Τhe ongoing update of the European Strategy for Particle Physics aims to identify the potential opportunities and challenges of the proposed research programme through an inclusive and evidence-driven process. Discussions focus on the parameter space that can be explored and how future searches could guide theoretical developments along with synergies on required R&D lines in detector development, accelerator technology, computing and other vital tools to progress in fundamental physics. Given the exploratory role of any post-LHC machine the question of synergies with other fields including astroparticle physics as well as cosmology and the gravitational wave community emerges. Finally, the strategy faces new challenges given the complexity of the proposed next-generation colliders and the interest of China and Japan to host a post-LHC collider. The different challenges are detailed in the Physics Briefing book that was published by the Physics Planning Group last October. The goal is to offer a coherent framework for understanding the possibilities offered by the proposed machines along with their complementarity with non-collider searches. This will inform future discussions during the drafting session scheduled for January 2020.

One of the top priorities remains the full exploitation of the LHC results and the high-luminosity (HL-LHC) upgrade of the machine, which remain key priorities for the global particle physics community. The outstanding performance of the LHC, confirms the CERN’s Council decision in December 1991 meeting that the LHC was the “right machine for the advancement of the subject and the future of CERN”. The ongoing upgrades of the LHC experiments will allow to measure the couplings of the Higgs boson to SM bosons and third-generation fermions at the percent level. This task calls for significant improvements in theory as discussed in this issue. Moreover, upgrades of LHCb, ATLAS and CMS will offer enhanced B-physics capabilities complementing searches of Belle II and of the high-transverse momentum programme. Similarly during dedicated heavy-ion runs the LHC experiments will continue exploring the nature of QCD and the physics describing the formation of hadrons. Furthermore, results from the LHC complement those of fixed-target, underground and astroparticle experiments allowing to test a significant part of the parameter space where new physics can lie.

Physics questions after the LHC

Results from the LHC and previous colliders have helped us to establish the Standard Model (SM) as the successful description of fundamental particles and their interactions. However, today we are still confronted with many unresolved puzzles - both experimentally and theoretically - that can be tackled only with a bold experimental programme based on substantial technological advances. This is why the next scientific tool should give us the broadest possible research programme allowing a smooth continuation after the completion of the LHC programme around 2040. Proposed future colliders can explore new physics extensively, up to multi-TeV scales, through direct and indirect searches. Lepton colliders like CLIC and FCC-ee tend to perform well in indirect searches in spite of the substantially lower centre-of-mass energy while hadron colliders like the proposed FCC-hh have a better reach for direct searches of new states, while profiting from its complementarity with FCC-ee in the FCC integrated programme.

Following the discovery of the Higgs boson a number of research areas call for a diverse experimental programme along with theoretical developments (Image courtesy of Prof. Jorgen D' Hondt, from the 105th Plenary ECFA meeting - CERN).

One of the main experimental challenges for next-generation particle colliders is to scrutinize the properties of the Higgs and delve into the physics of the electroweak sector. The remarkable self-consistency of the SM depends on the values of the coupling constants. Thus the collected inputs prioritize a lepton collider as the next project since it would serve as a Higgs factory. This would allow to study the Higgs interactions with all other known particles of the Standard Model but also with itself. New physics, it is argued, would influence the values of the Higgs couplings to the fundamental constituents of matter and interactions, and could be detected provided they are measured with very high precision to be sensitive to the relevant energy scales.

Future colliders could also probe the whether the Higgs is accompanied by other related spinless particles or not and whether it is a more composite rather than a fundamental particle (for an in-depth discussion see a previous EP article: https://ep-news.web.cern.ch/content/higgs-boson-probe-new-physics). In addition, high-energy colliders like the proposed FCC-hh will contribute in a complementary way to the Higgs, the electroweak precision and the flavour programmes where LHC experiments have made important contributions. One example is the study of rare Higgs decays (e.g. H → µµ, νν, Zγ) profiting from the high luminosity, while they will greatly improve precision measurements of the Higgs self-couplings compared to lepton machines and allow to measure the full Higgs potential.

Moreover, future colliders reaching energies of 100 TeV will be able to address the nature of the electroweak phase transition that took place in the early Universe. While the phase transition is a high-temperature phenomenon that cannot be recreated experimentally, precision measurements of Higgs properties— in particular of the triple-Higgs self-coupling—will give us decisive elements to reconstruct the dynamics that occurred when the Universe changed its vacuum state. According to the SM, the Higgs mechanism took place as a smooth crossover when the Universe cooled down to temperatures below 160 GeV, but the transition could be very different due to new physics. Testing the nature of the electroweak phase transition is an important task for future colliders that will considerably expand our knowledge about the early history of the Universe. Measuring a first order transition would open the door to the exciting prospect of explaining the cosmic baryon asymmetry with weak-scale physics or of observing with next-generation gravitational interferometers the primordial gravitational waves produced by the abrupt transition at that epoch.

