A new method to reject $g \rightarrow b\bar{b}$ jets in the ATLAS $b$-jet triggers

In this poster, we present a study of the rejection of jets containing more than one $b$ -hadron in the ATLAS “online" $b$-taggers, aiming to significantly reduce the readout rates of the ATLAS $b$-jet trigger system. It is important to be able to efficiently select events containing b-jets at the trigger level for analyses that involve many b-quarks in the final states, such as the search for $HH \rightarrow 4b$ production. However, in Run 2 the ATLAS $b$-tagger did not distinguish between jets containing a single $b$-hadron ($b$-jets) and jets containing 2 $b$-hadrons ($bb$-jets). Collision events involving small-angle $g \rightarrow b\bar{b}$ splitting, resulting in $bb$-jets are common in the LHC. Rejecting them in real-time would significantly reduce the readout rates of multi-$b$-jet triggers and ensure efficient signal extraction, which is particularly important for analyses that use multi-$b$-jet trigger chains. The proposed poster shows an approach to reject $bb$-jets in the ATLAS online $b$-taggers, the proposed implementation and the implications for the ATLAS Run 3 physics programme.