Academic Paper attributes
Lattice QCD with background magnetic fields is used to calculate the magnetic moments and magnetic polarizabilities of the nucleons and of light nuclei with Ale4, along with the cross-section for the M1 transition np dgamma, at the flavor SU(3)-symmetric point where the pion mass is mpisim 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 Letter. For the charged states, the extraction of the polarizability requires careful treatment of Landau levels, which enter non-trivially in the method that is employed. The nucleon polarizabilities are found to be of similar magnitude to their physical values, with betap=5.22(+0.66/-0.45)(0.23) times 10-4 fm3 and betan=1.253(+0.056/-0.067)(0.055) times 10-4 fm3, exhibiting a significant isovector component. The dineutron is bound at these heavy quark masses and its magnetic polarizability, betann=1.872(+0.121/-0.113)(0.082) times 10-4 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=pm 1 deuteron states, and is found to be betad,pm 1=4.4(+1.6/-1.5)(0.2) times 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, beta3rm He=5.4(+2.2/-2.1)(0.2) times 10-4 fm3, beta3rm H=2.6(1.7)(0.1) times 10-4 fm3, beta4rm He=3.4(+2.0/-1.9)(0.2) times 10-4 fm3. Mixing between the jz=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, L1, 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 npto dgamma 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.
