Constraints on a Light Singlet Scalar from Combined Exotic Higgs Decays

TL;DR

This paper analyzes constraints on a light singlet scalar from exotic Higgs decays, deriving bounds on decay rates and mixing angles using theoretical and experimental considerations.

Contribution

It provides analytical expressions for exotic Higgs decay rates into a light scalar and establishes upper bounds on decay widths and mixing angles.

Findings

Upper bound on \\cos heta < 0.12 - 0.13 for scalar masses up to 40 GeV.

Predicted decay rate \\Gamma_{h \\rightarrow \\phi \\phi} < 0.06 MeV.

Predicted decay rate \\Gamma_{h \\rightarrow \\phi \\phi \\phi} < 5 \\times 10^{-6} MeV.

Abstract

We investigate the phenomenology of the Standard Model extended by a real gauge-singlet scalar field, focusing on exotic Higgs decay channels. For a light scalar mass in the range \(0 < m_{\phi} < 40\) GeV, the Higgs boson can decay to both two and three scalar final states. We derive analytical expressions for these decay rates and impose a global constraint on the model parameters by requiring that their sum does not exceed the total Standard Model Higgs boson decay width. This requirement translates into a fourth-order inequality with respect to the singlet vacuum expectation value, \(v_{\phi}\). We demonstrate that satisfying this inequality imposes an upper bound of \(\cos \theta < 0.12 - 0.13\) across the entire mass range, providing a complementary constraint to existing direct search limits. Utilizing stronger independent constraints on the mixing (e.g., \(\cos \theta < 0.1\)), we then predict upper bounds on the individual exotic decay rates as a function of \(m_{\phi}\) as \(\Gamma_{h \rightarrow \phi \phi} < 0.06\) MeV and \(\Gamma_{h \rightarrow \phi \phi \phi} < 5 \times 10^{- 6}\) MeV, respectively.