Authors: Gianluigi Arduini, CERN, Kristiane Bernhard-Novotny, CERN, Joerg Jaeckel, University of Heidelberg, Gunar Schnell, UPV/EHU & Ikerbasque Bilbao, and Claude Vallée, CPPM-Marseille

The Physics Beyond Colliders (PBC) Study was launched in 2016 to explore the opportunities offered by CERN’s unique accelerator and experimental area complex and expertise to address some of the outstanding questions in particle physics through experiments complementary to the high-energy frontier. Together with the Large Hadron Collider (LHC) experiments, the PBC proposals form a synergistic partnership, which fosters an ecosystem beyond collider-based research and diversifies CERN’s science programme at the precision and intensity frontiers.

The fifth PBC annual workshop was held from 25 to 27 March at CERN to explore new ideas and avenues aiming to answer open questions of the Standard Model and beyond, and to provide updates of ongoing projects.

The Super Proton Synchrotron (SPS) North Area (NA) is one of the major fixed-target experimental facilities available at CERN and it is at the very heart of many present and proposed explorations for Beyond the Standard Model (BSM) physics. The NA includes an underground cavern (ECN3) for experiments requiring high-energy/high-intensity proton beams. Several proposals have been made for experiments to operate in ECN3 in the next decade and beyond. All of them require higher intensity proton beams than currently available. One of these proposals studied within PBC, SHiP (Search for Hidden Particles), aiming for a comprehensive investigation of the Hidden Sector in the GeV mass range at a dedicated Beam Dump Facility (BDF) [1], has been recently approved. Together with the activities of NA64, an experiment leading the searches for light dark particles with a versatile setup suited for electron [2], positron [3], muon [4] and hadron beams [5], this will significantly strengthen CERN’s focus towards dark-sector searches.

The FASER [6] and SND [7] experiments, now taking data at the LHC and originated in the first phase of the PBC initiative, contribute to both New Physics searches and to the study of very high-energy neutrinos. The proposed Forward Physics Facility (FPF), located in the line of sight of the interaction point 1 of the High Luminosity LHC (HL-LHC) 620 m away from it, could increase sensitivity to BSM physics by a factor of about 10,000 over FASER and it could allow for the detection of  thousands of neutrinos at TeV-energies per day with the potential of contributing to the measurement of parton-distribution functions with improved precision, benefitting the HL-LHC physics reach. The experiment consists of a series of sub-detectors of relatively small size. The FPF detectors’ layout definition and the corresponding integration studies have made significant progress as one of the main PBC-supported studies in view of the publication of a document describing the facility’s technical infrastructure by mid-2024.

proANUBIS [8], CODEX-beta [9] and MATHUSLA [10] are also actively being studied and would be located at large angles to the collision line of sight at the ATLAS, LHCb and CMS experiments.

Remaining in the realm of the Standard Model, a new NA60+[11] experiment with lead ions and NA61/SHINE[12] with light ions aim to uncover the onset of the Quantum Chromo Dynamics (QCD) phase transition at energy scales only accessible at the SPS, holding promise to decode the phases of nuclear matter in the non-perturbative regime of QCD. Understanding QCD means further to unravel the emergent properties of baryons and mesons. The AMBER [13] experiment plans to determine the charge radii of kaons and pions and to perform meson spectroscopy, in particular with kaons, within a wide range of experimental activities proposed beyond the next accelerator long Lshutdown (LS3). A substantial study has been carried out to enhance the number of identifiable kaons in the hadron beam delivered to AMBER. This could be achieved by improving the vacuum conditions and by the implementation of a dedicated optics in the beamline to the experiment.

To complement results obtained at AMBER’s predecessors COMPASS, HERA, and other experiments using a polarized beam and/or target, the LHCSpin collaboration  presented their proposal [14] to open a new frontier and to introduce spin physics at the LHC with a gaseous polarised target following the successful commissioning of the SMOG2 unpolarised-gas cell [15]. This would result in a new probe for studying collective phenomena at the LHC. Moreover, this would provide access to the multi-dimensional nucleon structure in a kinematic domain of hitherto limited exploration and make use of new probes, for instance by using charm mesons.

