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Uncovering light scalars with exotic Higgs decays to bbmumu

The search for exotic Higgs decays are an essential probe of new physics. In particular, the small width of the Higgs boson makes its decay uniquely sensitive to the existence of light hidden sectors. Here we assess the potential of an exotic Higgs decay search for h -> 2X -> bbmumu to constrain theories with light CP-even (X = s) and CP-odd (X = a) singlet scalars in the mass range of 15 to 60 GeV. This decay channel arises naturally in many scenarios, such as the Standard Model augmented with a singlet, the two-Higgs-doublet model with a singlet (2HDM+S) -- which includes the Next-to-Minimal Supersymmetric Standard Model (NMSSM) -- and in hidden valley models. The 2b2mu channel may represent the best discovery avenue for many models. It has competitive reach, and is less reliant on low-pT b- and tau-reconstruction compared to other channels like 4b, 4tau, and 2tau2mu. We analyze the sensitivity of a 2b2mu search for the 8 and 14 TeV LHC, including the HL-LHC. We consider three types of analyses, employing conventional resolved b-jets with a clustering radius of R ~ 0.4, thin b-jets with R=0.2, and jet substructure techniques, respectively. The latter two analyses improve the reach for m_X ~ 15 GeV, for which the two b-jets are boosted and often merged. We find that Br(h -> 2X -> 2b2mu) can be constrained at the few x 10^-5 level across the entire considered mass range of X at the HL-LHC. This corresponds to a 1 - 10% reach in Br(h->2X) in 2HDM+S models, including the NMSSM, depending on the type of Higgs Yukawa couplings.