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

TL;DR

This work analyzes a very light scalar h (100 MeV–10 GeV) mixing with the SM Higgs in a real singlet extension, focusing on the (m_h, sin^2 ρ) parameter space. It combines theoretical motivations (scale-invariant scenarios, inflation) with a comprehensive survey of experimental bounds from LEP, meson decays, and fixed-target experiments, and then explores LHC phenomenology. A key result is that h production is dominated by B-decays for m_h < m_B, yielding potentially thousands of displaced dimuon events, while for m_h > m_B the Vh channel offers the best LHC sensitivity; the study highlights significant uncertainties in h lifetime and hadronic branching in the 280 MeV–4 GeV region and urges renewed theoretical and experimental attention to displaced-signature searches. Overall, the paper maps out current constraints and identifies concrete LHC search strategies to probe h in currently unexplored parameter space.

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.

Figures (9)

• Figure 1: Branching fraction for a light scalar $h$ decaying into muons and its mean decay length for $\sin^2\rho=1$ (see Eq. \ref{['eqctau']}) as predicted by a number of models (see text) Voloshin:1985tcDuchovni:1989iiTruong:1989myDonoghue:1990xhGunion:1989we. The Duchovni et al. prediction is an application of the Raby & West result Raby:1988qf.
• Figure 2: $95\%$ C.L. upper bounds on $\sin^2\rho$ as a function of $m_h$ from OPAL, ALEPH and L3. Also shown is the Foot & Kobakhidze prediction (dashed).
• Figure 3: Kaon, B meson, and radiative $\Upsilon$ decays involving $h$.
• Figure 4: Limits on $(m_h,\sin^2\rho)$ parameter space from meson decays for $m_h<400$ MeV: $K\to \pi\mu^+\mu^-$ (blue solid), $K\to \pi+invisible$ (blue dot-dashed), $B\to K\mu^+\mu^-$ (red solid), $B\to K+invisible$ (red dot-dashed), $B\to K^{\ast0}\mu^+\mu^-$ dedicated search (magenta), and the CHARM beam dump experiment (green enclosed is excluded). Also shown are the predictions from the models of Foot & Kobakhidze and Bezrukov & Gorbunov descending (dashed).
• Figure 5: Upper limits on $\sin^2\rho\times Br(h\to l^+l^-)$ as a function of $m_h$ for $m_h>400$ MeV, where $l$ corresponds to either $\mu$ or $\tau$ depending on the channel: $B\to K\mu^+\mu^-$ (red), $\Upsilon\to \gamma h\to \gamma\mu^+\mu^-$ (blue), $\Upsilon\to \gamma h\to \gamma\tau^+\tau^-$ (orange), and $pp\to h \to \mu^+\mu^-$ via gluon fusion at CMS (magenta). Also shown is the level that dimuon (blue dashed) or ditau (orange dashed) bounds must reach to compete with L3 assuming branching ratios given by the perturbative approach in Sec. \ref{['secproperties']}.