Probing MeV to 90 GeV axion-like particles with LEP and LHC (1509.00476v1)

Abstract: Axion-like particles (ALPs), relatively light (pseudo-)scalars coupled to two gauge bosons, are a common feature of many extensions of the Standard Model. Up to now there has been a gap in the sensitivity to such particles in the MeV to 10 GeV range. In this note we show that LEP data on $Z\to\gamma\gamma$ decays provides significant constraints in this range (and indeed up to the $Z$-mass). We also discuss the sensitivities of LHC and future colliders. Particularly the LHC shows promising sensitivity in searching for a pseudo-scalar with $4 \lesssim m_a \lesssim 60$ GeV in the channel $pp \to 3 \gamma$ with $m_{3\gamma}\approx m_{Z}$.

• The paper introduces a novel strategy for probing axion-like particles (ALPs) in the MeV to 90 GeV range using LEP and LHC data.
• It analyzes exotic Z-boson decays into photon pairs and triple photon signatures to improve sensitivity to ALP couplings.
• The research enhances constraints on ALP interactions with gauge bosons, underscoring the potential of current and future collider experiments.

Overview of Research on Axion-Like Particles and Collider Sensitivity

The paper "Probing MeV to 90 GeV axion-like particles with LEP and LHC" by Joerg Jaeckel and Michael Spannowsky investigates the potential detection of axion-like particles (ALPs) within a mass range from mega-electronvolts (MeV) to 90 giga-electronvolts (GeV). ALPs are pseudo-scalar or scalar particles that couple with two gauge bosons, appearing commonly in numerous extensions of the Standard Model (SM) of particle physics. The research aims to bridge gaps in sensitivity for detecting these particles, particularly in the overlooked mass range between MeV and 10 GeV, leveraging data from the Large Electron-Positron Collider (LEP) and the Large Hadron Collider (LHC).

Context and Motivation

The quest for light, weakly interacting particles continues to be a priority because they offer plausible explanations for unobserved new physics, unlike heavy particles that have not been detected. Axion-like particles are especially interesting as they naturally emerge in models extending the SM and have implications for understanding dark matter. The challenge addressed by the paper lies in improving the sensitivity to ALPs in the mass range from MeV to GeV, where existing data and techniques have shown limitations.

Particle Models and Interactions

The interaction of ALPs with the SM is primarily through their coupling to gauge bosons like photons or hypercharge bosons, leading to decay modes such as ALP to two photons ($a\to\gamma\gamma$) or $Z\to a\gamma$. The paper primarily focuses on scenarios where ALP interactions are dominated by gauge bosons while neglecting potential interactions with fermions. The coupling strength and characteristics of these interactions are pivotal in determining the constraints and detection strategies for ALPs at particle colliders.

Methodology and Analysis

The paper utilizes existing LEP data on unusual $Z$-boson decays, particularly into two and three photons, to set constraints on ALP couplings. This approach is innovative as it considers not only direct constraints from three-photon decays ($Z \to 3\gamma$) but also leverages searches for tightly collimated photon pairs in $Z \to 2\gamma$ channels. These data sources offer a unified strategy to fill the sensitivity gap in the MeV to 10 GeV mass range.

For the LHC, the sensitivity analysis focuses on $Z$-boson decay into an ALP and a photon, producing a detectable signature of three photons with the combined system approximating the $Z$-boson mass. This detection method provides a mechanism to probe couplings that predominantly involve hypercharge bosons.

Results and Implications

The analysis yields new constraints on ALP couplings to both photons and hypercharge bosons, indicating significant coverage in previously unexplored mass regions. Particularly, the LHC has the potential to extend detection capabilities considerably owing to the substantial number of Z-bosons produced in hadronic collisions.

The paper also postulates future improvements in sensitivity could be offered by next-generation electron-positron colliders, such as FCC-ee, predicting advances in sensitivity by orders of magnitude.

Concluding Remarks

This research contributes to the ongoing efforts to discover or constrain light particles beyond the Standard Model. By effectively utilizing existing collider data and projecting future experimental capabilities, it prepares the ground for comprehensive ALP investigations, underscoring the importance of developing detection strategies for weakly interacting low-mass particle candidates. The results are significant for refining theoretical models involving ALPs, directly impacting our understanding of particle physics and dark matter phenomenology. Future explorations could include further refining detection techniques and applying these frameworks to a broader range of experimental setups.