Search for exotic decays of the Higgs boson to long-lived particles using displaced vertices in the ATLAS inner detector

A novel search for exotic decays of the Higgs boson to pairs of long-lived neutral particles, each decaying to a bottom quark pair, is performed using 139 fb$^{−1}$ of $\sqrt{s}=13$ TeV proton-proton collision data collected with the ATLAS detector at the LHC. Events consistent with the production of a Higgs boson in association with a $Z$ boson are analyzed, with the leptonic decay of the $Z$ mitigating the trigger challenges associated with displaced objects. Long-lived particle (LLP) decays are reconstructed from inner detector tracks as displaced vertices with high mass and track multiplicity relative to Standard Model processes. The analysis selection requires the presence of at least two displaced vertices, effectively suppressing Standard Model backgrounds. The residual background contribution is estimated using a fully data driven technique. No excess over the Standard Model prediction is observed, and upper limits are set on the branching ratio of the Higgs boson to pairs of LLPs. Branching ratios of 10% are excluded at the 95% confidence level for LLP mean proper lifetimes $c\tau$ as small as 4 mm and as large as 110 mm. For LLP masses below 40 GeV, these results represent the most stringent constraints to date for this range of proper lifetimes.