The Search for Hidden Particles (SHiP) experiment with the associated Beam Dump Facility (BDF), currently under consideration for implementation in the SPS ECN3 beam facility, is designed to perform a generic and exhaustive search for feebly interacting particles (FIPs) in a region of mass and coupling that is only accessible with a dedicated beam-dump configuration. “The high intensity and the high energy of the SPS, combined with the forward boost of FIP  production and decays, makes the full-acceptance beam-dump configuration the most suitable setup to generically explore the “coupling frontier” in the MeV to multiple-GeV mass range”, explains Richard Jacobsson, SHiP’s project leader/technical coordinator and co-convenor of BDF.

SHiP’s history started back in 2013 with the submission of an Expression of Interest (EoI) for a new experimental facility to the Super Proton Synchrotron Committee (SPSC). The main motivation was to develop a general-purpose intensity-frontier experimental facility operating in beam-dump mode at the CERN SPS accelerator to search for feebly interacting particles and to perform measurements in neutrino physics that cannot be done at conventional neutrino facilities. Alongside this baseline physics motivation, one of SHiP’s underpinning goals was to develop ideas around the full physics potential of the 4 × 10¹⁹ protons per year at 400 GeV. This has also led to several extensions and upgrades being considered.

Following a favorable decision by the SPSC in 2014, the members of the SHiP Collaboration started working towards a Technical Proposal on the detectors and the proposed SPS experimental facility. The successful review of the technical proposal in 2015, paved the way for the SHiP collaboration to move forward with Comprehensive Design Studies (CDS) of the detector as well of the beam dump facility (BDF), in the framework of the Physics Beyond Collider initiative, resulting in the birth of the BDF/SHiP project. The project proposal was successfully submitted to the 2020 European Particle Physics Strategy Update.

Andrey Golutvin, SHiP’s spokesperson explains: “In the last update of the European Strategy for Particle Physics, the breadth and the complementarity of the physics programme as well as the maturity of the detector and the proposed technologies were strongly recognised. The suitability of the SPS was also deemed unique to host the experimental facility. Although we met these three criteria, the project could not, as of 2020, be recommended for construction considering the budget constraints associated with the overall recommendations of the Strategy”. As a result, the team working on the design of the beam dump facility was mandated to review the implementation and the cost by exploring the possibility to host the project in one of CERN’s existing facilities. This effort was accompanied by a re-optimisation of the detector layout and key components by the SHiP collaboration. It is worth noting that a first location survey had already been performed in the framework of the original Expression of Interest, considering four locations, namely TT61, TCC4, TNC, and ECN3. The original survey concluded with the need for a new facility as TT61 was found unsuitable and as the BDF/SHiP project timeline would be interfering with the programmes that had been foreseen in the other facilities.  The development of the concepts for the new facility followed a strategy that makes a large part of the studies generic to other locations around the SPS.

The renewed effort of the BDF/SHiP collaboration in 2021 initially focused on a broad location study, again involving TCC4, TNC and ECN3. The primary goal of the experiment re-optimisation was to identify an alternative working point for the components involved in the suppression of background that allows preserving the original physics scope and physics reach of the facility while reducing in size the overall space required for the experiment, allowing integration into the alternative areas with only limited modifications of the infrastructure. The result of this location and layout optimisation study identified ECN3 as the most suitable and cost-effective option. 

Last year, the decision of the CERN management to review the post-LS3 physics programme in ECN3, prompted the BDF/SHiP collaboration to further pursue the studies of the facility and of the SHiP detector aimed at ECN3, and to verify, by full simulation, the physics performance. Jacobsson notes “in July 2022, CERN launched a decision process involving an in-depth assessment of the different initiatives proposing new physics programmes in ECN3 to start after Run 3”.

Discussing the Physics Potential of SHIP

The New Physics required to explain the fundamental mysteries of the Standard Model such as the flavour structure, matter-antimatter asymmetry, neutrino mass, etc may be associated with high energy scales, meaning it has escaped detection up to now by being too heavy to have been observed. At the same time, it appears that the Universe is also endowed with a sector in the form of “dark matter” that is not interacting at all, or at least hardly, with the Standard Model particles. This is  one way to approach the intriguing possibility that the New Physics is not necessarily heavy, but instead so feebly coupled that it has escaped detection because of limited luminosity at current facilities. Considering such options has brought a whole new spectrum of alternative solutions to the issues of the Standard Model that are within reach of experimental exploration.  Although the energy at the SPS is limited, bombarding a specialised long and high-A/Z target with the 4x10^19 protons per year would allow collecting integrated luminosities that are more than three orders of magnitude higher than at HL-LHC. This gives access to unprecedented yields of a wide variety of particles, e.g. SHiP would annually record O(10¹⁷) charm decays, O(10¹²) beauty decays, 10²⁰ photons over 100 MeV in the acceptance of the SHiP detector, that are known to be possible sources of feebly interacting particles. 

