QCD sum-rule determination of the axial-vector mixing angle in the \texorpdfstring{$B_c(1P)$}{Bc(1P)} sector

We determine the mixing angle between the $1^1{P}_1$ and $1^3{P}_1$ axial-vector states in the $B_c(1P)$ sector using QCD sum rules. The analysis gives ${\theta }_{B_c(1P)}=(43.3\pm 0.2{)}^{\circ }$, indicating sizable mixing between these two states. We also compare our result with theoretical studies available in the literature.