December 2018

The Physics Beyond Colliders Study at CERN

Jörg Jäckel, Mike Lamont and Claude Vallée *

PBC coordinators and contacts to the European Strategy Group

AbstractThe Physics Beyond Colliders (PBC) study was mandated by the CERN Man-agement in 2016 in order to prepare the next update of the European Strat-egy for Particle Physics. The present document reproduces the PBC mandate,presents the organization of the PBC activities and quotes the executive sum-mary of the PBC summary report. The full summary report is appended asaddendum.

*e-mails: jjaeckel@thphys.uni-heidelberg.de, Mike.Lamont@cern.ch, vallee@cppm.in2p3.fr

1 PBC Mandate

The initial PBC mandate given by the CERN Management early 2016 was updated in September 2018in order to extend the PBC activities until the end of the European Strategy update process. The currentmandate reads as the following.

CERN Management wishes to launch an exploratory study aimed at exploiting the full scientificpotential of its accelerator complex and other scientific infrastructure through projects complementaryto the LHC and HL-LHC and to possible future colliders (HE-LHC, CLIC, FCC). These projects wouldtarget fundamental physics questions that are similar in spirit to those addressed by high-energy colliders,but that require different types of beams and experiments.

This study should provide input for the future of CERN’s scientific diversity programme, whichtoday consists of several facilities and experiments at the Booster, PS and SPS, over the period until 2040.Complementarity with similar initiatives elsewhere in the world should be sought, so as to optimize theresources of the discipline globally, create synergies with other laboratories and intitutions, and attractthe international community.

Scientific goal

The main goal of the Study Group is to explore the opportunities offered by the CERN accelerator com-plex to address some of today’s outstanding questions in particle physics through experiments comple-mentary to high-energy colliders and other initiatives in the world. These experiments would typically:(i) enrich and diversify the CERN scientific program, (ii) exploit the unique opportunities offered byCERN’s accelerator complex and scientific infrastructure, (iii) complement the laboratory’s collider pro-gramme (LHC, HL-LHC and possible future colliders). Examples of physics objectives include searchesfor rare processes and very-weakly interacting particles, measurements of electric dipole moments, etc.

Structure of the Study Group and deliverables

The group will be led by three coordinators representing the scientific communities of accelerator, ex-perimental, and theoretical particle physics: Joerg Jaeckel (Heidelberg), Mike Lamont (CERN), ClaudeVallée (CPPM, Marseille).

Following consultation with the relevant communities, they will define the structure and the mainactivities of the group and appoint convenors of thematic working groups as needed. They will call akick-off meeting in 2016, organize regular meetings, and monitor the overall scientific activity. Thescientific findings will be collected in a report to be delivered by the end of 2018. This document willalso serve as input to the next update of the European Strategy for Particle Physics.

It is expected that the group will continue its activity throughout the ESPP process, which willbe completed in May 2020, so as to follow up the development of the various studies and provide anyadditional input the ESPP may need.

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2 PBC Process

As introduction to the starting point of PBC an overview of the current CERN accelerator complex isshown in figure 1. The LHC injectors are hosting a vigorous particle physics program gathering arounda thousand physicists within 2 dozens of experiments. The main evolutions in the past decade were:

– the successful completion of the CNGS neutrino beam program and of the associated OPERA andICARUS experiments at Gran Sasso;

– the end of the DIRAC experiment at the PS, and the steady continuation of the NA61 and COM-PASS programs devoted to QCD;

– the successful development of the antimatter factory including breakthroughs in precision mea-surements of antiprotons and antihydrogen atoms;

– the start of operation of the NA62 and NA64 experiments devoted to BSM searches;– the completion of the baseline programs of the non-accelerator experiments CAST and OSQAR.

In order to prepare the future of these fields of research the PBC study group has organized twosurveys of the community around its kickoff workshop of September 2016 and the follow-up of Novem-ber 2017, and has structured its activities accordingly.

