Search for decays of the Higgs boson into scalar particles decaying into four or six $b$-quarks using $pp$ collisions at $\sqrt{s}=13\mathrm{T}\mathrm{e}\mathrm{V}$ 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, $H\to a_1{a}_2\to 3a_1\to 6b$. The analysis uses proton-proton collision data at $\sqrt{s}=13$ TeV collected with the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of $140{\mathrm{f}\mathrm{b}}^{-1}$. 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 $\sigma (ZH)\mathrm{/}{\sigma }_{\text{SM}}(ZH)\times \mathcal{B}$($H\to 2a\to 4b$), between 24% and 38% for $\sigma (ZH)\mathrm{/}{\sigma }_{\text{SM}}(ZH)\times \mathcal{B}$($H\to a_1{a}_2\to 4b$), and between 10% and 20% for $\sigma (ZH)\mathrm{/}{\sigma }_{\text{SM}}(ZH)\times \mathcal{B}$($H\to a_1{a}_2\to 3a_1\to 6b$), depending on the masses of the scalar particles.