Feebly-Interacting Particles:FIPs 2020 Workshop Report (2102.12143v2)

Abstract: With the establishment and maturation of the experimental programs searching for new physics with sizeable couplings at the LHC, there is an increasing interest in the broader particle and astrophysics community for exploring the physics of light and feebly-interacting particles as a paradigm complementary to a New Physics sector at the TeV scale and beyond. FIPs 2020 has been the first workshop fully dedicated to the physics of feebly-interacting particles and was held virtually from 31 August to 4 September 2020. The workshop has gathered together experts from collider, beam dump, fixed target experiments, as well as from astrophysics, axions/ALPs searches, current/future neutrino experiments, and dark matter direct detection communities to discuss progress in experimental searches and underlying theory models for FIPs physics, and to enhance the cross-fertilisation across different fields. FIPs 2020 has been complemented by the topical workshop "Physics Beyond Colliders meets theory", held at CERN from 7 June to 9 June 2020. This document presents the summary of the talks presented at the workshops and the outcome of the subsequent discussions held immediately after. It aims to provide a clear picture of this blooming field and proposes a few recommendations for the next round of experimental results.

• The paper consolidates interdisciplinary research by summarizing experimental and theoretical approaches to study feebly-interacting particles.
• It reviews diverse detection methods—including collider, beam dump, and fixed-target experiments—to probe particle interactions relevant to dark matter.
• The report outlines future research directions, urging collaborative efforts to enhance detector sensitivity and refine theoretical models.

Overview of the FIPs 2020 Workshop Report

The "Feebly-Interacting Particles: FIPs 2020 Workshop Report" is a comprehensive document summarizing the discussions and outcomes of the FIPs 2020 workshop. The workshop brought together scientists from various fields such as collider physics, astrophysics, and particle phenomenology to discuss the physics of Feebly-Interacting Particles (FIPs) — a class of particles hypothesized to interact very weakly with Standard Model (SM) particles. This report captures important insights into ongoing research, current experimental results, and theoretical models concerning FIPs.

The motivation behind exploring FIPs arises from several unsolved mysteries in particle physics and cosmology, including the nature of dark matter, the asymmetry between matter and antimatter, and the strong CP problem. FIPs offer potential solutions to these problems, and the workshop addressed how these particles might be explored experimentally.

Key Areas of Discussion

1. Cosmology and FIPs: The report addresses the impact of FIPs on cosmological and astrophysical observations. One significant aspect is their potential contribution to dark matter. The research discussed involves studying cosmological bounds on FIPs and exploring how their interactions could explain observed phenomena such as the cosmic microwave background (CMB) anisotropies or dark matter relic density.
2. Detection and Experimental Approaches: Various strategies for detecting FIPs are explored. These include direct searches through collider experiments like the Large Hadron Collider (LHC), which can produce FIPs in high-energy collisions, and indirect methods involving astrophysical observations where FIPs could contribute to star and neutron star cooling processes. Beam dump and fixed target experiments such as those conducted at CERN are also highlighted as critical to probing the parameter space for FIPs.
3. Theoretical Models and Renormalization: The theoretical underpinning of FIPs includes their classification as light particles with suppressed interactions often characterized by higher-dimensional operators. Models like those involving dark photons, axion-like particles (ALPs), and heavy neutral leptons (HNLs) are discussed. The report emphasizes the need for a deeper understanding of these models and their couplings with the SM particles to devise effective experimental strategies.
4. Dark Matter Candidates: Among the FIPs, attention is given to those that could serve as dark matter candidates, particularly those with masses between a few MeV and GeV. The freeze-out or freeze-in mechanisms, which describe how FIPs could have achieved their current abundance in the universe, are explored.
5. Future Prospects and Recommendations: The workshop participants underscored the necessity for collaborative efforts to integrate theoretical and experimental research. The report provides recommendations for future research directions, including the development of more sensitive detectors, the design of new experimental setups, and the pursuit of innovative theoretical models.

Conclusion

In summary, the "FIPs 2020 Workshop Report" provides a detailed overview of the current state and future potential of research on feebly-interacting particles. FIPs offer intriguing possibilities for addressing some of the most fundamental questions in physics. The workshop consolidates the interdisciplinary approaches required to unravel these particles' properties and impacts, highlighting the synergy needed between theory and experiment. Further investigations prompted by this workshop will be pivotal in potentially observing FIPs and understanding their role in our universe.