Magnetic structure of light nuclei from lattice QCD

Lattice QCD with background magnetic fields is used to calculate the magnetic moments and magnetic polarizabilities of the nucleons and of light nuclei with A≤4, along with the cross section for the $M1$ transition np→dγ, at the flavor SU(3)-symmetric point where the pion mass is m_π~806 MeV. These magnetic properties are extracted from nucleon and nuclear energies in six uniform magnetic fields of varying strengths. The magnetic moments are presented in a recent article [S. Beane et al., Phys. Rev. Lett. 113, 252001 (2014)]. For the charged states, the extraction of the polarizability requires careful treatment of Landau levels, which enter nontrivially in the method that is employed. The nucleon polarizabilities are found to be of similar magnitude to their physical values, with βₚ=5.22(0+0.66-0.45)(0.23)×10^-4 fm³ and βₙ=1.253(0+0.056-0.067)(0.055)×10^-4 fm³, exhibiting a significant isovector component. The dineutron is bound at these heavy quark masses, and its magnetic polarizability, βₙₙ=1.872(0+0.121-0.113)(0.082)×0ex0ex10^-4 fm³, differs significantly from twice that of the neutron. A linear combination of deuteron scalar and tensor polarizabilities is determined by the energies of the jz=±1 deuteron states and is found to be β_d,±1=4.4(0+1.6-1.5)(0.2)×10^-4 fm³. The magnetic polarizabilities of the three-nucleon and four-nucleon systems are found to be positive and similar in size to those of the proton, ${{β}}_{³He}={{0ex}{0ex}}5.4({}{}{0}{}{+2.2}{{-}2.1})(0.2)×{}{10}^{{-}4} {fm}³$, ${{β}}_{{-0.22em}³H}=2.6(1.7)(0.1)×{}{10}^{{-}4} {fm}³$, and ${{β}}_{⁴He}=3.4({}{}{0}{}{+2.0}{{-}1.9})(0.2)×{}{10}^{{-}4} {fm}³$. Mixing between the ${j}z=0$ deuteron state and the spin-singlet $np$ state induced by the background magnetic field is used to extract the short-distance two-nucleon counterterm, ${L̄}₁$, of the pionless effective theory for $NN$ systems (equivalent to the meson-exchange current contribution in nuclear potential models) that dictates the cross section for the $np{→}d{γ}$ process near threshold. Combined with previous determinations of $NN$ scattering parameters, this enables an ab initio determination of the threshold cross section at these unphysical masses.