The Physics Beyond Colliders (PBC) initiative, launched in 2016, explores the opportunities offered by the CERN accelerator complex and infrastructure that are complementary to high-energy collider experiments and other initiatives worldwide. It takes place in an exciting and quickly developing physics landscape. To quote a contribution by theorist Jonathan Feng at the recent ICFA seminar in Ottawa: “In particle theory, this is a time of great creativity, new ideas, and best of all, new proposals for experiments and connections to other fields.”
Following a kick-off workshop in September 2016 (CERN Courier November 2016 p28), the second general PBC workshop took place at CERN on 21–22 November. With more than 230 physicists in attendance, it provided an opportunity to review the progress of the studies and to collect further ideas from the community.
During the past year, the PBC study was organised into working groups to connect experts in the various relevant fields to representatives of the projects. Two physics working groups dealing with searches for physics beyond the Standard Model (BSM) and QCD measurements address the design of the experiments and their physics motivation, while several accelerator working groups are pursuing initiatives ranging from exploratory studies to more concrete plans for possible implementation at CERN. The effort has already spawned new collaborations between different groups at CERN and with external institutes, and significant progress is already visible in many areas.
The potential performance increase for existing and new users of the upgraded HL-LHC injector chain, following the culmination of the LHC injector upgrade project (CERN Courier October 2017 p22), is being actively pursued with one key client being the SPS North Area at CERN. The interplay between potential future operation of the existing SPS fixed-target experiments (NA61, NA62, NA64, COMPASS) and the installation of new proposed detectors (NA64++, MUonE, DIRAC++, NA60++) has started to be addressed in both accelerator and physics respects. The technical study of the SPS proton beam dump facility and the optimisation of the SHiP detector for investigating the hidden sector are also advancing well.
Different options for fixed-target experiments at the LHC, for instance using gas targets or crystal extraction, are under investigation, including feasibility tests with the LHC beams. The novel use of partially stripped ions (PSI) to produce high-energy gamma rays in a so-called gamma factory (CERN Courier November 2017 p7) is also gaining traction. Having taken PSI into the SPS this year, near-term plans include the injection of partially stripped lead ions into the SPS and LHC in 2018.
The design study of a storage ring for a proton electric-dipole-moment (EDM) measurement is progressing, and new opportunities to use such a ring for relic axion searches through oscillating EDMs have been put forward. In the loop are the COSY team at Jülich who continue to break new ground with polarised deuteron experiments (CERN Courier September 2016 p27).
Last but not least are non-accelerator projects that wish to benefit from CERN’s technological expertise. One highlight is the future IAXO helioscope, proposed as a successor of the CERN CAST experiment for the search of solar axions. Recently IAXO has formed as a full collaboration and is in discussion with DESY as a potential site. IAXO and a potential precursor experiment (Baby-IAXO) benefit from CERN PBC support for the design of their magnets.
The workshop also included a session devoted to the presentation of exciting new ideas, following a call for contributions from the community. One noticeable new idea consists of the construction of a low-energy linac using CLIC technology for electron injection and acceleration in the SPS. A slow extracted SPS e-beam in the 10–20 GeV energy range would allow hidden sector searches similar to NA64 but at higher intensity, and the linac would provide unique R&D possibilities for future linear accelerators. Another highlight is the prospect of performing the first optical detection of vacuum magnetic birefringence using high-field magnets under development at CERN. New projects are also being proposed elsewhere, including a first QED measurement in the strong field regime at the DESY XFEL (LUXE project) and a search for η meson rare decays at FNAL (REDTOP experiment).
The presentations and discussions at the workshop have also shown that, beyond its support to the individual projects, the PBC study group provides a useful forum for communication between communities with similar motivations. This will be an important ingredient to optimise the scope of the future projects.
The PBC study is now at a crucial point, with deliverables due at the end of 2018 as input to the European Strategy for Particle Physics Update the following year. The PBC documents will include the results of the design studies of the accelerator working groups, with a level of detail matched to the maturity of the projects, and summaries of the physics motivation of the proposed experiments in the worldwide context by the BSM and QCD physics groups. One overview document will provide an executive summary of the overall landscape, prospects and relevant issues. It should also be emphasised that the goal of the PBC study is to gather facts on the proposed projects, not to rank them.
A follow-up plenary meeting of the PBC working groups is foreseen in mid-2018, and the main findings of the PBC study will be presented to the community in an open closeout workshop towards the end of the year.