The TWOCRYST collaboration aims to demonstrate the feasibility and the performance of a possible fixed-target experiment in the LHC to measure electric and magnetic dipole moments (EDMs and MDMs) of charmed baryons [16], offering a complementary platform for the study of Charge-Parity (CP) violation in the Standard Model. These baryons would be generated by the collision of the protons of the secondary beam halo channelled by a crystal onto a target. MDM and EDM would be determined by measuring the baryon spin precession in the strong electric field of a crystal installed immediately downstream of the target.

The conceptual design of a beamline to produce a tagged neutrino beam to improve the precision of neutrino cross-section measurements has been developed combining the ENUBET [17] and NuTag [18] proposals. This design would significantly increase the amount of tagged neutrinos generated within a given geometric acceptance and energy band.

The Gamma Factory (GF) collaboration, which aims to demonstrate the principle of the Gamma Factory in the SPS, reported the progress achieved at IJCLab (France) in the development of the laser system required for this facility. The GF scheme is based on resonant excitation of ultra-relativistic partially stripped ions (that could be made available at the SPS and LHC) with a laser beam tuned to the atomic transition frequencies, followed by the process of spontaneous emission of photons. The resonant excitation of atomic levels of highly ionised atoms (ions) is possible due to the large energies of the ions generating a Doppler frequency boost of the counter-propagating laser beam photons by a factor of up to 2g, where g is the relativistic factor. Spontaneously-emitted photons produced in the direction of the ion beam, when seen in the laboratory frame, have their energy boosted by a further factor of 2g. As a consequence, the process of absorption and emission results in a frequency boost of the incoming photon of up to 4g ². In the GF scheme, the SPS (LHC) atomic beams play the role of photon “frequency converters” of eV-photons into keV (MeV) X-rays (γ-rays). These intense and quasi-monochromatic beams could be used in a variety of atomic, nuclear and particle physics experiments [19] and they could potentially find application to energy production or nuclear-waste transmutation as well as the generation of intense positron and muon beams for future accelerator facilities.

High quality factor superconducting radio-frequency cavities, similar to those used for the acceleration of charged particles in accelerators, can also be used to detect axions (hypothetical particles that might be able to explain both the strong CP violation problem and account for dark matter) and even gravitational waves, and they can also be of interest for developing multi-qubit systems. The design and fabrication of a superconducting cavity for the heterodyne detection of axion-like particles over a wide range of masses [20] is the subject of a joint project between PBC and the CERN Quantum Technology Initiative. Atom Interferometry is another subject of common interest between the two CERN initiatives and PBC has demonstrated the technical feasibility of installing an atom interferometer with a baseline of 100 m in one of the LHC access shafts [21].

The charged-particle EDM collaboration presented the status of their approach to build a prototype ring that would validate the main concepts of a ring required to perform the first direct measurement of a proton EDM [22] and evaluate the sensitivity reach of such measurement.

The proposed injectors of the Future Circular electron-positron Collider (FCC-ee) [23] will significantly expand the variety of the offer of the CERN accelerator complex in terms of beam types and parameters, potentially opening up the possibility of new experiments. New ideas have been also presented, ranging from the measurement of molecular EDMs at the ISOLDE (Isotope Separator On Line DEvice) Radioactive Ion Beam Facility, over the prospects for antiproton physics at the Antiproton Decelerator (AD) and the Extra Low ENergy Antiproton (ELENA) ring, to the measurement of the gravitational effect of the LHC beam.

With these highlights in stock, many fruitful discussions, the annual workshop concluded as a resounding success. The PBC community thanked Claude Vallée (CPPM, Marseille), who retired as PBC co-coordinator and co-founder of the PBC initiative, after almost a decade of integral work, and welcomed Gunar Schnell (UPV/EHU & Ikerbasque, Bilbao) who will take on this role.

