[Paper Review] New Charged Particles from Higgs Couplings

Andrew G. Cohen, Martin Schmaltz|arXiv (Cornell University)|Jul 15, 2012

Particle physics theoretical and experimental studies15 references4 citations

TL;DR

This paper proposes a novel assay using Higgs couplings to gluons and photons to probe new charged particles beyond the Standard Model, particularly those linked to electroweak symmetry breaking. By analyzing the ratio of Higgs couplings to photons and gluons, the method extracts the effective squared charge-to-Casimir ratio of new fermions, demonstrating its utility with a model of charge 5/3 quarks that can be pair-produced at the LHC and decay via cascade processes into multiple W bosons and b-jets.

ABSTRACT

The recently reported observation of a new particle with mass about 125 GeV and couplings generally resembling those of the Standard Model Higgs boson provides a potential probe of the physics of electroweak symmetry breaking. Although the current data only provides hints, we suggest a particular combination of Higgs couplings as an assay for new charged particles connected with electroweak symmetry breaking, and construct a simple model with charge 5/3 quarks as a demonstration of its use.

Motivation & Objective

• To develop a model-independent assay using Higgs couplings to detect new charged particles associated with electroweak symmetry breaking.
• To identify a measurable observable—specifically the ratio δcγ/δcg—that encodes information about the electric charge and representation of new fermions.
• To construct a concrete realization of the assay using a model of heavy quarks with electric charge +5/3, transforming via cascade decays into multiple W bosons and b-jets.
• To assess the phenomenological viability of such new particles within LHC constraints, particularly from fourth-family B-quark searches.
• To explore the impact of varying Yukawa coupling strength on the stability and decay signatures of the lightest new quark.

Proposed method

• Parameterize quantum corrections to Higgs couplings to gluons (hGG) and photons (hFF) via δcg and δcγ, derived from one-loop contributions of new fermions.
• Derive analytical expressions for δcg and δcγ in terms of the fermion's electric charge Q, color representation R, and the logarithmic derivative dlnmψ/dlnv.
• Introduce the observable āq² = 2δcγ/δcg as a model-independent discriminator of the effective squared charge per Casimir, enabling identification of new particle quantum numbers.
• Construct a minimal model with a heavy SU(2) doublet of quarks carrying charge +5/3, transforming under color triplet representation.
• Implement a Yukawa coupling to the top quark (λdecay) to allow the lightest quark to decay into SM particles, while preserving heavy quark baryon number.
• Simulate cascade decays: heavier quarks decay via W/Z/h to the lightest +5/3 quark, which then decays via W⁺t → W⁺W⁺b, producing final states with 4–8 W bosons and 2 b-jets.

Experimental results

Research questions

• RQ1 Can the ratio δcγ/δcg serve as a direct probe of the electric charge and color representation of new charged particles coupling to the Higgs?
• RQ2 What are the phenomenological signatures of a new heavy quark with charge +5/3 at the LHC, particularly when decaying via cascade processes?
• RQ3 How does the strength of the Yukawa coupling λdecay affect the stability and detectability of the lightest new quark?
• RQ4 To what extent do precision electroweak observables constrain this model, and are the contributions to S and T parameters suppressed?
• RQ5 How do existing LHC searches for fourth-family B quarks constrain the mass of the new +5/3 quarks?

Key findings

• The ratio δcγ/δcg is proportional to Q²/C₂(R), providing a direct measurement of the effective charge-to-Casimir ratio of new fermions from Higgs coupling data.
• For a single color-triplet fermion, āq² = Q², so measuring δcγ/δcg directly reveals the electric charge of the new particle.
• The model of +5/3 quarks produces distinctive final states with 4 to 8 W bosons and 2 b-jets, arising from cascade decays through W, Z, and Higgs bosons.
• The lightest +5/3 quark can be long-lived if λdecay ≲ 10⁻⁸, leading to exotic hadronic bound states such as charge-2 mesons and charge-3 baryons.
• For moderate λdecay (10⁻⁶ to 10⁻¹), the lightest quark decays promptly but slower than heavier ones, yielding clean signatures with multiple W bosons.
• Existing ATLAS and CMS searches for fourth-family B quarks, which decay similarly (B → W⁻t → WWb), set lower mass bounds of ~480 GeV and ~611 GeV, respectively, constraining the model.