Abstract
High-luminosity fixed target experiments provide impressive sensitivity to new light weakly coupled degrees of freedom. We revisit the minimal case of a scalar singlet $S$ coupled to the Standard Model through the Higgs portal, that decays visibly to leptons for scalar masses below the di-pion threshold. The dataset from the LSND experiment is found to impose the leading constraints within two mass windows between $m_S \sim 100$ and 350 MeV. In the process, we analyze a number of scalar production channels in the target, finding that proton bremsstrahlung provides the dominant channel at LSND beam energies.
Highlights
The empirical evidence for physics beyond the Standard Model (SM), notably for dark matter and neutrino mass, may point to the presence of a more complex hidden sector [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17]
HIGGS PORTAL We extend the SM by adding a scalar singlet S, for which the leading relevant or marginal couplings to the Higgs doublet H constitute the Higgs portal [42], PHYS
Utilizing only the splitting function calculation (10) as a conservative approximation for the total rate, the total number of scalars NS produced through the bremsstrahlung channel can be estimated numerically, where we normalize the rate to the number of πþ produced, Nπ, which is given at LSND energies by the Burman-Smith distribution [77]
Summary
The empirical evidence for physics beyond the Standard Model (SM), notably for dark matter and neutrino mass, may point to the presence of a more complex hidden (or dark) sector [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17]. The strongest existing constraints on the Higgs portal, in the low mass range where BrðS → lþl−Þ ∼ 1, arise from searches for leptonic decays at the CHARM fixed target experiment at CERN [43,44,45], and analysis of Kþ → πþS signatures at the Brookhaven E949 experiment, with S escaping the detector before decaying and being counted as missing energy in the search for Kþ → πþνν [44,45,46].
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