QCD sum rules for magnetically induced mixing between $\eta_c$ and   $J/\psi$
  We investigate the properties of charmonia in strong magnetic fields by using QCD sum rules. We show how to implement the mixing effects between $\eta_c$ and $J/\psi$ on the basis of field-theoretical approaches, and then show that the sum rules are saturated by the mixing effects with phenomenologically determined parameters. Consequently, we find that the mixing effects are the dominant contribution to the mass shifts of the charmonia in strong magnetic fields. 
Sungtae Cho, Koichi Hattori, Su Houng Lee, Kenji Morita and Sho Ozaki
hep-ph hep-lat nucl-th