Search for decays of the Higgs boson into scalar particles decaying into four or six b quarks using pp collisions at √s= 13 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 𝑍⁢𝐻 production with several decay scenarios for the Higgs boson; first to a pair of identical scalars, 𝐻 →2⁢𝑎 →4⁢𝑏, second to a pair of scalars with different masses (𝑚_𝑎⁢1 <𝑚𝑎⁢2), either directly, 𝐻 →𝑎₁⁢𝑎2 →4⁢𝑏, or via a longer decay chain, 𝐻 →𝑎₁⁢𝑎₂ →3⁢𝑎₁ →6⁢𝑏. The analysis uses proton-proton collision data at √𝑠 = 13  TeV collected with the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 140  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 4⁢𝑏 or 6⁢𝑏, between 4% and 25% for 𝜎⁡(𝑍⁢𝐻)/𝜎_SM⁡(𝑍⁢𝐻)×ℬ⁡(𝐻→2⁢𝑎→4⁢𝑏), between 24% and 38% for 𝜎⁡(𝑍⁢𝐻)/𝜎_SM⁡(𝑍⁢𝐻)×ℬ⁡(𝐻→𝑎₁⁢𝑎₂→4⁢𝑏), and between 10% and 20% for 𝜎⁡(𝑍⁢𝐻)/𝜎_SM⁡(𝑍⁢𝐻)×ℬ⁡(𝐻→𝑎₁⁢𝑎₂→3⁢𝑎₁→6⁢𝑏), depending on the masses of the scalar particles.