Probing the Rare Four-Bottom Higgs Decay $H\to b\bar{b}b\bar{b}$ at the HL-LHC and ILC

Abstract

We propose the rare SM Higgs decay $H\to b\bar{b}b\bar{b}$ as a probe of the structure of Higgs interactions with bottom quarks and gauge bosons, and as a baseline for searches for new physics producing four-bottom final states in Higgs decays. We compute the leading contributions to this decay, including the dominant $H\to b\bar{b}g\to b\bar{b}b\bar{b}$ topology, the sizeable $H\to Z{Z}^{\ast }\to b\bar{b}b\bar{b}$ channel, and the loop-induced $H\to gg\to b\bar{b}b\bar{b}$ contribution. We find a branching ratio of order $1.6\times 10^{-3}$ and show that destructive interference among the leading amplitudes is phenomenologically relevant. We demonstrate that this decay can be probed in associated Higgs production at both the HL-LHC and the ILC. For $pp\to WH\to Wb\bar{b}b\bar{b}$ at $\sqrt{s}=14$ TeV, we use a multivariate analysis based on boosted decision trees to exploit correlations among the four-$b$ kinematic observables. At $3000{\mathrm{fb}}^{-1}$, the statistical significance reaches about $3.5$, while a tighter high-purity working point gives $S/B\simeq 5\%$ with significance close to $3\sigma$. A combined high-luminosity LHC dataset could therefore make this rare decay observable. For $e^{+}{e}^{-}\to ZH\to Zb\bar{b}b\bar{b}$ at the ILC with $\sqrt{s}=250$ GeV, we demonstrate that the cleaner collider environment gives a high-purity signal sample. In the nominal setup, the multivariate analysis gives a significance above $5\sigma$ already at $300{\mathrm{fb}}^{-1}$. At integrated luminosities of order $1-3{\mathrm{ab}}^{-1}$, the branching ratio can be measured with several-percent precision.

Comments: 33 pages, 20 figures