Search for the decay of the Higgs boson to a pair of light pseudoscalar bosons in the final state with four bottom quarks in proton-proton collisions at $\sqrt{s}$ = 13 TeV

A search is presented for the decay of the 125 GeV Higgs boson (H) to a pair of new light pseudoscalar bosons (a), followed by the prompt decay of each a boson to a bottom quark-antiquark pair, H $\to$ aa $\to$ $\mathrm{b}\bar{\mathrm{b}}\mathrm{b}\bar{\mathrm{b}}$. The analysis is performed using a data sample of proton-proton collisions collected with the CMS detector at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb${}^{-1}$. To reduce the background from standard model processes, the search requires the Higgs boson to be produced in association with a leptonically decaying W or Z boson. The analysis probes the production of new light bosons in a 15 $<$ $m_{\mathrm{a}}$ $<$ 60 GeV mass range. Assuming the standard model predictions for the Higgs boson production cross sections for pp $\to$ WH and ZH, model independent upper limits at 95% confidence level are derived for the branching fraction $\mathcal{B}$(H $\to$ aa $\to$ $\mathrm{b}\bar{\mathrm{b}}\mathrm{b}\bar{\mathrm{b}}$). The combined WH and ZH observed upper limit on the branching fraction ranges from 1.10 for $m_{\mathrm{a}}=$ 20 GeV to 0.36 for $m_{\mathrm{a}}=$ 60 GeV, complementing other measurements in the $\mu \mu \tau \tau$, $\tau \tau \tau \tau$ and bb$\mathrm{\ell }\mathrm{\ell }$ ($\mathrm{\ell }=$ $\mu$, $\tau$) channels.