Fig. 1: Overview of the CERN accelerator complex circa 2018

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2.1 Community surveys

The community surveys performed by the PBC study group collected a wealth of suggestions on howto further exploit the CERN complex in the future. An overview of the initial ideas is presented here.Projects originating from past or present Collaborations are given their initial experiment name to whicha generic "++" tag is added.

Most of the ongoing experiments have proposed detector upgrades and extensions of their datataking beyond the currently approved programs: as regards QCD measurements, COMPASS++ wishesto further operate as a long term QCD facility devoted to hadron structure, spin and spectroscopy, witha focus on hadron beams including higher intensity antiproton and kaon RF-separated beams; NA61++plans to further study the QCD phase transition with open charm production; as regards searches NA62++plans part-time operation in beam-dump mode to investigate the hidden sector, and NA64++ wants tooperate at higher intensity with electrons, as well as to extend its searches of dark vectors to muon andhadron beams in order to explore alternative BSM models.

New experiments have also been proposed with conventional (upgraded) beams: A new idea,MUonE, is proposed to measure the hadronic vacuum polarization (the dominant theory uncertainty onthe muon g-2 prediction) in the t-channel with a high energy muon beam, complementary to the e+e−

method used up to now. NA60++ plans to revive the NA60 concept to explore the QCD phase diagramthrough an energy scan of low-E dimuon production in Pb-Pb collisions. DIRAC++ wishes to install aDIRAC-like detector on a SPS proton beam, which would boost the production of mesonic atoms by afactor 20 compared to the previous DIRAC location at the PS. For searches the KLEVER project aims toextend the NA62 investigations of ultra-rareK+ decays to ultra-rareK0 decays using a high intensityK0

beam, whereas REDTOP is motivated to investigate ultra-rare η decays using a high intensity continuouslow energy proton beam.

New Fixed Target facilities based on the SPS are considered. The Beam Dump Facility (BDF)based on a high intensity, high energy and slow extracted SPS proton beam, is proposed as a new gen-eral purpose high intensity facility: within the BDF, the SHiP detector would perform a comprehensiveinvestigation of the hidden sector, and a small fraction of the upstream beam is planned to be used by theTauFV experiment to investigate forbidden τ decays. Complementary to the BDF, an extension of theAWAKE set-up could provide a high intensity medium energy pulsed electron beam for investigation ofthe hidden sector with a detector located in the former CNGS decay tunnel. A primary high intensity/lowenergy electron beam could also be delivered by the SPS implementing a new e-injector based on CLICtechnology (project eSPS).

Fixed Target physics is also proposed to be developed at the LHC using internal targets locatednear the LHCb or ALICE collision points, as was pioneered by LHCb with the SMOG system. Thisopens a unique new kinematical domain to proton and heavy ion measurements, primarily for QCDphysics. Several methods based on internal gas targets, including polarisation options, as well as onbeam halo crystal extraction using UA9 developments, are being explored.

Other new long term facilities have been proposed within the PBC study. One highlight is anelectrostatic storage ring for a high precision measurement of the proton electric dipole moment. Thefeasibility and motivation of a "γ-factory", a very high intensity γ-beam produced by conversion of alaser beam through partially stripped ions stored in the LHC, has been studied. A CERN implementationof nuSTORM, a high intensity ν beam based on a muon storage ring, is also proposed.

Finally, several groups have proposed to further exploit the synergies between CERN technologicalexpertise and projects developed in external institutes for searches and precision measurements. Somehighlights include the CERN contribution to the design of the large scale magnet of IAXO, the nextgeneration axion helioscope considered as a successor of CAST, and the potential use of high fieldmagnets developed for HL-LHC and the FCC in other domains.

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2.2 Organization of the PBC studies

Following the community surveys, working groups have been set-up to pursue studies in the areas ofinterest. They involve around a hundred of contributors including CERN accelerator experts, project pro-ponents and independent physicists. The PBC working groups (figure 2) are structured along acceleratorworking groups focused on technical studies of the required beams and facilities, and physics-orientedgroups investigating the physics reach of the projects in the worldwide landscape.

