Search for nonresonant Higgs boson pair production in the 4 leptons plus 2 b jets final state in proton-proton collisions at $\sqrt{s} =$ 13 TeV

A search for nonresonant pair-production of Higgs bosons (HH) with one Higgs boson decaying into four leptons and the other into a pair of b quarks is presented. The analyzed data correspond to an integrated luminosity of 137 $\text{fb}^{-1}$ of proton-proton collisions at $\sqrt{s} = 13~\text{TeV}$ collected with the CMS detector during the years 2016 to 2018. An upper limit of 30 at 95$\%$ confidence level (CL) is set on the signal strength modifier $\mu$, defined as the ratio of the observed double-Higgs boson production rate in the $\text{HH} \rightarrow ZZ^{*}b\bar{b} \rightarrow 4\ell b\bar{b}$ decay channel to the standard model (SM) expectation. Possible modifications of the SM Higgs trilinear coupling are investigated and constrained to be within the observed (expected) range $-9 (-10.5) < k_{\lambda} < 14 (15.5)$ at 95$\%$ CL.