Physics Beyond Colliders at CERN: Beyond the Standard Model Working Group Report

Abstract: The Physics Beyond Colliders initiative is an exploratory study aimed at exploiting the full scientific potential of the CERN's accelerator complex and scientific infrastructures through projects complementary to the LHC and other possible future colliders. These projects will target fundamental physics questions in modern particle physics. This document presents the status of the proposals presented in the framework of the Beyond the Standard Model physics working group, and explore their physics reach and the impact that CERN could have in the next 10-20 years on the international landscape.

• The paper presents CERN's strategic plan to use its infrastructure for exploring new physics beyond traditional collider experiments.
• It demonstrates systematic approaches to detect low-mass, weakly interacting particles, addressing gaps left by current high-energy searches.
• It outlines novel proposals, such as IAXO, NA62++ and SHiP, that leverage high-intensity beams to extend sensitivity to unexplored energy scales.

Overview of "Physics Beyond Colliders at CERN: Beyond the Standard Model Working Group Report"

This paper provides a detailed exploration of the Physics Beyond Colliders (PBC) initiative at CERN, aimed at harnessing the scientific capabilities of CERN’s infrastructure to address fundamental questions in particle physics beyond the scope of the LHC and potential future colliders. It delineates the status and potential impact of various proposals under the Beyond the Standard Model (BSM) physics working group on the international physics landscape over the next two decades.

Key Focus Areas and Proposals

1. New Physics (NP) Exploration:
   • The paper underscores the gap in current experimental physics, where the Large Hadron Collider (LHC) and other initiatives have yet to register unambiguous signals of NP. Theoretical models do not provide clear guidance, necessitating a broader experimental search to explore different mass and coupling ranges.
2. Low-Mass and Weakly Coupled Particles:
   • The report highlights low-mass, weakly interacting particles as a promising research frontier. These particles, such as axions and axion-like particles, necessitate systematic exploration at both accelerator experiments and through terrestrial detection proposals.
3. High Energy Scales:
   • Extremely high energy NP, potentially out of direct reach of traditional colliders, can be investigated through rare decay processes and searches for electric dipole moments (EDMs), offering insights into physics at energy scales over 100 TeV.
4. CERN’s Contributions:
   • The strength of CERN’s high-intensity beams and diverse infrastructure is pivotal to the mission of the PBC. These facilities provide a platform for several novel experiments aiming to complement existing LHC programs and expand the scope of the international physics experiment agenda.

Notable Projects and Their Potential Impact

• IAXO and JURA: Aim to explore axions and ALPs through novel detection methods and improved sensitivity to axion-photon coupling, potentially exceeding current limits by significant margins.
• NA62$^{++}$, NA64$^{++}$, and SHiP: Utilize high-intensity beams to search for Dark Matter candidates and other weakly interacting particles, pushing the frontier with enhanced sensitivity and precision over current capabilities.
• REDTOP: Investigates ultra-rare $\eta$ and $\eta'$ decays to cite possible BSM effects, offering unique insights distinct from other NP searches.
• KLEVER: Seeks high precision in measuring rare Kaon decays, extending sensitivity to potential new physics signatures.
• TauFV: Will explore lepton-flavor-violating tau decays, leveraging high yields in proton interactions to achieve unprecedented sensitivity levels.

Implications and Future Outlook

The initiatives within the PBC framework are poised to fill critical gaps in our exploration of BSM physics. They offer unique methodological approaches and enable exploration across wide parameter spaces previously unattainable. Importantly, by exploring low-mass, weakly interacting particles and probing high energy scales, these projects may elucidate the mechanisms underlying unsolved problems in particle physics such as neutrino masses, dark matter, and baryon asymmetry.

The success of these efforts hinges on fostering robust collaborations and securing the necessary resources to build and operate these ambitious experiments. As CERN gears up to execute the PBC strategy, the potential to shift the landscape of fundamental physics research on a global scale is significant. These endeavors are set to enhance our understanding of the universe and uncover new physics that could redefine the paradigms established by the Standard Model.