Fig. 2: Structure of the PBC working groups

The PBC working groups meet regularly with a total score of more than 200 meetings up to now.Two general working group meetings have been scheduled in March 2017 and June 2018 for globalreviews of the studies. As a follow-up of the kickoff workshop of September 2016, two PBC publicworkshops have been organized in November 2017 and January 2019 to inform the community about theprogress. The PBC activities can be continuously monitored on the public web site pbc.web.cern.ch.

The studies of the past two years are reported in a summary report [1] based on detailed documentsof the PBC working groups [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]. The full summary report is appended asaddendum and its executive summary reproduced in the next section.

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3 Executive summary of the PBC summary report

CERN operates the only high-energy frontier collider worldwide, the LHC, and hosts a vigorous particlephysics program on the LHC injector complex with a thousand of physicists involved. Communitysurveys performed to prepare the future of this program have identified a wealth of projects covering abroad range of physics domains. The PBC studies address the suitability of the CERN complex to hostthe projects, their general relevance in the physics landscape, their competitiveness versus other projectsworldwide, as well as their implementation issues.

CERN complex specificities and opportunities

The LHC injectors (including, e.g. SPS, PS) form a very flexible complex serving diverse communitieswith both low- and high-energy beams to address a broad physics spectrum.

Low energy facilities: The low energy part (< 26 GeV) of the complex has a lower powerand duty cycle than high intensity frontier machines such as FNAL or J-PARC, but hosts three uniquefacilities worldwide: the ISOLDE radioactive ion beams, the n_TOF pulsed neutron beam, and the verylow energy antiproton beam of the Antimatter Factory. These facilities were not considered within PBCbecause recent upgrades (HIE-ISOLDE, n_TOF’s 2nd experimental area, ELENA) secure their future forat least the next decade. The hosted experiments have a unique fundamental physics reach and theirshort evolution time scales may generate promising new ideas on short notice. It is therefore importantto maintain adequate flexibility in the usage of these facilities.

A low-energy storage ring had previously been proposed to improve the measurements of theproton and deuteron EDM. CERN joined with the proponents within a new CPEDM collaboration inorder to consolidate the ring design. Systematic effects received close attention and results suggest thata prototype ring be exploited before the full-size ring can be built. The prototyping could be done in afacility associated to a CPEDM institute, e.g. COSY in Jülich.

High energy Fixed Target: The work-horse of high energy Fixed Target physics at CERN isthe SPS, which provides a worldwide unique combination of high energy up to 400 GeV, high intensityand high duty cycle. The SPS can serve many users in parallel with slow or fast extraction and highflexibility. All kinds of particles including ions are available.

Completion of the CNGS neutrino beam program in 2012, together with injector upgrades for HL-LHC, leaves room for a new high intensity facility at CERN as regards proton yield. This opportunitymotivated the proposal of the Beam Dump Facility (BDF) to exploit unique possibilities to enter a newera of beam-dump/fixed-target physics experiments at the intensity frontier. The design of the BDF hasbeen consolidated within PBC and is now ready for further studies towards a technical design report.The technically driven baseline schedule would allow start of operation after LS3.

The ongoing CERN accelerator R&D offers opportunities for other new facilities at the SPS onthe long term. Two options were proposed and studied within PBC: a new primary high intensity e-beam(eSPS) based on a CLIC linac, SPS acceleration up to 16 GeV and slow extraction; and a pulsed e-beamof up to 50 GeV produced in the former CNGS target area with the AWAKE plasma acceleration method.

Other opportunities: Promising feasibility studies of a gamma factory, a novel concept ex-ploiting the unique LHC high energy to produce a high intensity γ-ray beam, have been performed. Thepossible layout of a nuSTORM implementation at CERN has also been studied.

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CERN physics potential beyond colliders

There is growing interest for BSM models involving either new particles with low masses or new forceswith very low effective couplings. This stems from the non-observation of new high-mass states at theLHC up to now and, more importantly, from the realization that these new forms of BSM physics are wellmotivated by both theory and phenomenology. The PBC projects cover the full range of alternatives tohigh-energy frontier direct searches: precision and rare decay experiments, beam-dump experiments andnon-accelerator low-mass axion searches. In addition a number of PBC projects are devoted to specificQCD measurements.

