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Revealing the electroweak properties of a new scalar resonance

Ian Low, Joseph Lykken

TL;DR

The paper develops a custodial SU(2)_C–based, model‑independent framework to identify the electroweak quantum numbers of a newly discovered neutral CP‑even scalar via its decays into WW, ZZ, Zγ, and γγ. By classifying scalars as custodial singlets, 5‑plets, or EW singlets with dimension-five couplings, it derives distinctive patterns in VV branching ratios that reveal whether the state is a SM Higgs look‑alike or a Higgs impostor. It provides explicit relations for decay widths and branching fractions, including off-shell decays, and shows how measurements across all four channels can discriminate between representations and multiplet sizes. The approach complements angular analyses and can significantly constrain the scalar sector’s role in electroweak symmetry breaking at the LHC and future colliders.

Abstract

One or more new heavy resonances may be discovered in experiments at the CERN Large Hadron Collider. In order to determine if such a resonance is the long-awaited Higgs boson, it is essential to pin down its spin, CP, and electroweak quantum numbers. Here we describe how to determine what role a newly-discovered neutral CP-even scalar plays in electroweak symmetry breaking, by measuring its relative decay rates into pairs of electroweak vector bosons: WW, ZZ, γγ, and Zγ. With the data-driven assumption that electroweak symmetry breaking respects a remnant custodial symmetry, we perform a general analysis with operators up to dimension five. Remarkably, only three pure cases and one nontrivial mixed case need to be disambiguated, which can always be done if all four decay modes to electroweak vector bosons can be observed or constrained. We exhibit interesting special cases of Higgs look-alikes with nonstandard decay patterns, including a very suppressed branching to WW or very enhanced branchings to γγand Zγ. Even if two vector boson branching fractions conform to Standard Model expectations for a Higgs doublet, measurements of the other two decay modes could unmask a Higgs imposter.

Revealing the electroweak properties of a new scalar resonance

TL;DR

The paper develops a custodial SU(2)_C–based, model‑independent framework to identify the electroweak quantum numbers of a newly discovered neutral CP‑even scalar via its decays into WW, ZZ, Zγ, and γγ. By classifying scalars as custodial singlets, 5‑plets, or EW singlets with dimension-five couplings, it derives distinctive patterns in VV branching ratios that reveal whether the state is a SM Higgs look‑alike or a Higgs impostor. It provides explicit relations for decay widths and branching fractions, including off-shell decays, and shows how measurements across all four channels can discriminate between representations and multiplet sizes. The approach complements angular analyses and can significantly constrain the scalar sector’s role in electroweak symmetry breaking at the LHC and future colliders.

Abstract

One or more new heavy resonances may be discovered in experiments at the CERN Large Hadron Collider. In order to determine if such a resonance is the long-awaited Higgs boson, it is essential to pin down its spin, CP, and electroweak quantum numbers. Here we describe how to determine what role a newly-discovered neutral CP-even scalar plays in electroweak symmetry breaking, by measuring its relative decay rates into pairs of electroweak vector bosons: WW, ZZ, γγ, and Zγ. With the data-driven assumption that electroweak symmetry breaking respects a remnant custodial symmetry, we perform a general analysis with operators up to dimension five. Remarkably, only three pure cases and one nontrivial mixed case need to be disambiguated, which can always be done if all four decay modes to electroweak vector bosons can be observed or constrained. We exhibit interesting special cases of Higgs look-alikes with nonstandard decay patterns, including a very suppressed branching to WW or very enhanced branchings to γγand Zγ. Even if two vector boson branching fractions conform to Standard Model expectations for a Higgs doublet, measurements of the other two decay modes could unmask a Higgs imposter.

Paper Structure

This paper contains 5 sections, 61 equations, 2 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Scalar Couplings with $V_1V_2$
  3. Implications of Custodial Invariance
  4. Partial Widths of $S\to V_1V_2^{(*)}$
  5. Discussion and outlook

Figures (2)

  • Figure 1: Ratio of branching fractions into $WW$ and $ZZ$, $Br(ZZ/WW)$, for an $SU(2)_C$ singlet and a 5-plet, as a function of the scalar mass.
  • Figure 2: The predicted ratios of branchings, as a function of $Br(ZZ/WW)$, for an electroweak singlet scalar. The red (gray) curves are for $Br(\gamma\gamma/WW)$ and black curves for $Br(Z\gamma/WW)$. In this plot we assume the branching into $WW$ is nonzero.