  A measurement of $b$-hadron pair production is presented, based on a data set
corresponding to an integrated luminosity of 11.4 fb$^{-1}$ of proton--proton
collisions recorded at $\sqrt{s}=8$ TeV with the ATLAS detector at the LHC.
Events are selected in which a $b$-hadron is reconstructed in a decay channel
containing $J/\psi \rightarrow \mu\mu$, and a second $b$-hadron is
reconstructed in a decay channel containing a muon. Results are presented in a
fiducial volume defined by kinematic requirements on three muons based on those
used in the analysis. The fiducial cross section is measured to be $17.7 \pm
0.1 ($stat.$) \pm 2.0 ($syst.$)$ nb. A number of normalised differential cross
sections are also measured, and compared to predictions from the Pythia8,
Herwig++, MadGraph5\_aMC@NLO+Pythia8 and Sherpa event generators, providing new
constraints on heavy flavour production.
