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

Abstract

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 $m_h \lesssim m_B$ 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 $Vh$ production channel has the best sensitivity for $m_h \gtrsim m_B$ and that future bounds in this region could conceivably compete with those of LEP.