Ordering Lusztig's families in type $B_n$

Abstract: Let $W$ be a finite Coxeter group and $L$ be a weight function on $W$ in the sense of Lusztig. We have recently introduced a pre-order relation $\preceq_L$ on the set of irreducible characters of $W$ which extends Lusztig's definition of "families" and which, conjecturally, corresponds to the ordering given by Kazhdan--Lusztig cells. Here, we give an explicit description of $\preceq_L$ for $W$ of type $B_n$ and any $L$. (All other cases are known from previous work.) This crucially relies on some new combinatorial constructions around Lusztig's "symbols". Combined with previous work, we deduce general compatibility results between $\preceq_L$ and Lusztig's $\ba$-function, valid for any $W,L$.