Precision measurements and rare decays: Precision measurements and rare decays probehigher masses than accessible with LHC direct searches, through the contribution of loop diagrams. TherareK decays to be investigated by NA62 (K+ → π+νν̄) and KLEVER (K0

L → π0νν̄) are complemen-tary to each other and to B decays for BSM searches, and are accessible in a novel in-flight techniquethanks to unique high-intensity/high-energy CERN hadron beams. TauFV@BDF has also a leading po-tential for 3rd-generation LFV decays (τ → 3µ) thanks to the unique characteristics of the BDF beam.REDTOP devoted to η rare decays could have access to complementary BSM models provided sufficienthigh luminosity can be collected. The unique high energy µ-beam also offers an opportunity for a CERNcontribution to the exploitation of the (g-2)µ experiments, with a direct measurement by MUonE of theterm responsible for the dominant theory uncertainty.

High energy beam dumps: Beam dump like experiments probe a specific MeV-GeV mass rangeof the hidden sector parameter space of special interest to solve open questions in cosmology. The PBCprojects make full use of the CERN opportunities with e-beams (NA64++(e) and LDMX@eSPS), µ-beams (NA64++(µ)) and proton beams (NA62++ and SHiP@BDF). The BDF high energy beam providesextended access to the high-mass range of the targeted region. Using both appearance and disappearancesignatures, as well as different types of particle beam, maximizes sensitivity in couplings and masses.Hidden sector benchmark models have been defined to compare the reach of the PBC projects with eachother and with the worldwide competition including dedicated long-lived particle projects at the LHC.

Low energy hidden sector: In line with past CERN non-accelerator experiments, new PBCprojects propose to further explore axion models with a unique reach in the sub-eV range (IAXO helio-scope and JURA light regeneration experiment), motivated by the QCD axion as well as astrophysicalhints. The EDM ring could also potentially probe the very low mass region with oscillating EDMs.

QCD measurements: The growing number of QCD-oriented facilities across the world, eitherin operation (JLab, RHIC, J-PARC Hadron Experimental Facility), in construction (NICA, FAIR), orin discussion (EIC) reduces the windows of opportunities for competitive measurements at CERN inthe future, but several unique cases were identified within the PBC projects for both hadron and QGPmeasurements.

At the SPS: The CERN high-energy µ-beam provides an opportunity for a meaningful contri-bution to the proton radius puzzle by the COMPASS++(Rp) program. Chiral QCD could be tested inthe full SU(3) sector with DIRAC++ mesonic πK atoms and a COMPASS++ K polarisability measure-ment. COMPASS++ could also perform unique pion structure measurements with DY and revisit strangespectroscopy with high statistics. The SPS kinematic domain gives a unique access to the QGP phasetransition region where a Critical Point is expected. This is proposed to be revisited by NA61++ withopen charm and by NA60++ with low-E dimuons.

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At the LHC: There is a unique kinematic domain that can be explored by LHC Fixed Targetmeasurements. Precision quark/gluon high-x PDFs, a crucial input to exploit HL-LHC for high-massnew particle searches, as well as spin asymmetries, could be extracted. The LHC-FT kinematics alsocorresponds to the QGP cross-over between the low-µB LHC/RHIC region and the SPS Critical Pointregion, where high statistics measurements would be important to consolidate predictions for high-µB .

PBC projects competitiveness and implementation

Short-term opportunities: There are significant discovery potentials of NA64++(e, µ) and ofNA62++ in beam dump mode which are unrivaled on the Run 3 timescale. The NA64++(e) efficiencyshould be maximized to cover as much as possible of the DM-favored region. The NA62++ beam dumpoperation will also provide valuable input to finalize the SHiP design.

