Doubly heavy baryons (QQq) and singly heavy antimesons $( \bar{Q} q)$ are related by the heavy quark-diquark (HQDQ) symmetry because in the heavy quark limit, the light degrees of freedom in both the hadrons are expected to be in identical configurations. Hyperfine splittings of the ground states in both systems are nonvanishing at ${\cal O}(1 / m_Q)$ in the heavy quark mass expansion and HQDQ symmetry relates the hyperfine splittings in the two sectors. In this talk, I will point out the existence of both perturbative and nonperturbative corrections to leading hyperfine splitting from HQDQ symmetry. The perturbative correction scales in powers of $\alpha_S$ and nonperturbative correction scales in powers of $\Lambda_QCD/m_Q$. In the extreme heavy quark limit in which the potential between the quarks in the diquark is Coulombic, the perturbative correction to the hyperfine splittings is only a few percents or smaller for values of $\alpha_S$ relevant to doubly charm and doubly bottom systems. We also argue that nonperturbative corrections to the prediction for the hyperfine splittings are suppressed by $(Λ_QCD/m_Q)^2$ which leads to hyperfine splitting corrections of order 10% in the charm sector and smaller in heavier systems. Eventually, I will compare my results to the lattice calculations for hyperfine splittings.
