Collider Probes of Axion-Like Particles (1708.00443v2)

Abstract: Axion-like particles (ALPs), which are gauge-singlets under the Standard Model (SM), appear in many well-motivated extensions of the SM. Describing the interactions of ALPs with SM fields by means of an effective Lagrangian, we discuss ALP decays into SM particles at one-loop order, including for the first time a calculation of the $a\to\pi\pi\pi$ decay rates for ALP masses below a few GeV. We argue that, if the ALP couples to at least some SM particles with couplings of order $(0.01-1) \mbox{TeV}^{-1}$, its mass must be above 1 MeV. Taking into account the possibility of a macroscopic ALP decay length, we show that large regions of so far unconstrained parameter space can be explored by searches for the exotic, on-shell Higgs and $Z$ decays $h\to Za$, $h\to aa$ and $Z\to\gamma a$ in Run-2 of the LHC with an integrated luminosity of 300 fb$^{-1}$. This includes the parameter space in which ALPs can explain the anomalous magnetic moment of the muon. Considering subsequent ALP decays into photons and charged leptons, we show that the LHC provides unprecedented sensitivity to the ALP-photon and ALP-lepton couplings in the mass region above a few MeV, even if the relevant ALP couplings are loop suppressed and the $a\to\gamma\gamma$ and $a\to\ell^+\ell^-$ branching ratios are significantly less than 1. We also discuss constraints on the ALP parameter space from electroweak precision tests.

• The paper presents a comprehensive analysis of ALP interactions with the Standard Model using effective Lagrangians to calculate novel decay rates.
• It demonstrates that LHC Run-2 experiments can improve constraints on ALP couplings by up to six orders of magnitude in the low-mass region.
• The study highlights the complementary role of collider, low-energy, and cosmological probes in advancing the search for physics beyond the Standard Model.

Collider Probes of Axion-Like Particles: An Expert Summary

The paper of Axion-Like Particles (ALPs) represents a significant avenue for probing extensions of the Standard Model (SM) of particle physics. In the paper "Collider Probes of Axion-Like Particles," Bauer, Neubert, and Thamm provide a detailed analysis of how ALPs can be explored using collider experiments, particularly focusing on the interactions between ALPs and SM fields via effective Lagrangians.

Key Concepts and Methodology

The authors examine ALPs, which are gauge singlets in many SM extensions, via their interactions described by an effective Lagrangian at one-loop order. These interactions enable the ALP to decay into SM particles, such as photons and leptons. A novel aspect of this work is the comprehensive calculation of decay rates, including the decay of ALPs into three pions ($a \to \pi\pi\pi$) for masses below a few GeV. The authors suggest that ALP mass must exceed 1 MeV if it couples to SM particles with couplings of order $(0.01-1)\,\mbox{TeV}^{-1}$.

The paper identifies multiple parameters and coupling constants pertinent for these interactions and analyzes the macroscopic ALP decay length. This allows them to cover significant areas of parameter space through Run-2 of the LHC, specifically through exotic decays such as $h \to Z a$, $h \to a a$, and $Z \to \gamma a$. These decays are pivotal for exploring ALPs that could potentially elucidate phenomena like the anomalous magnetic moment of the muon.

Numerical Strengths and Claims

The authors provide strong numerical results indicating that LHC searches could improve existing constraints on ALP--photon and ALP--lepton interactions by up to six orders of magnitude. Remarkably, the paper emphasizes the unprecedented sensitivity of the LHC to ALP couplings in mass regions just above a few MeV. This includes scenarios where ALPs can account for the muon's anomalous magnetic moment, offering a robust probe into these couplings even at loop-suppressed levels.

Implications and Future Directions

This analysis has several implications for future experimental strategies in particle physics. The LHC's ability to explore large sections of previously unconstrained parameter spaces suggests a promising direction for uncovering new particles and forces beyond the SM. The paper highlights the complementarity of collider experiments with low-energy observables and cosmological probes, suggesting a multi-faceted approach to uncovering ALP characteristics.

Looking forward, the paper speculates on the potential for future developments, particularly in scenarios where heavy particles predominantly gain mass via electroweak symmetry breaking. This could reveal novel insights into UV completions of the SM, making collider experiments even more crucial in unraveling these facets.

Overall, "Collider Probes of Axion-Like Particles" offers a comprehensive framework for understanding and investigating ALPs, thus serving as a pivotal guide for ongoing and future experiments in this dynamic field. The detailed calculations and theoretical foundation laid by Bauer, Neubert, and Thamm present substantial opportunities for advancing particle physics, with implications reaching beyond collider experiments to broader astrophysical phenomena.