Physics Division Research Highlights

Electromagnetic Moments and Transitions in A ≤ 9 Nuclei with Meson-Exchange Currents Derived from χEFT

Quantum Monte Carlo calculations of electromagnetic moments and transitions were made for A ≤ 9 nuclei with nonzero magnetic moments [1]. The Argonne v18 two-nucleon and Illinois-7 three-nucleon potentials were used to generate the nuclear wave functions.  Contributions of two-body meson-exchange current (MEC) operators were included for both moments and M1 transitions.  The MEC operators are from both an older standard nuclear physics approach and a newer chiral effective field theory (EFT) formulation.  The latter includes a number of low-energy constants, which have been fit to the A=2,3 magnetic moments.  Energies, charge radii, quadrupole moments, and E2 transitions were also evaluated, but without MEC contributions.

In some cases, the two-body MEC contributions provide significant corrections to the magnetic moments computed in just the impulse approximation (IA), which includes only one-body current operators.  For example the MECs provide ~40% of the total in ⁹C, while they are very small in other cases, like ⁹Be, where the IA is already close to experiment.  The two different MEC approaches give qualitatively similar contributions but the chiral EFT approach is quantitatively superior.

Computed moments are shown in the top figure.  Black stars mark experimental magnetic moments; blue circles, our IA Green function Monte Carlo (GFMC) calculations; and red diamonds, the full GFMC calculations including MEC currents from χEFT: agreement with experiment is excellent.  Results for M1 (and some E2) transitions are shown on in the bottom figure, as a ratio of theory to experiment; the solid black lines denote the experimental errors.  Again, with inclusion of MEC contributions, agreement with experiment is very good wherever the transitions have been well-measured.  We also predict transitions in ⁹Li which should be measurable at a rare-ion beam facility such as ATLAS with Gammasphere.

Reference

[1] Quantum Monte Carlo calculations of electromagnetic moments and transitions in A <= 9 nuclei with meson-exchange currents derived from chiral effective field theory, S. Pastore, Steven C. Pieper, R. Schiavilla, R.B. Wiringa, arXiv:1212.3375 [nucl-th]

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