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$h \to Υγ$ Decay: Smoking Gun Signature of Wrong-Sign $hb\bar{b}$ Coupling

Aditya Batra, Sanjoy Mandal, Rahul Srivastava

TL;DR

This paper addresses how to detect a wrong-sign $h b\bar{b}$ coupling by examining the rare decay $h \to \Upsilon\gamma$, whose rate in the SM is suppressed by a near-cancellation between direct and indirect amplitudes. The authors perform a model-independent analysis of possible new physics that could distort this cancellation, focusing on both direct modifications via the $h b\bar{b}$ vertex and indirect modifications through $h\to\gamma\gamma$ loops from new particles. They show that only a wrong-sign $h b\bar{b}$ coupling can boost the $h \to \Upsilon\gamma$ rate by about two orders of magnitude, while a wide range of other new-physics scenarios (vector-like leptons, heavy quarks, charged scalars, and $W'$ bosons) either suppress the rate or are tightly constrained by $\mu_{\gamma\gamma}$ measurements. Consequently, an observed large enhancement in $h \to \Upsilon\gamma$ would serve as a smoking gun for an extended Higgs sector with a wrong-sign $h b\bar{b}$ coupling, with HL-LHC and FCC-hh offering substantial discovery potential.

Abstract

We perform a model-independent study of new physics effects in the Higgs decay $h \to Υγ$, focusing on scenarios that spoil the accidental cancellation between the direct and indirect amplitudes. After imposing all existing constraints from Higgs production and decay measurements, we find that a wrong-sign $h b\bar b$ coupling is the only viable scenario capable of enhancing the $h \to Υγ$ decay width by nearly two orders of magnitude. Therefore, an observation of a significantly enhanced $h \to Υγ$ rate at the LHC or future colliders would provide unambiguous evidence for a wrong-sign $h b\bar b$ coupling, directly pointing to the presence of an extended Higgs sector.

$h \to Υγ$ Decay: Smoking Gun Signature of Wrong-Sign $hb\bar{b}$ Coupling

TL;DR

This paper addresses how to detect a wrong-sign coupling by examining the rare decay , whose rate in the SM is suppressed by a near-cancellation between direct and indirect amplitudes. The authors perform a model-independent analysis of possible new physics that could distort this cancellation, focusing on both direct modifications via the vertex and indirect modifications through loops from new particles. They show that only a wrong-sign coupling can boost the rate by about two orders of magnitude, while a wide range of other new-physics scenarios (vector-like leptons, heavy quarks, charged scalars, and bosons) either suppress the rate or are tightly constrained by measurements. Consequently, an observed large enhancement in would serve as a smoking gun for an extended Higgs sector with a wrong-sign coupling, with HL-LHC and FCC-hh offering substantial discovery potential.

Abstract

We perform a model-independent study of new physics effects in the Higgs decay , focusing on scenarios that spoil the accidental cancellation between the direct and indirect amplitudes. After imposing all existing constraints from Higgs production and decay measurements, we find that a wrong-sign coupling is the only viable scenario capable of enhancing the decay width by nearly two orders of magnitude. Therefore, an observation of a significantly enhanced rate at the LHC or future colliders would provide unambiguous evidence for a wrong-sign coupling, directly pointing to the presence of an extended Higgs sector.
Paper Structure (21 sections, 47 equations, 30 figures, 12 tables)

This paper contains 21 sections, 47 equations, 30 figures, 12 tables.

Table of Contents

  1. INTRODUCTION
  2. Measuring the couplings of the Higgs boson
  3. Higgs decay to $\Upsilon \gamma$
  4. Possible modifications to the direct diagram
  5. The Two Higgs Doublet Model
  6. Wrong-sign $h b \bar{b}$ coupling
  7. Possible modification to indirect diagram
  8. New Heavy Charged Leptons
  9. Vector-like singlet leptons
  10. Vector-like doublet leptons
  11. New Heavy Quarks
  12. Vector-like singlet up-type quarks
  13. Vector-like singlet down-type quarks
  14. Vector-like doublet quarks
  15. New Heavy Charged Scalars
  16. ...and 6 more sections

Figures (30)

  • Figure 1: Feynman diagrams for some leading order contributions to the SM Higgs boson decays.
  • Figure 2: Branching ratios of various Higgs decay processes in the SM versus the mass of the Higgs boson. The experimentally measured mass is $125.20 \pm 0.11$ GeV ParticleDataGroup:2024cfk.
  • Figure 3: Indirect (left) and direct (right) Feynman diagrams of the $h\rightarrow\Upsilon\gamma$ process.
  • Figure 4: Modification to direct diagram (overall sign change) due to wrong-sign solution.
  • Figure 5: Branching Ratio of $h \to \Upsilon \gamma$ as a function of $\xi_b$ with $\xi_b = 1$ corresponding to the SM case. The black lines show the wrong-sign and right-sign predictions for $|\xi_b| = 1$. The horizontal grey region depicts the region excluded by direct search of $h \to \Upsilon \gamma$ATLAS:2022rej while the red regions are ruled out from $h \to X X$ measurements listed in Table. \ref{['tab:mu']}CMS-PAS-HIG-21-018ATLAS-CONF-2025-006.
  • ...and 25 more figures