The Physics Beyond Colliders (PBC) initiative, launched in 2016, has submitted a summary report to the European Particle Physics Strategy Update (EPPSU). The projects considered under the PBC initiative will use the laboratory’s existing accelerator complex to broaden the experimental quest for new physics in different ranges of interaction strengths and particle masses. The reports documents a number of opportunities and emphasizes the complementarity of the proposed experiments with searches at the LHC and other existing or planned initiatives worldwide.

Despite the tremendous progress in fundamental physics with the completion of the particle content of the Standard Model a number of questions remains unanswered while the LHC experiments test the limits of the SM description. The discovery of the Higgs boson, whilst being a tremendous success for particle physics also represents a tremendous challenge as we still need to understand how it interacts with other particles and with itself. Moreover, certain astrophysical and cosmological evidence point to the existence of physics beyond the Standard Model (BSM) though we have few clues about how this physics could look like. There are two possible reasons for that. Perhaps any new particles are heavier and thus beyond the reach of the LHC, or new physics may be weakly coupled to the Standard Model and residing in what is often called a hidden sector; the latter is also motivated by our knowledge about dark matter and dark energy that must be very weakly coupled to the Standard Model.

The Physics Beyond Collider (PBC) study is exploring how CERN’s existing accelerator complex and the rich scientific infrastructure can be used for experiments that will complement the scientific programme of the LHC and possibly profit from developments for future colliders.

CERN’s North Area already provides beams for dark matter and precision searches while further pushing the exploration of the strong interaction through a number of QCD based experiments. A variety of options for new experiments and their technological readiness were considered and the PBC report identifies opportunities for new experiments – and the required resources - that will cover the full range of alternatives to energy frontier direct searches.

Foreseen proton-production capabilities of the complex. (Image credit: Giovanni Rumolo) 

The workhorse for high-energy fixed target experiments at CERN is the SPS. It provides a variety of beams of energies up to 400 GeV with high intensity and a high duty cycle to several different experiments in parallel.  Furthermore, there is a proposal for a new SPS Beam Dump Facility (BDF) that could serve both beam dump like and fixed target experiments.

The design of BDF was developed as part of the PBC’s mandate and is now ready to move on towards preparation for a technical design report. The BDF high-energy beam provides extended access to the high-mass range of the targeted region. In the first instance, exploitation is envisaged for the SHiP and TauFV experiments. The first will perform a comprehensive investigation of the hidden sector with discovery potential in the MeV-GeV range and TauFV will search for forbidden τ decays. TauFV has a leading potential for third generation lepton flavour violation decays (τ -> 3μ) thanks to the characteristics of the BDF beam.

The high-energy muon beam from the SPS could contribute to the explanation of the (g-2)μ via the proposed MUonE experiment which could directly measure one of the terms responsible for the theoretical uncertainty. Moreover, a high-energy muon beam will make a meaningful contribution to address the proton radius puzzle as part of the COMPASS programme.

Other options for a more diverse exploitation of the SPS beams have also been considered, including the proton driven plasma wakefield acceleration of electrons to a dark matter experiment (AWAKE++); the acceleration and slow extraction of electrons to light dark matter experiments (eSPS); and the production of well-calibrated neutrinos via a muon decay ring (nuSTORM).

PBC also considered a number of experiments that will allow the precision measurements of rare decays as they can indirectly probe higher energies compared to those directly accessible at the LHC. Two such experiments are the already operational NA62 and the planned KLEVER – both focusing on kaon decays. KLEVER’s motivation is to extend to neutral kaons the current NA62 measurement of ultra-rare charged kaon decays and will depend on NA62 results, the evolution of B-anomalies, and the KOTO competition in Japan. The two experiments are complementary to each other, as well as to experiments looking for B decays.

Furthermore, NA61 could measure the QCD parameters close to the expected critical regime, while upgrades to NA62 and a few months of operation in beam dump mode would explore an interesting domain of the hidden parameter space. In addition, the revival of the former DIRAC and NA60 concepts would also provide unique insights and could fit together in the underground hall currently occupied by NA62.

