comment A roadmap for the future The particle physics community refreshes the roadmap for the feld in Europe, taking into account the worldwide context, in the so-called European Strategy for Particle Physics update, which happens every seven years. Fabiola Gianotti and Gian Francesco Giudice

hat are the scientific priorities the way certain materials behave The value of exploring the unknown and future objectives of particle as superconductors below a critical does not lie in the number of hypothetical Wphysics? Which instruments temperature. discoveries that are promised, but in the offer the best potential to achieve these It was fascinating to realize that the same knowledge acquired through investigation objectives? And which technologies need phenomenon operates at cosmic scales, and the guidance obtained on future to be developed to build such instruments? and made the early Universe undergo research directions. From more than a These are the main questions addressed by a phase transition that transformed the century of research we have learned that the European Strategy for Particle Physics nature of empty space. Nevertheless, nature hides its fundamental laws at the (ESPP) following a bottom-up approach, believing that the Higgs boson discovery smallest scales — and that is what drives us driven by the scientific community. has completed our understanding of this to explore the innermost structure of matter. The process is based on the physics complex phenomenon is too simplistic. Hence, for the longer term, the 2020 knowledge at the time of the update, the On the contrary, much remains to be update of the ESPP identifies a proton– results from existing facilities such as the understood about this very special particle, proton collider reaching the highest energies Large Hadron Collider (LHC) at CERN, the including whether it is an elementary allowed by technology (at least 100 TeV) as status of the relevant technologies and the or composite object, how it leads to the the most powerful tool for the exploration prospects for future advances, proposals peculiar pattern of quark and lepton masses of uncharted territories. Even if it is and design studies for future experiments observed, what determines the stability of difficult to anticipate what these studies and facilities, and ideas for promising new the vacuum and what triggered the phase will reveal, we can be certain that this path avenues of investigation. transition in the early Universe. of exploration will greatly expand our This input is scrutinised by the These questions are still largely knowledge on many fronts. European Strategy Group, which includes unexplored experimentally and raise deep A good example of a guaranteed result is representatives of all CERN’s Member conceptual concerns theoretically. That is dark matter. A proton collider operating at States and the directors of the major why the ESPP update has identified the energies around 100 TeV will conclusively particle physics laboratories in Europe. detailed study of the Higgs boson as the probe the existence of weakly interacting The ESPP provides the scientific vision most pressing priority for the field. Since dark-matter particles of thermal origin. This and priorities of the field for the short, the Higgs boson discovery in 2012 the will lead either to a sensational discovery medium and long terms, while also general-purpose LHC experiments, ATLAS or to an experimental exclusion that will covering subjects of wider interest, and CMS, have made extraordinary profoundly influence both particle physics such as education and public outreach, progress in pinning down the features and astrophysics. Another example is the career prospects for young scientists, of this particle and, by the end of LHC precision measurement of the Higgs boson technology transfer, and the societal and operation in 2038 — thanks to the self-interaction which, by confirming environmental impact of particle physics. high-luminosity upgrades of the collider or refuting the underlying theory — the The Strategy update is a thorough and and the detectors — they should be able to Standard Model — will significantly advance rigorous process, engaging the physics measure the Higgs boson properties with our understanding of the phase transition community for almost two years from greatly improved precision. that occurred in the early Universe. beginning to end. To gain even deeper insights into the In June 2020, CERN’s governing Higgs boson and its role in fundamental Executing the strategy body, the Council, unanimously adopted physics, the ESPP recommends an electron– Given the size, complexity and cost of the resolution to update the European positron collider as the next facility, followed such a project, the ESPP recommends Strategy, which provides a realistic and by a high-energy proton–proton collider in first to perform an assessment of its prudent approach to ambitious, visionary the longer term. technical and financial feasibility. If this is scientific objectives. successful, approval may be granted by the A strategy for the future end of this decade. Construction would The Higgs boson as a starting point Humanity’s thirst for knowledge, curiosity most likely take place in stages, and the Recent experimental results, in particular and spirit of exploration have always been cost of around $30 billion, including the from the LHC, have radically transformed the engines that drive particle physics. construction of a 100 km tunnel, the proton– the status of particle physics and form Unsurprisingly, the more we dive into proton collider and an electron–positron the basis for future research directions. uncharted territory the more difficult it Higgs and electroweak factory as the The discovery of the Higgs boson has becomes to predict what future experimental possible first stage, would be spread over been a turning point, unveiling a particle endeavours could find. This is the very 50 years or more. with unprecedented characteristics and essence of research: if we knew for certain Such an ambitious project can only be shedding new light on a phenomenon what future experiments will discover, we implemented through the collaboration of that has surprising similarities with would not need to build them. scientists from all over the world. It will

