Phenomenology of a very light scalar (100 MeV < m h < 10 GeV) mixing with the SM Higgs

In this paper we investigate the phenomenology of a very light scalar, h, with mass 100 MeV < m h < 10 GeV, mixing with the SM Higgs. As a benchmark model we take the real singlet scalar extension of the SM. We point out apparently unresolved uncertainties in the branching ratios and lifetime of h in a crucial region of parameter space for LHC phenomenology. Bounds from LEP, meson decays and fixed target experiments are reviewed. We also examine prospects at the LHC. For m h ≲ m B the dominant production mechanism is via meson decay; our main result is the calculation of the differential p T spectrum of h scalars originating from B mesons and the subsequent prediction of up to thousands of moderate (triggerable) p T displaced dimuons possibly hiding in the existing dataset at ATLAS/CMS or at LHCb. We also demonstrate that the subdominant V h production channel has the best sensitivity for m h ≳ m B and that future bounds in this region could conceivably compete with those of LEP.