A small part of the community who contributes with lively discussions and innovative proposals and projects to the success of PBC.
Credit: K. Bernhard-Novotny (CERN)

[1] SHiP Collaboration, BDF/SHiP at the ECN3 high-intensity beam facility, CERN-SPSC-2022-032 ; SPSC-I-258

[2] Yu. M. Adreev et al. , Search for Light Dark Matter with NA64 at CERN,     Phys.Rev.Lett. 131 (2023) 16, 161801

[3] Yu. M. Adreev et al. , Probing light dark matter with positron beams at NA64,     Phys.Rev.D 109 (2024) 3, L031103

[4] Yu. M. Adreev et al. , Exploration of the Muon g−2 and Light Dark Matter explanations in NA64 with the CERN SPS high energy muon beam, arxiv:2401.01708 ; accepted by PRL

[5] S. Gninenko et al., Test of vector portal with dark fermions in the charge-exchange reactions in the NA64 experiment at CERN SPS, arxiv:2312.01703

[6] H. Abreu et al., First Direct Observation of Collider Neutrinos with FASER at the LHC, Phys.Rev.Lett. 131 (2023) 3, 031801

[7] R Albanese et al., Observation of Collider Muon Neutrinos with the SND@LHC Experiment, Phys.Rev.Lett. 131 (2023) 3, 031802

[8] A Shah et al., Searches for long-lived particles with the ANUBIS experiment, PoS EPS-HEP2023 (2024) 051 / A Shah et al., Installation of proANUBIS – a proof-of-concept demonstrator for the ANUBIS experiment, PoS LHCP2023 (2024) 168

[9] C Aielli et al., The Road Ahead for CODEX-b, arXiv:203.07316

[10] C Alpigani et al., An Update to the Letter of Intent for MATHUSLA: Search for Long-Lived Particles at the HL-LHC, arXiv:2009.01693

[11] NA60+ Collaboration, Letter of Intent: the NA60+ experiment, CERN-SPSC-2022-036; SPSC-I-259, Geneva, 2022, https://cds.cern.ch/record/2845241

[12] NA61/SHINE Collaboration, Addendum to the NA61/SHINE Proposal: A Low-Energy Beamline at the SPS H2, CERN-SPSC-2021-028 / SPSC-P-330-ADD-12, Geneva 2021, https://cds.cern.ch/record/2783037/files/SPSC-P-330-ADD-12.pdf

[13] C Quintas et al., The New AMBER Experiment at the CERN SPS, Few Body Syst. 63 (2022) 4, 72

[14] P. Di Nezza et al., The LHCspin Project, Acta Phys.Polon.Supp. 16 (2023) 7, 7-A4

[15] C. Boscolo Meneguolo, et al., Study of beam-gas interactions at the LHC for the Physics Beyond Colliders fixed-target study, JACoW proceedings (2019)

[16] S. Aiola et al., Progress towards the first measurement of charm baryon dipole

moments, Phys. Rev. D 103, 072003 (2021).

[17] F Acerbi et al., Design and performance of the ENUBET monitored neutrino beam, Eur.Phys.J.C 83 (2023) 10, 964

[18] A Baratto-Roldan et al., NuTag: proof-of-concept study for a long-baseline neutrino beam, arXiv:2401.17068

[19] D. Budker, M. Gorchtein, M. W. Krasny, A. Pálffy, A. Surzhykov (editors), Physics Opportunities with the Gamma Factory, Annalen der Physik, Volume 534, Issue 3 (2022)

[20] A Berlin et al., Heterodyne Broadband Detection of Axion Dark Matter, Phys. Rev. D 104, L111701

[21] G. Arduini et al., A Long-Baseline Atom Interferometer at CERN: Conceptual Feasibility Study, arXiv:2304.00614", CERN-PBC-REPORT-2023-002, Geneva, 2023, https://cds.cern.ch/record/2851946

[22] F. Abusaif, et al., Storage ring to search for electric dipole moments of charged particles: Feasibility study, CERN Yellow Reports: Monographs, CERN-2021-003, Geneva, 2021, https://cds.cern.ch/record/2654645, doi=10.23731/CYRM-2021-003

[23] M. Benedikt et al. (editors), Future Circular Collider Study. Volume 2: The Lepton Collider (FCC-ee) Conceptual Design Report, CERN-ACC-2018-0057, Geneva, December 2018. Published in Eur. Phys. J. ST.