Nature Physics | VOL 16 | October 2020 | 997–998 | www.nature.com/naturephysics 997 comment require the development of cutting-edge Continued, reinforced collaboration place in its early stages, show intriguing technologies in several domains, from between European laboratories with similarities with unexplained aspects related new-generation superconducting magnets partners in other regions of the world and to the Higgs boson. to cryogenics, vacuum, electronics and with neighbouring fields such as nuclear The focus of particle physics has thus the handling of big data with significant and astroparticle physics is emphasised in evolved towards addressing structural technological spin-offs for society. the Strategy update as an indispensable tool questions about spacetime, fundamental Over the past 70 years, accelerators for tackling extremely complex scientific interactions and the origin of the Universe. have contributed immensely to shaping questions. Last but not least, environmental Some of these are as old as civilization our understanding of fundamental protection is identified as a top priority itself and it is fascinating that we have physics and the structure and evolution with the recommendation that the impact today reached the maturity and developed of the Universe. This came about not of all future projects on the environment the technologies to address them. The only through fascinating discoveries, but be carefully assessed and minimised, urge to seek answers to such questions is also through precise measurements of and that efforts be redoubled to save part of what defines us as humans. The known particles, forces and phenomena. and re-use energy. ambitious task that lies ahead entails global Precision measurements are not only a collaboration on a courageous experimental way of testing and consolidating known Outlook venture, involving high-energy colliders, theories, but also an extremely powerful Today, humanity is facing pressing low-energy precision tests, observational tool for detecting hints of new phenomena challenges including climate change, cosmology, cosmic rays, dark-matter in a way that is complementary to and — combatting epidemics and poverty, searches, gravitational waves, terrestrial and in some cases — more far-reaching than confronting food and water shortages cosmic neutrinos, and much more. direct exploration. in many parts of the world, ensuring Many different fields will enrich A remarkable example is provided by the sustainable and clean energy production, one another by sharing insights and Large Electron–Positron collider at CERN and protecting the environment. Science is experimental techniques, as the boundaries and the Stanford Linear Collider at SLAC, key to addressing these and other challenges that used to separate them become which transformed our understanding of and should be supported in all its facets. increasingly blurred. In this global context, the Standard Model without discovering any Investments in science are comparatively high-energy colliders will continue to be an new particles. The 2020 update of the ESPP small on the macro-economic level, but their indispensable and irreplaceable microscope reaffirms the crucial importance of precision impact on the future of humanity is vast. In to scrutinise nature at the smallest scales, measurements as discovery tools. This particular, history shows that fundamental providing knowledge that cannot be motivates a vast experimental programme research is a driver of innovation, and obtained through any other means. ❐ not only at high-energy colliders, but accelerator-based particle physics has also through a variety of lower-energy spawned numerous technological advances Fabiola Gianotti ✉ and projects capable of probing known and new that have benefitted society, from the World Gian Francesco Giudice ✉ phenomena, for example, rare-decay or Wide Web to technologies for medical CERN, Geneva, Switzerland. transition processes. imaging and cancer treatment. Research ✉e-mail: dg.ofce@cern.ch; [email protected] The ESPP also recommends support at future colliders will boost technological for accelerator-based neutrino projects developments in many fields in ways that are Published online: 24 September 2020 in the US and Japan, theoretical research, unimaginable without the driving force of https://doi.org/10.1038/s41567-020-01054-6 and searches for very light, feebly fundamental science. interacting particles. Our understanding of particle physics Competing interests The authors declare no competing interests. Another priority in the 2020 update of and cosmology has today reached an Open Access Tis article is licensed under the ESPP is a reinforced accelerator research unprecedented level of maturity, and this a Creative Commons Attribution 4.0 and development programme, with great has dramatically changed the targets of International License, which permits use, potential for future applications in particle research. Although we have equations that sharing, adaptation, distribution and reproduction in any physics and other fields, focusing on describe most observed phenomena, we medium or format, as long as you give appropriate credit new-generation superconducting high-field lack a true understanding of the principles to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes magnets and a variety of other technologies underlying those equations and the physical were made. Te images or other third party material in this such as high-gradient accelerating origin of their many free parameters. article are included in the article’s Creative Commons structures, plasma wakefield acceleration, Moreover, many of the open questions in license, unless indicated otherwise in a credit line to bright muon beams or energy-recovery the micro-world and the cosmos appear to the material. If material is not included in the article’s linear accelerators. The ESPP also calls for be closely intertwined. For instance, some Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted intensified research and development on of the puzzles encountered in describing use, you will need to obtain permission directly from instrumentation, detectors, computing the current accelerating expansion of the the copyright holder. To view a copy of this license, and software. Universe, and that which presumably took visit http://creativecommons.org/licenses/by/4.0/.

998 Nature Physics | VOL 16 | October 2020 | 997–998 | www.nature.com/naturephysics