RSS Few-Body Systems
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Momentum Dependent Nucleon–Nucleon Contact Interactions and Their Effect on $$p-d$$ Scattering Observables
Abstract
Starting from a complete set of relativistic nucleon–nucleon contact operators up to order \(O(p^4)\) of the expansion in the soft (relative or nucleon) momentum p, we show that non-relativistic expansions of relativistic operators involve twenty-six independent combinations, two starting at \(O(p^0)\) , seven at order \(O(p^2)\) and seventeen at order \(O(p^4)\) . This demonstrates the existence of two low-energy free constants that parameterize interactions dependent on the total momentum of the pair of nucleons P. The latter, through the use of a unitary transformation, can be removed in the two-nucleon fourth-order contact interaction of the Chiral Effective Field Theory, generating a three-nucleon interaction at the same order. Within a hybrid approach in which this interaction is considered together with the phenomenological potential AV18, we show that the LECs involved can be used to fit very accurate data on the polarization observables of the low-energy \(p-d\) scattering, in particular the \(A_y\) asymmetry.
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Meson Exchange Currents in the Clothed-Particle Representation: Calculation of the Deuteron Magnetic Form Factor
Abstract
We show an original way to build up a new family of electromagnetic meson exchange current operators via the method of unitary clothing transformations. Being introduced in such a way they do not depend on the choice of states with which we calculate the matrix elements. The new expressions for meson exchange currents has been compared with ones derived within the previous explorations. Special attention is paid to the deuteron eigenvalue problem and finding the proper deuteron states in a moving frame. An effective technique of ensuring the gauge independence based upon generalization of Siegert’s theorem is proposed. Magnetic form factor of the deuteron is calculated with both one-body and two-body mechanisms. The influence of relativistic effects in the one-body calculation has been considered. Besides, separate contributions from the different meson exchange mechanisms are discussed.
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Abnormal Solutions of Bethe–Salpeter Equation with Massless and Massive Exchanges
Abstract
We summarize the main properties of the so called “abnormal solutions” of the Wick–Cutkosky model, i.e. two massive scalar particles interacting via massless scalar exchange (“photons”), within the Bethe–Salpeter equation. These solutions do not exist in the non-relativistic limit, in spite of having very small binding energies. They present a genuine many-body character dominated by photons, with a norm of the valence constituent wave function (two-body norm) that vanishes in the limit of zero binding energy. We present new results concerning the massive-exchange case, in particular determine under which conditions is it possible to obtain such peculiar solutions without spoiling the model by tachyonic states ( \(M^2<0\) ).
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Low-Energy Dipole Strength in $$^8$$ He
Abstract
In this work, we present new ab initio coupled-cluster calculations of dipole-excited state properties of \(^8\) He based on the chiral effective field theory interaction 1.8/2.0 (EM). We focus on the dipole polarizability, and compare the results to our previous study [Phys. Rev. C 105, 034313 (2022)] and subsequent theoretical work. With the aim of connecting the presence of low-lying dipole strength to structure properties of \(^8\) He, we compute the point-neutron radius, finding excellent agreement with available experimental data, and investigate its correlation with the dipole polarizability.
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Universality for Three Bosons with Large, Negative Effective Range: Aspects and Addenda
Abstract
Resummed-range Effective Field Theory is the consistent non-relativistic Effective Field Theory of point interactions in systems with large two-body scattering length a and an effective range \(r_0\) large in magnitude but negative. Its leading order is non-perturbative, and its observables depend only on the dimensionless ratio \(\xi :=2r_0/a\) once \(|r_0|\) is chosen as base unit. This presentation highlights aspects for three identical spinless bosons and adds details to a previous discussion (Griesshammer and van Kolck in Eur Phys J A 59:289, 2023). At leading order, no three-body interaction is needed. A ground state exists only in the range \(0.366\ldots \ge \xi \ge -8.72\ldots \) , and excited states display self-similarity and Discrete Scale Invariance, with small corrections for nonzero \(r_0\) .