A common property of FIPs is that the distance they travel before decay grows fast with weaker coupling and when their mass is smaller. “As a result, the low-mass region of the parameter space, in the range of few GeVs, remains practically inaccessible to collider experiments. Beam-dump experiments, like the proposed SHiP, can cover this region. Jacobsson adds “In the beam-dump configuration, the production and decay is very much forward focused and the decay volume length may be made very long. It can easily be several tens of meters to cover a much larger lifetime acceptance than at collider detectors. Moreover, we explore a number of technical solutions to efficiently reduce backgrounds by placing the decay volume behind a chain of components designed to suppress the beam-induced particle backgrounds, such as a hadron absorber and a muon deflector”. This considerably reduces the need to impose strict signal selection criteria, which in turn guarantees a search that is independent of particular models.

In this way, BDF/SHiP complements the world-wide program of New Physics searches by exploring a large region of parameter space which cannot be addressed by other experiments, and which reaches several orders of magnitude below existing bounds. The SHiP detector is sensitive both to decay and scattering signatures for a wide variety of models predicting feebly interacting particles (FIPs) including heavy neutral leptons, dark photons, dark scalars, axion-like particles, light dark matter, but also feebly interacting particles originating from models involving high-energy scales such as SUSY. FIPs can play a direct role in BSM phenomena, like e.g. heavy neutral leptons or HNLs in the sub-electroweak mass range that could explain neutrino masses via the see-saw mechanism, as well as the matter-antimatter asymmetry via their out-of-equilibrium kinetics in the early Universe at a temperature above 100 GeV. FIPs can also be a ”portal” that connects the SM sector with a dark sector, e.g. as in the case in which the dark sector particles only interact with ordinary matter via the FIP as mediator. “This coupling frontier of particle physics is constituted by a whole class of SM extensions” explains Jacobsson.

The purpose of BDF/SHiP is to make a break-through in this direction during the next 10-15 years, in parallel with the HL-LHC programme. In case of discovery, SHiP’s sensitivity allows not only to establish the existence of a new particle but also identify its properties such as branching ratios of various decay channels, precise mass, etc in a large region of the unexplored parameter space. “It’s difficult to overestimate the impact a discovery would have on the future directions of the field”, explains Nico Serra, SHiP’s physics coordinator. If nothing is discovered, SHiP would set the stage for the future colliders that could drastically advance the intensity frontier, as increasing collision energies also implies increasing the number of W, Z, Higgs bosons and heavy flavour mesons — particles that can also produce FIPs in their decays. SHiP would also provide invaluable insights in interpreting astrophysics and cosmological data that will arrive in the coming decade. The data from cosmic and ground-based telescopes such as GAIA, DESI, EUC- LID, Vera Rubin Observatory, E-ELT, JWST and later SKA might bring significant progress in the understanding of the properties of dark matter particles. “If these planned telescopes discover a self-interacting feature of dark matter, then SHIP could offer further insights, even characterization of particle properties.” notes Serra and adds “Combining this with the discoveries or limits on interactions between hidden sectors and the SM sector that can be obtained at BDF/SHiP becomes a powerful tool for guiding the future strategies and technology developments in both fields”.

Finally, BDF/SHiP can also contribute to neutrino physics by enabling unprecedented measurements with tau neutrinos and neutrino-induced charm production. The BDF/SHiP target system would  also give unique access to a high-intensity neutron spectrum that is not easily accessible at spallation facilities. This makes it possible to implement a user platform for studying neutron-induced reactions on short-lived isotopes that is relevant for astrophysics and nuclear physics, as well as material testing and radiation-to-electronics (R2E) studies. 

Pushing the technological boundaries 

The proposed setup of the BDF/SHiP facility is shown below. It consists of a high-density proton target located in the target bunker, followed by a hadron stopper and a muon shield. The proton target system is one of the most critical components of BDF/SHiP, presenting a number of outstanding challenges. While the target must be highly optimised for the physics signal over background, it has to sustain the 2.6 MJ of protons impinging on it every 7.2s second. “The safety, reliability, and maintainability aspects put the design of the target and the target complex at the forefront of technology and strong synergy with what is also required for neutrino facilities and spallation sources”, explains Jacobsson.

