Two-pion exchange electromagnetic current in chiral effective field theory using the method of unitary transformation

Abstract

Abstract. Using the method of unitary transformation in combination with chiral effective field theory we derive the two-pion exchange contributions to the two-nucleon electromagnetic current. A formal definition of the current operator in this scheme and the power counting is presented. We discuss the implications of additional unitary transformations that have to be present to ensure the renormalizability of the one-pion exchange current. Further, we give explicit and compact results for the current in coordinate-space.

In-medium similarity renormalization group for closed and open-shell nuclei

“…The inclusion of 3N interactions in the initial Hamiltonian is crucial for achieving this good reproduction of the experimental level data: It stabilizes the spacing between the neutron 0d 3/2 orbital and the other levels in the sd-shell as neutrons are added, which governs the energies of low-lying excitations and the location of the neutron drip line [55,128,160,163]. We note that the excited states in 23,24 O are overestimated to some degree, but this is expected because our Shell model calculations do not explicitly treat the continuum coupling at present, and the N N +3N (400) Hamiltonian produces a too-pronounced shell closure at N = 16 (see section 6.1).…”

“…We will find further examples of enhanced shell closures in other isotopic chains. The interplay of nuclear interactions, manybody and continuum effects that causes the flat trend of the experimental groundstate resonance energies beyond 24 O suggests that the oxygen isotopes will remain an important testing ground for nuclear Hamiltonians and many-body methods for the foreseeable future.…”

“…If we use an 24 O reference state instead, both the excitation energies and neutron separation energies change by 80 keV or less (not shown). Figure 18 illustrates that the TNO improves the core energy, single-particle energies, and two-body matrix elements, in decreasing order of importance.…”

“…Instead, we start from nuclear interactions that describe low-energy QCD observables in the N N and 3N systems, like scattering data or binding energies. Nowadays, such interactions are derived in Chiral Effective Field Theory (EFT), which provides a constructive framework and organizational hierarchy for N N , 3N , and higher many-nucleon forces, as well as consistent electroweak operators (see, e.g., [14][15][16][17][18][19][20][21][22][23][24][25]). Since Chiral EFT is a lowmomentum expansion, high-momentum (short-range) physics is not explicitly resolved by the theory, but parametrized by the so-called low-energy constants (LECs).…”

Parity and time-reversal violation in A=2-4 nuclei

“…The last feature has been clearly demonstrated when studying electromagnetic (EM) processes [12][13][14][15].…”