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Decay and Detection of a Light Scalar Boson Mixing with the Higgs

Martin Wolfgang Winkler

TL;DR

This work resolves long-standing uncertainties in the hadronic decays of a GeV-scale scalar mixing with the Higgs by performing a two-channel dispersive analysis (ππ and KK) with updated phase data, aligning the results with Donoghue et al. and correcting a sign issue in prior analyses. The authors bridge nonperturbative QCD with perturbative spectator modeling to provide a unified decay-rate prediction across the full mass range, and they deliver meson form factors applicable to non-universal couplings. They re-evaluate accelerator, cosmological, and astrophysical constraints, highlighting notable shifts in beam-dump sensitivities and identifying viable parameter windows for Higgs-relaxion and dark-mediator scenarios. The results have direct implications for current and future searches (e.g., CHARM, SHiP, NA62) by refining decay-length predictions and exclusion bounds. Overall, the paper enhances the reliability of light scalar searches and broadens the scope for discovering Higgs-portal new physics.

Abstract

The simplest extension of the standard model consists in adding one singlet scalar field which mixes with the Higgs boson. O(GeV) masses of the new scalar carry strong motivation from relaxion, dark matter and inflation models. The decay of a GeV scalar is, however, notoriously difficult to address since, at this mass scale, the chiral expansion breaks down and perturbative QCD does not apply. Existing estimates of the GeV scalar decay rate disagree by several orders of magnitude. In this work, we perform a new dispersive analysis in order to strongly reduce these uncertainties and to resolve discrepancies in earlier results. We will update existing limits on light scalars and future experimental sensitivities which are in some cases strongly affected by the new-found decay rates. The meson form factors provided in this work, can be used to generalize our findings to non-universally coupled light scalars.

Decay and Detection of a Light Scalar Boson Mixing with the Higgs

TL;DR

This work resolves long-standing uncertainties in the hadronic decays of a GeV-scale scalar mixing with the Higgs by performing a two-channel dispersive analysis (ππ and KK) with updated phase data, aligning the results with Donoghue et al. and correcting a sign issue in prior analyses. The authors bridge nonperturbative QCD with perturbative spectator modeling to provide a unified decay-rate prediction across the full mass range, and they deliver meson form factors applicable to non-universal couplings. They re-evaluate accelerator, cosmological, and astrophysical constraints, highlighting notable shifts in beam-dump sensitivities and identifying viable parameter windows for Higgs-relaxion and dark-mediator scenarios. The results have direct implications for current and future searches (e.g., CHARM, SHiP, NA62) by refining decay-length predictions and exclusion bounds. Overall, the paper enhances the reliability of light scalar searches and broadens the scope for discovering Higgs-portal new physics.

Abstract

The simplest extension of the standard model consists in adding one singlet scalar field which mixes with the Higgs boson. O(GeV) masses of the new scalar carry strong motivation from relaxion, dark matter and inflation models. The decay of a GeV scalar is, however, notoriously difficult to address since, at this mass scale, the chiral expansion breaks down and perturbative QCD does not apply. Existing estimates of the GeV scalar decay rate disagree by several orders of magnitude. In this work, we perform a new dispersive analysis in order to strongly reduce these uncertainties and to resolve discrepancies in earlier results. We will update existing limits on light scalars and future experimental sensitivities which are in some cases strongly affected by the new-found decay rates. The meson form factors provided in this work, can be used to generalize our findings to non-universally coupled light scalars.

Paper Structure

This paper contains 19 sections, 45 equations, 8 figures, 1 table.

Table of Contents

  1. Introduction
  2. Standard model extensions with light scalars
  3. Connection to Dark Matter
  4. Relaxion
  5. Scalar Decay Rates
  6. Status of Hadronic Decay Rates
  7. Chiral Perturbation Theory
  8. Dispersive Analysis
  9. Perturbative Spectator Model
  10. Experimental Constraints and Future Sensitivities
  11. Rare Decays
  12. Collider Searches
  13. Beam Dump Experiments
  14. Cosmology and Astrophysics
  15. Conclusion
  16. ...and 4 more sections

Figures (8)

  • Figure 1: Evaluations of the light scalar decay rate to pions by Voloshin Voloshin:1985tc, Raby & West Raby:1988qf, the Higgs Hunter's Guide Gunion:1989we, Truong & Willey Truong:1989my, Donoghue et al. Donoghue:1990xh and Monin et al. Monin:2018lee. In this figure $s_\theta$ has been set to unity.
  • Figure 2: Modulus (upper panel) and phase (lower panel) of the pion and kaon form factors.
  • Figure 3: Light scalar decay rate into pions from this work, from Truong & Willey Truong:1989my and from Donoghue et al. Donoghue:1990xh. We also show Truong & Willey's decay rate after correcting a sign error in their $T$-matrix parameterization (see text).
  • Figure 4: Hadronic and leptonic decay rates of a light scalar mixing with the Higgs. The decay rates scale with $s_\theta^2$ which was set to unity in this plot.
  • Figure 5: Radiative $\Upsilon$ decays and flavor changing $B$ decays mediated by a light scalar.
  • ...and 3 more figures