Co-Conveners: Richard Milner (MIT), Eric Prebys (UCD), Mike Lamont (CERN)

A number of fundamental physics questions require exploration of rare processes and what is called Physics Beyond Colliders (PBC). These are similar in spirit to those addressed by high-energy colliders, but require different types of beams and experiments. Modifications of existing accelerator complexes and future dedicated scientific infrastructure are considered for the next two decades through projects complementary to the LHC/HL-LHC and other possible future colliders.

There are clearly links to the work of the Rare Processes and Precision Measurement Frontier

The areas we address include:

Motivated by the QCD axion as well as astrophysical hints, the low energy hidden sector is potentially accessible via number of sub-eV Axion/ALP search techniques. For example:

• Helioscopes (e.g. BabyIAXO/IAXO)
• Haloscopes using resonant cavities (e.g. ADMX) or other methods (e.g. MADMAX)
• Light-shining-through-walls experiments (e.g. JURA, STAX)

Many of these initiatives can profit from ongoing advances in accelerator technology e.g. high field superconducting magnets, superconducting RF.

Light Dark Matter searches in the MeV - GeV mass range target a parameter space of the Hidden Sector of special relevance to open questions in cosmology. Options include:

• Direct detection WIMP searches (primarily addressed by the Cosmic Frontier).
• Proton beam dump experiment: new proposals (e.g. BDF/SHiP), re-purposed existing experiments (e.g. NA62, MiniBooNE, SeaQuest)
• Electron beam dump experiments: NA64, LDMX, BDX… Proposals could include use of novel use of existing facilities (LCLS-II, CEBAF, FAST/IOTA) or the development of new facilities.
• Long lived particles at colliders (LHC, SuperKEKB)

Precision measurements and rare decays can probe higher masses than accessible with LHC direct searches, via searches for the possible influence of the contribution of loop diagrams in a number of scenarios. For example:

• Ultra-rare or forbidden decays/reactions:
  • Kaon sector (NA62, KOTO, KLEVER}
  • Lepton sector (TauFV, Mu3e, MEG,mu2e/mu2e-II)
• Precision measurements:
  • Permanent EDM
    • in protons/deuterons (CPEDM)
    • in strange/charmed baryons (LHC-FT)
  • Anomalous magnetic moment of muon (g-2)

Use of existing accelerator technologies or accelerator technology under development for novel physics applications

Technologies to be considered include:

• High field magnets
  • e.g. axion and dark matter searches
• Superconducting RF
  • e.g. thin film SRF cavities for axion searches
• ERLs
• Induction LINACs
  • e.g. rare muon processes
• Quantum sensors, ultra-sensitive opto-mechanical force sensors (e.g. KWISP), SQUID based measurement devices
• Carbon Nanotubes (CNT) (e.g. directional detection of DM candidates)
• Targetry, FFAs

Physics applications might include various types of axion/ALP searches (mentioned above), vacuum magnetic birefringence (VMB), exploration of Ultra-Light Dark Matter and Mid-Frequency Gravitational Waves (e.g. AION, MAGIS)

The Physics Beyond Collider study at CERN was set-up in 2016 and reported to the European Strategy for Particle Physics in 2019. Although it naturally had a CERN/European bent, it did attempt to evaluate things in a world wide context. A couple of potentially interesting references follow.

• Beyond the Standard Model Working Group Report https://arxiv.org/abs/1901.09966
• Summary Report of Physics Beyond Colliders at CERN https://arxiv.org/abs/1902.00260

Here is the list of submitted LOIs to this topic. First index before “/” corresponds to the primary frontier used for the submission.