To control the beam-induced background from muons, the flux in the detector acceptance must be reduced from O(10¹¹)Hz (at > 1GeV) to less than O(10⁵ )Hz. Despite the aim to cover the long lifetimes associated with the FIPs, the detector volume should be situated as close as possible to the proton target to ensure optimal acceptance. Hence, the muon flux should be reduced over the shortest possible distance. To this end, an active muon shield entirely based on magnetic deflection has been developed. As Golutvin explains: “We are working on designing a muon shield where the field map is derived from machine learning that is fed a very large sample of muons from simulation. The simulation has been tuned with a real spectrum measured with a prototype of the BDF/SHiP proton target at the SPS. This technique is something that many experiments in the future could profit from”. To further shorten the muon shield, SHiP is now also studying a superconducting magnet to replace a part of the resistive magnet system constituting the original muon shield design. 

In the years since the Comprehensive Design Study, the SHIP team has been collaborating with TE-MSC on a new type of coil for the spectrometer magnets that could significantly reduce the power consumption of such large-aperture magnets. The concept would be beneficial for other future experiments as well as potentially allowing retro-fitting existing resistive magnets for energy efficiency. 

Another key challenge, among several, for the SHiP detector is the electromagnetic calorimetry with requirements on timing and the need to reconstruct the shower axis of photons in order to reconstruct axion-like particles decaying to two ordinary photons. To address this challenge, the collaboration has developed a calorimeter configuration incorporating high-precision tracking layers at different depths of the electromagnetic showers. 

The BDF/SHiP collaboration is also pursuing the use of nuclear emulsion combined with SciFi target trackers in a high-occupancy environment for reconstructing tau neutrino interactions and light dark matter scattering in the SHiP neutrino target based on tungsten. “Although it may sounds like a technology of the past, the micrometric resolution of the nuclear emulsion in 3D is unsurpassed. The high resolution makes emulsion a unique tool for reconstructing tau neutrinos in the tungsten layers”, explains Antonia Di Crescenzo, SHiP’s deputy physics coordinator: “Automated microscopes, improved in resolution and speed, convert the information in the emulsion to electronic data to be used in the global reconstruction as for any vertex detector”. The whole concept of SHiP’s scattering and neutrino detector is today successfully operated at the LHC by the SND@LHC collaboration. At the same time the two collaborations are jointly looking at the possibility of replacing some of the nuclear emulsion films with silicon detectors in the future.

Schematic layout of the Scattering and Neutrino Detector (SND) and Hidden Sector Decay Spectrometer (HSDS).

Building an international collaboration and steps ahead

The BDF/SHiP project has attracted significant attention over the years and contributions from a large number of groups in various countries in Europe and Asia, that make up the collaboration since 2014”, says Jacobsson. Although all subsystems of BDF/SHiP have involvement of experts in the fields, new groups are invited to fill all aspects of the development and the construction of the detector. The facility is also expecting to see significant interest from around the world in international collaboration on the target, target complex and magnets. For Richard Jacobsson “The construction of BDF/SHiP is an important undertaking, lying at a scale between NA62 and LHCb in size and complexity. This means that the project will offer opportunities for groups to apply technologies and maintain the expertise gained from the upgrades of the LHC experiments, as well as build on the current R&D activities for future detectors, in order to bridge the gap up to the next large collider”. This need clearly emerged during the EPPSU discussions in Granada where many colleagues expressed their interest in a medium-size and medium-term project.

In continuing building the collaboration, SHiP’s main goal of this year is to raise interest in the ECN3 decision process throughout the community and in the discussions on the future physics programmes in CERN’s North Area. This will allow identifying the groups that are interested to join the project after the approval process to secure all parts of the implementation of the detector.

Currently the collaboration is busy preparing a proposal document related to the adaptation of BDF/SHiP in ECN3, requested by the SPSC as part of the decision process. For this document SPSC has asked the collaboration to elaborate on the physics prospects with 10-15 years of operation. “This is even within the period during which BDF/SHiP can continue pushing the exploration without saturating the sensitivity from the expected background level”, explains Serra. The document will also include a chapter on possible future extensions and upgrades of the facility that can be implemented without affecting the principal goal of BDF/SHiP. “Some of these extensions can be implemented immediately at the start of the facility but there are others that could be implemented during future long-shutdowns by other collaborations”. 

As for steps beyond the approval process Golutvin notes: “Given adequate funding, we believe that the TDR phase for BDF/SHiP will take us about three-four years, followed by production and construction, and aim for commissioning the facility towards the end of 2030, and real data taking from 2031. This will allow two years of data-taking during Run 4, before the start of Long Shutdown 4”, which would be the obvious opportunity to improve or consolidate, if necessary following the experience of the first years of data taking. 

The work carried out by the SHIP collaboration not only constitutes an important contribution to the full physics exploitation of the CERN’s injector complex, but are of strategic interest for future particle experiments and will benefit the high-energy physics community at large by complementing the research programme of intensity and energy frontier colliders.