Session E13: Nuclear Theory I

Quantum Monte Carlo calculations of electron Scattering from 12C in the Short-Time Approximation

The Short-time approximation has been developed in the context of quantum Monte Carlo calculations to calculate nuclear responses in nuclei with A≥12. This algorithm exploits a factorization scheme to consistently retain two-body physics, both in two-body currents and correlations.

In this talk, we will present developments in the calculations of response densities in ¹²C, as well as recent results for longitudinal and transverse electromagnetic response functions.

We will also discuss the removal of the elastic contribution from the response densities calculated in the STA, necessary to reproduce experimental data for exchange momenta down to q=300 MeV.   

Cold Neutron Capture on the Deuteron in Pionless Effective Field Theory

Cold neutron capture on the deuteron to create the triton and emit a photon ($nd\to \gamma t$) at low energies is an important process for big bang nucleosynthesis; the time reversed process is relevant for experiments at the Triangle Universities Nuclear Laboratory (TUNL)/High Intensity Gamma Source (HIGS). For momentum transfer well below the pion mass, pionless effective field theory (EFT) is valid and provides a systematic expansion for the calculation of amplitudes. In this work, we calculate the cross section of $nd\to \gamma t$ to next-to-leading order in pionless EFT strictly perturbatively. We will discuss the context of this calculation and its potential impact on polarization-induced asymmetries.   

E1 strength distribution of 11Li in Halo Effective Field Theory

One of the signatures of a halo nucleus is a low-energy enhancement of the E1 strength distribution, dB(E1)/dE. This observable can be measured in Coulomb dissociation experiments. The E1 strength distribution is sensitive to long-distance properites of the halo state, but it is also affected by final-state interactions (FSI). 

We calculate dB(E1)/dE for the two-neutron halo nucleus ¹¹Li in Halo Effective Field Theory (EFT) using the ⁹Li core and the two halo neutrons as degrees of freedom. By evaluating the multiple-scattering series order-by-order we show that the neutron-neutron final-state interaction is the dominant FSI at low energies. The comparison of leading-order Halo EFT results with experimental data from RIKEN [T. Nakamura et al., Phys. Rev. Lett. 96, 252502 (2006)] shows reasonable agreement.    

Low-energy effective field theory of the deuteron

The deuteron is the only known bound state of two nucleons at physical quark masses. Yet, it provides an important testing ground for nuclear theory to better understand few-nucleon systems. We use a consistent effective field theory (EFT) developed in Ref. [1] to study proton-deuteron and deuteron-deuteron elastic scatterings in the S=2 channel at low energies below the deuteron breakup momentum (∼45.7 MeV). In this EFT, the deuteron is considered a fundamental particle. Therefore, interactions such as the Coulomb force can be treated analytically. Our study may provide valuable tests for use in the full four-body calculations.

 

[1] G. Rupak et al. “Fate of the neutron–deuteron virtual state as an Efimov level.” In: Phys. Lett. B791(2019), pp. 414–419.

    

Two-pion exchange as a leading-order contribution in chiral effective field theory

Pion exchange is the central ingredient to nucleon-nucleon interactions used in nuclear structure calculations, and one pion exchange (OPE) enters at leading order in chiral effective field theory. In the ^2S+1L_J=¹S₀ partial wave, however, OPE and a contact term needed for proper renormalization fail to produce the qualitative, and quantitative, features of the scattering phase shifts. Cutoff variation also revealed a surprisingly low breakdown momentum of about 330 MeV in this partial wave. Here we show that potentials consisting of OPE, two pion exchange (TPE), and a single contact address these problems and yield accurate and renormalization group (RG) invariant phase shifts in the ¹S₀ partial wave. We demonstrate that a leading-order potential with TPE can be systematically improved by adding a contact quadratic in momenta. For momentum cutoffs below about 500 MeV, the removal of relevant physics from TPE loops needs to be compensated by additional contacts to keep RG invariance. Inclusion of the Δ isobar degree of freedom in the potential does not change the strong contributions of TPE.   

Single-nucleon spectroscopic overlaps from the ab initio symmetry-adapted no-core shell model

Ab initio symmetry-adapted no-core shell model (SA-NCSM) calculations of single-nucleon spectroscopic observables are a first step to description of low-energy nuclear reactions and can be used as input for analyzing existing or future experimental data as well as for fitting ab-initio-based optical potentials. The SA-NCSM utilizes emergent symmetries in nuclei to reduce the dimensionality of the model space. This, in turn, allows one to reproduce the low-energy nuclear dynamics with only a small fraction of the model space, and hence making solutions to heavier nuclei and ultralarge model spaces feasible. This work discusses calculations of spectroscopic overlap functions, spectroscopic factors and asymptotic normalization coefficients (ANC’s) for Li isotopes.   

Pole position of the a1(1260) resonance in a three-body unitary framework

Masses, widths, and branching ratios of hadronic resonances are quantified by their pole positions and residues with respect to transition amplitudes on the Riemann sheets of the complex energy plane. We utilize a manifestly 3-body unitary model to determine the pole position of the a1(1260) meson from the ALEPH experiment by allowing for πρ coupled channels in S- and D-wave. We find it to be (1232+15+9 −0−11 − i266+0+15 −22−27) MeV.