A second main research line in particle physics is the study of strong interactions. Though the creation of QGP as an almost perfect liquid has been experimentally established, studies of heavy-ion collisions at the LHC (by the dedicated heavy-ion ALICE experiment as well as by the other LHC experiments) and at RHIC (Brookhaven) have been a constant source of surprises. Open questions include the mechanism for the transition from QCD to long-distance phenomena and the characterisation of the collective behaviour emerging under extreme conditions similar to those that existed just fractions of a second after the Big Bang. Experimental measurements are needed to understand the behavior of strong interactions in the non-perturbative regime where theoretical calculations become very demanding. Furthermore, the observation of collective effects opened a new area of studies for the heavy-ion community. A high-energy AA/pA/pp research programme offered at a circular collider would be unique to Europe and would lead to a profound understanding of hot and dense QCD matter. The lower-energy research programme of QCD matter at the SPS at CERN, is complementary to other emerging facilities worldwide in the US (BES at BNL), in Germany (FAIR), in Russia (NICA at JINR) or in Japan (J-PARC), and brings valuable contributions in the exploration of the QCD phase diagram

Flavour physics is another regime where future experiments will shed light, allowing to study new pathways to searching for new physics. The document identified that “Experimental hints for deviations from SM predictions in flavour processes are one of our best hopes to direct research towards the right energy scale where new physics may lurk”. From both the experimental and the theory side, a novel synergy between the searches for flavour violating decays and for feebly interacting and dark particles is emerging. The next generation of flavour physics experiments, will inaugurate a completely new realm of sensitivity using many different observables available in future experiments. We do not know which approach will discover evidence of New Physics first, the highly sensitive search for deviations from SM predictions in precision flavour physics or direct observation of new particles. Input from both will be needed to understand the physics that lies beyond the Standard Model. In the mid-term future much can be gained from the possible upgrade of the LHCb experiment for the HL-LHC in addition to the hope that the pending question of lepton number universality will be fully resolved with more data. On the longer term, the Tera-Z option of the FCC-ee also offers an attractive program of exploring flavour physics with high precision.

Another experimental goal for a post-LHC machine is the search for Dark Matter candidate particles. Historically direct-detection DM experiments have been dominated by WIMP searches. However taken into account the limits from multiple overlapping direct detection experiments, there is a paradigm shift focusing on a broader set of particles that could be anything from as light as 10⁻²² eV to as heavy as primordial black holes of tens of solar masses. In particular, the search for ultralight DM particles like the axion and axion-like particles has gained significant momentum. The study of the Higgs (given its unique characteristics) and sterile neutrinos, or the recently proposed long-lived particles (LLPs) are also promising candidates to shed light on the dark matter mystery. Future colliders (ILC/ CLIC, FCC-ee/hh/eh) have an excellent potential to explore models of thermal DM in the GeV to 10TeV mass range complementing other experimental searches based on accelerators, solar haloscopes or light shining-through-wall experiments. Discussing DM searches we also have to consider the comlementarity of future collider searches with those from other approaches including dedicated underground experiments but also by large astroparticle detectors like H.E.S.S., Antares or IceCube, and in the near future the CTA observatory expected to start operations in 2022. Therefore while updating the EPPSU one has to consider the opportunity for Europe to play a leading role in DM searches using CERN’s accelerator complex and the potential of a post-LHC collider but also by contributing to the axion research programme in other laboratories.

The physics briefing book includes a discussion on neutrino physics given the surging interest from the global community and facilities planned both in the US and Japan as well as astrophysics and underground experiments targeting the study of this particle. Neutrino masses offer today one of the strongest experimental signs for the existence of new physics beyond the Standard Model. Therefore, we need to continue exploring the neutrino sector with accelerator, reactor, solar and atmospheric neutrino experiments. CERN’s Neutrino Platform (NP) brings together as of today about 90 European institutions to support and participate in detector R&D and construction for neutrino facilities. EP department has also launched in 2016, the neutrino group, to coordinate activities within the department and ensure that Europe will continue playing a vital role in the mid-term future. Furthermore, the physics briefing book suggests/explores a range of alternative approaches not limited to colliders, but complemented by beam-dump, fixed-target and other experiments. Feebly-interacting and long-lived particles are two recent examples demonstrating how the experimental community can come up with new ideas as a healthy response to the LHC results.

Finally, discussing about very precise measurements poses a clear need for better independent determination of the proton structure to feed theoretical calculations. This motivates the proposed programme based on fixed target experiments and on dedicated ep machines has been proposed in Europe, in the US and in China. The high-energy end of the proposed facilities at CERN such as the LHeC and/or FCC-eh have in addition the potential to complement the programme of BSM physics discussed above.

The future demands diversity

The way forward involves challenges that cannot be addressed without constant progress in advancing accelerator science, designing better detectors, and developing proper computer infrastructures. The Physics Briefing book emphasizes the need to retain a strong focus on instrumentation R&D and develop an environment that stimulates innovation, with the primary goal of addressing the well-defined technological challenges of future experimental programmes. Technology innovation emerges from synergies within the fields of physics as well as with industry and any future research infrastructure will act as hub for co-innovation between academia and industry. The continued R&D on solid-state sensors has led to the possibility to add timing detectors to LHC experiments upgrades - an improvement that was not originally foreseen - enabling better pile-up rejection while paves the way for using these technologies for medical purposes and other applications beyond HEP.

Typical path for a successful decision requires to consider the physics questions that we target, compare the physics opportunities offered by the different experimental approaches along with the performance of the proposed research infrastrcutrues, select a scenario and plan early the next steps. (Image from Prof. Jorgen D' Hondt's presentation during the 105th ECFA plenary meeting at CERN).

The goal for next year is to turn the results documented in the Physics Briefing Book into a coherent strategy that will enable Europe to continue leading in the field of particle physics and will guide CERN - as a truly international laboratory - into the future. The European Strategy for Particle Physics revision will conclude in May 2020, and will hopefully identify a concrete vision for the future of the field. The outcome of the strategy will guide the complex preparatory efforts for a post-LHC collider and allow to establish the required level of co-operation at European and international level. The realisation of any future project relies not only on the experimentalists, currently involved in the LHC experiments and elsewhere, but also on the strong support of the theory community and, last but not least, new advances in technologies, computing software and infrastructure.