Coming to the LHC, fixed target studies are ongoing. LHCb is already well engaged in fixed target physics with SMOG and SMOG2. The gas storage cell approach has made significant advances beyond the pioneering LHCb SMOG program, and on the crystal front, there have been impressive results with beam, and a number of options are developed. LHCb and ALICE may focus on different physics signals due to different acceptances, data acquisition rates and operation modes. For both experiments, the physics reach will depend strongly on the feasibility of simultaneous Fixed Target and collision operation of the LHC. For ALICE there is a possibility of dedicated Fixed Target operation during LHC proton running.

Another interesting area of research concerns long-lived particles. PBC reviewed proposals for future experiments that can search for unstable particles with lifetimes much longer than what one would expect by estimating the decay rate from the mass of the decaying particle and currently there are several reasons that can provide parametrically longer lifetimes. Collider searches for BSM phenomena motivated by the problems of the SM have largely assumed that decays  of  new  particles  occur  quickly  enough  that  they  appear  prompt.   This expectation has impacted the design of the detectors, as well as the reconstruction and identification techniques and algorithms.  However, there are several mechanisms by which particles may be metastable or even stable, with decay lengths that are significantly larger than the spatial resolution of a detector at a collider, or larger than even the scale of the entire detector.  The impact of such mechanisms can be seen in the wide range of lifetimes of the particles of the SM. Thus, it is possible that BSM particles directly accessible to experimental study are long-lived, and that exploiting such signatures would discover them in collider data. Phenomenologically lifetimes greater than 10^-8 seconds and shorter than a few minutes are particularly interesting as they are less constrained by the LHC experiments and big bang nucleosynthesis and they can be the target of specific experiments considered. Two such experiments are FASER and MATHUSLA.

The Gamma Factory has also shown successful developments in 2018 as the team injected and accelerated partially stripped ions in the LHC. The goal is to produce gamma ray beams with a break-through in intensity by up to seven orders of magnitude, at very high γ-energies up to 400 MeV, compared to the current state of art for fundamental QED measurements, vector mesons, neutrons and radioactive ions. Following the first successful results, the team is now working towards a proof-of-principle experiment in the SPS.

Moreover, the existing EDM storage ring community, based mainly in Germany (JEDI) and Korea (srEDM), joined their efforts with a fledgling CERN effort under the PBC auspices to form a loose collaboration known as CPEDM, with the aim of converging towards a common design of an EDM storage ring and beyond. Searching for new sources of CP violation is one of the most pressing tasks in fundamental physics. Electric dipole moments are one of the most sensitive probes while the proton – with its unshielded monopole charge – is considered experimentally very challenging. However, it is an interesting target as a proton ring gives the prospect to measure proton and nucleon EDMs with a sensitivity level of 10^-29 e. cm, an attractive target at one or two orders of magnitude better precision than that of hadronic EDM experiments. Moreover, it will give the opportunity to search for oscillating EDMs that are a direct probe of dark matter consisting of axions or axion-like particles.

The projects with short term prospects at CERN (NA61, COMPASS(R_p), MUonE, NA62, NA64) have now been handed over to the SPSC Committee for detailed implementation review and recommendations.

Efforts have been made to provide some coherent leverage of CERN’s technological skills base to novel experiments. Experiment synergies were explored, leading to collaboration in applied technologies – in particular, technological synergies between light-shining-through walls and QED vacuum-birefringence measurements.

Finally, some PBC projects are likely to flourish outside CERN: the IAXO axion helioscope, now in consideration at DESY; the proton EDM ring, which could be prototyped at the Jülich laboratory, also in Germany; and the REDTOP experiment devoted to η meson rare decays, for which Fermilab in the US seems better suited.

The PBC mandate has been extended until May 2020 as a support to the EPPSU process: the design of the proposed long-term facilities will continue within the PBC accelerator working groups until the strategy guidelines are known. The PBC study group will also provide any additional input the EPPSU may need. The next years will offer exciting possibilities for novel experiments exploring fundamental physics and getting a glimpse at what lies beyond our current understanding.