The Standard Model of particle physics is currently the best description of the world we live in, but it offers no explanation of dark matter. However, no direct proof has been found in physics experiments yet. To address this challenge, novel experiments are needed to search for new particles with, which are assumed to couple very weakly to Standard Model particles, hence also known as feebly interacting particles (FIPs), by exploring energy ranges that are complementary to those at the Large Hadron Collider (LHC).
To carry out some of these experiments, CERN will use its Super Proton Synchrotron (SPS). This is the first time the SPS will be used at its full intensity potential since the CERN Neutrino to Gran Sasso (CNGS) experiment finished collecting data in 2012.Now, a new experiment will exploit the full intensity of the proton beam in the underground Experimental Cavern North 3 (ECN3), located in the SPS North Area, which was originally designed for high-intensity beams. The ECN3 facility currently hosts the NA62 programme, which studies rare kaon decays.
Two outstanding experimental proposals from the Physics Beyond Collider (PBC) Initiative at CERN were in consideration to be hosted at the ECN3: One of them is the Beam Dump Facility (BDF) with its associated Search for Hidden Particles (SHiP) detector. BDF and the associated SHiP will search for Hidden Sector Particles with the help of scattering and decay signatures. The detector system for scattering signatures is also ideal for neutrino physics.
The other proposal was the High Intensity Kaon Experiment (HIKE) with the off-axis Search for Hidden and Dark Objects With the SPS (SHADOWS) detector. HIKE proposed an extension of the current NA62 programme with charged kaons at higher intensity in the first phase and neutral kaons in a second phase. The experiment aimed to improve precision in measuring rare kaon decays, explore unobserved channels, and detect kaon decays forbidden by the Standard Model. SHADOWS, in turn, would have enhanced HIKE's ability to search for FIPs from kaon decays.
Both proposals underwent an extensive review by the Super Proton Synchroton Comittee and the Scientific Policy Committee, both attesting to the excellent quality of their respective physics cases. The committees were presented a challenging decision-making process, as both projects demonstrate a strong technical feasibility and align with the physics goals recommended in the 2020 update of the European Strategy for Particle Physics. Both proposals have unique strengths and characteristics, and they both have the potential to contribute significantly to the search for new physics, regardless of the final decision.
After evaluating all possible options and weighing all arguments, the CERN Research Board decided on 6 March to pursue the SHiP experiment, providing a significant boost to dark-matter searches in the North Area together with the NA64 experiment.
The accelerator-based search for FIPs is an essential activity in the hunt for dark matter and provides access to a particularly interesting kinematical domain. The sensitivity of SHiP to FIPs extends over one order of magnitude in mass into a regime not explored by any other accelerator-based experiment that can be realised within existing infrastructures in the upcoming decade. SHiP will also include a neutrino detector that will collect, among others, two orders of magnitude more tau neutrino interactions than any other experiment. Tau neutrinos are the protoype of FIPs within the Standard Model, as they are neutral, long-lived and couple to heavy-flavour particles. Only the proposed Forward Physics Facility at the LHC would be able to detect tau neutrinos at a comparable scale and higher energies in the foreseeable future.
The investment in a new experiment at the highest intensity ECN3 cavern is a clear and strong sign that CERN is focusing on the diversity of both on the high-energy and the intensity frontiers, and reaffirming its role as a world-leading laboratory.