Search for a new resonance in the $bj\mu\mu$ final state at a dimuon invariant mass around 28 GeV with ATLAS $pp$ data collected at $\sqrt{s} = $ 8 and 13 TeV

A search for a resonance decaying to two muons in the dimuon invariant mass range between 26-30 GeV and produced in association with a $b$-jet and an additional jet, is presented. The analysis uses proton-proton collision datasets recorded by the ATLAS detector at the centre-of-mass energies of 8 and 13 TeV, corresponding to integrated luminosities of 20~$\text{fb}^{-1}$ and 139~$\text{fb}^{-1}$, respectively. The events are categorised into two mutually exclusive search regions - the first one with exactly one central jet and at least one forward jet, and the second with at least two central jets and no forward jets. No significant excess above the Standard Model predictions is observed.