MUonE, COMPASS++(Rp) and NA64++(µ) all have unique motivations but are in competitionon the same µ-beam during Run 3. The feasibility and achievable precision of the projects still needquantification. Anticipating positive outcomes, optimal use of the high-intensity µ-beam requires tostrengthen the studies to fit the programs together. The open charm measurement planned by NA61++in the QCD Critical Point region is unique and has no new competition expected on the beam line.

Long-term opportunities: The PBC benchmark models comparisons show that SHiP@BDFhas a unique discovery potential compared to the worldwide competition in the next two decades. In-cluding a small upstream experimental hall in the BDF baseline design, as proposed by TauFV, wouldincrease the BDF potential by paving the way to a long term rare decay facility with a reach exceedingBELLE-II for the 3rd flavor generation.

AWAKE++ and eSPS offer attractive options of new high intensity e-beams to go much beyondthe reach of current CERN e-beams for dark photon searches in visible and invisible modes, respectively.LDMX proposed on eSPS has a shorter term implementation opportunity at SLAC, pending approval ofLCLS-II beam extraction. The planned DESY XFEL upgrades may also offer unique opportunities forsuch searches at unrivaled intensity.

The KLEVER detector and high intensity K0-beam can in practice only be implemented in theNA62 hall. KLEVER phasing after NA62 will depend on NA62K+ results, on the progress of the KOTOcompetitor at J-PARC, as well as on the evolution of the B anomalies and their possible explanations.

COMPASS++ as a long term QCD factory needs to prioritize its physics goals. Measurementssuch as the π-structure can make use of existing beams, but others are pending on significant beamupgrades, e.g. a RF-separated K-beam for strange spectroscopy. Competition in the experimental hallwill depend on the future of MUonE and NA64++(µ).

LHC Fixed Target measurements have already started with LHCb. Contributions from ALICEwould extend the potential thanks to the different acceptances, strengths and operation modes of the twodetectors. High statistics is essential for many measurements, and the overall physics reach will dependstrongly on the possibility to combine LHC Fixed Target and collision operation in an efficient way.

A few projects with unique physics reach face specific difficulties for implementation at CERN:DIRAC++ and NA60++ can reasonably fit only in the current NA62 hall and could therefore start im-plementation only when the hall is freed; CERN beams are not optimal for REDTOP and would havedifficulties to produce the required luminosity for a discovery. Considering the time needed to finalizethe detector design, a REDTOP implementation at FNAL sounds more efficient.

CERN participation in the design of non-accelerator experiments such as IAXO proves decisiveto these projects. Several technological domains (high field magnets, RF-cavities, cryogenics, vacuum,optics) have been identified as particularly suited for future exchange of expertise around non-acceleratorprojects, for the mutual benefit of CERN and outside laboratories.

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4 References

[1] PBC working groups. “Summary Report of Physics Beyond Colliders at CERN”. In: CDS link(2018).

[2] PBC-PHYS QCD WG. “PBC QCD Working Group Report”. In: CDS link (2018).

[3] PBC-PHYS BSM WG. “PBC BSM Working Group Report”. In: CDS link (2018).

[4] PBC-ACC BDF WG. “BDF Comprehensive Design Study”. In: CDS link (2018).

[5] PBC-ACC Conventional Beams WG. “PBC Conventional Beam Working Group Report”. In:CDS link (2018).

[6] PBC-ACC LHC-FT WG. “PBC LHC-FT Working Group Report”. In: CDS link (2018).

[7] PBC-ACC LIU study group. “PBC LIU Report”. In: CDS link (2018).

[8] PBC-ACC EDM WG. “PBC EDM Ring Working Group Report”. In: CDS link (2018).

[9] PBC Technology WG. “PBC Technology Working Group Report”. In: CDS link (2018).

[10] PBC-ACC eSPS Study Group. “PBC eSPS Report”. In: CDS link (2018).

[11] PBC-ACC AWAKE Study Group. “PBC AWAKE Report”. In: CDS link (2018).

[12] PBC-ACC νSTORM Study Group. “PBC νSTORM Report”. In: CDS link (2018).

[13] PBC-ACC Gamma Factory Study Group. “PBC Gamma Factory Report”. In: CDS link (2018).

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