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Spin interactions in mesons in strong magnetic field

Spin interactions in relativistic quark-antiquark system in magnetic field is considered in the framework of the relativistic Hamiltonian, derived from the QCD path integral. The formalism allows to separate spin-dependent terms from the basic spin-independent interaction, contained in the Wilson loop, and producing confining and gluon exchange interaction. As a result one obtains relativistic spin-spin interaction $V_{ss}$, generalizing its nonrelativistic analog. It is shown, that in large magnetic field $eB, V_{ss}$ modifies and produces hyperfine shifts which grow linearly with $eB$ and preclude the use of perturbation theory. We also show, that tensor forces for $eB \neq 0$ are active in all meson states, but do not grow with $eB$