Search for decays of the Higgs boson into scalar particles decaying into four or six $ b$ quarks using $ pp$ collisions at $ \sqrt{s}=13\text{}\text{}\mathrm{TeV}$ with the ATLAS detector

A search for exotic decays of the Higgs boson $ H$ into new scalar or pseudoscalar particles that subsequently decay into $ b$-quarks is presented. The search considers $ ZH$ production with several decay scenarios for the Higgs boson; first to a pair of identical scalars, $ H\to 2a\to 4b$, second to a pair of scalars with different masses ($ {m}_{a1}<{m}_{a2}$), either directly, $ H\to {a}_{1}{a}_{2}\to 4b$, or via a longer decay chain, $ H\to {a}_{1}{a}_{2}\to 3{a}_{1}\to 6b$. The analysis uses proton-proton collision data at $ \sqrt{s}=13\text{}\text{}\mathrm{TeV}$ collected with the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of $ 140\text{}\text{}{\mathrm{fb}}^{-1}$. No significant excess above the Standard Model prediction is observed. The search sets upper limits at 95% confidence level on the ratio of the Higgs boson production cross section to the SM prediction times the branching ratio of Higgs bosons decaying into $ 4b$ or $ 6b$, between 4% and 25% for $ \sigma (ZH)/{\sigma }_{\mathrm{SM}}(ZH)\times \mathcal{B}(H\to 2a\to 4b)$, between 24% and 38% for $ \sigma (ZH)/{\sigma }_{\mathrm{SM}}(ZH)\times \phantom{\rule{0ex}{0ex}}\mathcal{B}(H\to {a}_{1}{a}_{2}\to 4b)$, and between 10% and 20% for $ \sigma (ZH)/{\sigma }_{\mathrm{SM}}(ZH)\times \mathcal{B}(H\to {a}_{1}{a}_{2}\to 3{a}_{1}\to 6b)$, depending on the masses of the scalar particles.