  We apply a new mass formula which is derived analytically in the relativistic
flux tube model to the mass spectra of $\Lambda_Q$ and $\Xi_Q$ (\emph{Q} =
\emph{c} or \emph{b} quark) baryons. To this end, the heavy quark-light diquark
picture is employed. We find that all masses of the available $\Lambda_Q$ and
$\Xi_Q$ states can be understood well. The assignments to these states do not
appear to contradict the strong decay properties. $\Lambda_c(2760)^+$ and
$\Xi_c(2980)$ are assigned to the first radial excitations with $J^P = 1/2^+$.
$\Lambda_c(2940)^+$ and $\Xi_c(3123)$ might be the 2\emph{P} states. The
$\Lambda_c(2880)^+$ and $\Xi_c(3080)$ are the good 1\emph{D} candidates with
$J^P = 5/2^+$. $\Xi_c(3055)$ is likely to be a 1\emph{D} state with $J^P =
3/2^+$. $\Lambda_b(5912)^0$ and $\Lambda_b(5920)^0$ favor the 1\emph{P}
assignments with $J^P = 1/2^-$ and $3/2^-$, respectively. We propose a search
for the $\tilde{\Lambda}_{c2}(5/2^-)$ state which can help to distinguish the
diquark and three-body schemes.
