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 mπ∼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 βp=5.22(+0.66/−0.45)(0.23)×10−4 fm3 and
βn=1.253(+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(+0.121/−0.113)(0.082)×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=±1 deuteron states, and is found to be
βd,±1=4.4(+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, β3He=5.4(+2.2/−2.1)(0.2)×10−4 fm3, β3H=2.6(1.7)(0.1)×10−4 fm3, β4He=3.4(+2.0/−1.9)(0.2)×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,
Lˉ1, 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→dγ 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.Comment: 49 pages, 24 figure