Search for Higgs boson pairs decaying to WW*WW*, WW*$\tau\tau$, and $\tau\tau\tau\tau$ in proton-proton collisions at $\sqrt{s}$ = 13 TeV

The results of a search for Higgs boson pair ($\textrm{HH}$) production in the $\textrm{WW}^{*}\textrm{WW}^{*}$, $\textrm{WW}^{*}\tau\tau$, and $\tau\tau\tau\tau$ decay modes are presented. The search uses $138~\text{fb}^{-1}$ of proton-proton collision data recorded by the CMS experiment at the LHC at a center-of-mass energy of $13~\mathrm{TeV}$ from 2016 to 2018. Analyzed events contain two, three, or four reconstructed leptons, including electrons, muons, and hadronically decaying tau leptons. No evidence for a signal is found in the data. Upper limits are set on the cross section for nonresonant $\textrm{HH}$ production, as well as resonant production in which a new heavy particle decays to a pair of Higgs bosons. For nonresonant production, the observed (expected) upper limit on the cross section at $95\%$ confidence level (CL) is $21.3$ ($19.4$) times the standard model (SM) prediction. The observed (expected) ratio of the trilinear Higgs boson self-coupling to its value in the SM is constrained to be within the interval $-6.9$ to $11.1$ ($-6.9$ to $11.7$) at $95\%$ CL, and limits are set on a variety of new-physics models using an effective field theory approach. The observed (expected) limits on the cross section for resonant $\textrm{HH}$ production range from $0.18$ to $0.90$ ($0.08$ to $1.06$)$~\mathrm{pb}$ at $95\%$ CL for new heavy-particle masses in the range $250$--$1000~\mathrm{GeV}$.