  We study the $\Xi_b(6100)$ using the methods of QCD sum rules and light-cone
sum rules within the framework of heavy quark effective theory. Our results
suggest that the $\Xi_b(6100)$ can be well interpreted as the $P$-wave bottom
baryon of $J^P=3/2^-$, belonging to the $SU(3)$ flavor $\mathbf{\bar 3}_F$
representation. It has a partner state of $J^P=1/2^-$, labelled as
$\Xi_b(1/2^-)$, whose mass and width are predicted to be
$m_{\Xi_b(1/2^-)}=6.08^{+0.13}_{-0.11}$~GeV and
$\Gamma_{\Xi_b(1/2^-)}=4^{+29}_{-~4}$~MeV, with the mass splitting $\Delta
M=m_{\Xi_b(6100)}-m_{\Xi_b(1/2^-)}=9\pm3$~MeV. We propose to search for it in
the $\Xi_c({1/2}^-)\to \Xi_b^{\prime}\pi$ decay channel. Our results also
suggest that the $\Lambda_b(5912)$ and $\Lambda_b(5920)$ are their partner
states with $J^P=1/2^-$ and $3/2^-$ respectively, and moreover, the
$\Lambda_c(2595)$, $\Lambda_c(2625)$, $\Xi_c(2790)$, and $\Xi_c(2815)$ are
their charmed partner states.
