ALPs Effective Field Theory and Collider Signatures (1701.05379v3)

Abstract: We study the leading effective interactions between the Standard Model fields and a generic singlet CP-odd (pseudo)Goldstone boson. Two possible frameworks for electroweak symmetry breaking are considered: linear and non-linear. For the latter case, the basis of leading effective operators is determined and compared with that for the linear expansion. Associated phenomenological signals at colliders are explored for both scenarios, deriving new bounds and analyzing future prospects, including LHC and High Luminosity LHC sensitivities. Mono-$Z$, mono-$W$, $W$-photon plus missing energy and on-shell top final states are most promising signals expected in both frameworks. In addition, non-standard Higgs decays and mono-Higgs signatures are especially prominent and expected to be dominant in non-linear realizations.

• The paper distinguishes linear and non-linear EFT approaches for ALP interactions, offering new predictions for collider signatures.
• It identifies unique non-linear couplings like aZh^n that enable promising mono-Higgs and non-standard Higgs decay observations.
• Collider analyses, including mono-Z and top final states, set experimental bounds with future upgrades expected to enhance sensitivity.

ALPs Effective Field Theory and Collider Signatures: An Expert Overview

This paper investigates the effective interactions between the Standard Model (SM) fields and a generic singlet CP-odd (pseudo)Goldstone boson, commonly known as an Axion-Like Particle (ALP). The paper explores two frameworks for electroweak symmetry breaking (EWSB), namely the linear and non-linear effective field theory (EFT) approaches. The authors extensively examine phenomenological implications for collider experiments, particularly in relation to signals detectable at the Large Hadron Collider (LHC) and its high-luminosity upgrade.

Key Findings and Numerical Highlights

• Frameworks and Operator Bases: The research distinguishes between linear and non-linear realizations of EWSB. In the non-linear case, the paper constructs a complete, non-redundant set of CP-even bosonic operators that include ALP interactions. The linear EFT, often termed the Standard Model Effective Field Theory (SMEFT), traditionally organizes interactions in towers of gauge-invariant operators ordered by mass dimension.
• Distinctive Non-linear Couplings: A significant result is the identification of interactions in the non-linear framework—such as $aZh^n$ vertices—that appear at leading order, as opposed to their absence at lower orders in the linear framework. This includes prominent mono-Higgs and non-standard Higgs decay signatures that are absent or highly suppressed in the SMEFT approach.
• Collider Signatures: Mono-$Z$, mono-$W$, $W$-photon plus missing energy, and on-shell top final states are identified as promising collider signatures. Specifically, the paper calculates sensitivities and prospective bounds for these signals across current and future collider experiments. For instance, non-standard Higgs decays are constrained with current limits suggesting a branching ratio into exotic channels of less than approximately 34% (at 95% confidence level).
• Implications for LHC and Future Colliders: The paper scrutinizes existing LHC data for possible exclusion limits or discovery potential concerning the considered operators, revealing that several are already constrained by rare decay processes and astrophysical observations. Future collider experiments are expected to significantly enhance sensitivity, particularly with the potential to probe up to 20 TeV scales for certain operator coefficients at the high-luminosity LHC.

Theoretical and Practical Implications

• Relevance to Dark Matter and Beyond Standard Model Physics: The exploration of ALPs serves as a potential avenue to address long-standing issues such as the strong CP problem and dark matter. ALPs potentially bridge gaps between observed phenomena and theoretical predictions, making them a cornerstone for beyond Standard Model (BSM) physics explorations.
• Model Dependence and EFT Validity: This work emphasizes the necessity of matching kinematic scales to EFT applicability, reinforcing desires to exceed current framework boundaries while acknowledging their inherent approximations. The robustness of these theoretical predictions, contingent upon appropriate boundary conditions and underlying model assumptions, guides the scope of phenomenological applicability and experimental verifications.
• Future Directions: One of the salient features of non-linear EFT highlighted by this paper is its flexibility and broader applicability compared to linear models. Prospective research should expand the investigation to include potential flavor-changing neutral currents induced by these operators and explore the comprehensive coupling impacts of fermions within this framework.

This comprehensive analysis of ALPs within an EFT framework meaningfully expands the scope of theoretical and experimental exploration in particle physics, particularly regarding the intricate structure and interactions of light scalar particles in potential new physics scenarios. The present paper sets a firm ground for both current experimental inquiries and theoretical refinements in the ongoing journey towards an enriched understanding of fundamental physics.