasis on the sensitivity to the size of the quark antiquark condensate ., Comment: contribution to QCD99, 4 pages, Latex, using espcrc2.sty (included), 2 PS figure

These are the notes of a lecture given at the Pisa summer school on Basic nuclear interactions and their link to nuclear processes in the cosmos and on earth. Taking a historical perspective we will describe the gauge principle, which lies at the basis of all known fundamental interactions.