# Explore the future of particle physics

&ball; Physics 13, 156

Particle physicists in the United States are embarking on a year-long community study to examine options for the future landscape of their field.

All scientists look to the future, on time scales ranging from the immediate to the long term. Particle physicists are no different, but their perspective is colored by projects that span decades, instruments that require large investments, and research teams that range from a few individuals to thousands. Accordingly, the global particle physics community has a particular need for inclusive and coherent planning. Over the next year, US particle physicists, joined by international colleagues, will engage in an in-depth community study, Snowmass 2021, to define the questions that matter most to the field and identify the greatest opportunities. more promising to answer these questions in a global context. the context.

The name of this community study comes from a particle physics meeting in Snowmass, Colorado, organized in the summer of 1982 by the Division of Particles and Fields of the American Physical Society. This gathering was the first modern attempt to bring together an eclectic group of active researchers to envision an optimal national program. Snowmass 2021 is gearing up now with a community planning meeting October 5-8. In my view, the first goal of this study should be to broaden the horizons of individual physicists and the field – to look beyond our current research problems, collaborations, institutions, and to learn from others. I would like to see each initiative emerge from Snowmass better understood, improved and taken more seriously, so that new possibilities can take shape in all sub-disciplines. Then we can move forward to establish priorities, which will eventually be recommended to funding agencies through the Advisory Group on High Energy Physics.

Snowmass follows a similar European study that lasted nearly two years. Informed of this effort, the CERN Council unanimously decided in June this year to update the European Strategy for Particle Physics, setting priorities for European ambitions in a global context and taking into account developments in neighboring disciplines [1]. The update identifies detailed exploration of the Higgs boson as the most urgent priority for the field. Projecting what will be known when the Large Hadron Collider (LHC) shuts down in 2038, he cites an electron-positron collider “the Higgs factory” as the most appropriate new instrument to advance this agenda. The Eurostrategy also recommends an assessment of the technical and financial feasibility of a 100 TeV proton-proton collider, considered the most powerful instrument for exploring new scientific terrain.

Snowmass 2021 will certainly consider future accelerators, as well as many other projects of interest to particle physicists and their close scientific neighbors. The technical aspects can be summarized in a few questions. What are the most important and timely issues for our science? Which instruments – accelerators, detectors, observatories – offer the best potential to achieve these goals? What technologies do we need to develop to build these instruments? There is no shortage of engaging questions, from the most specific to the most marginally metaphysical, to which we would like to know the answers. I have covered a few in a brief essay [2]and more extensively in seminars [3]where I asked 120 questions.

Many burning questions are related to the phenomenon of electroweak symmetry breaking and the Higgs boson, detected in 2012 at the LHC. In the short term, we want to know more precisely the properties of this new particle, to see to what extent they correspond to the expectations of the electroweak theory and to know if other particles of this type exist. This requirement will set performance targets for a Higgs plant, to be studied at Snowmass. We need to go to higher energies than a Higgs factory or the LHC can provide to study how the Higgs boson interacts with itself and whether it alone prevents weak boson interactions from growing too rapidly at high energies. What type of future machine and what experimental conditions could guarantee definitive answers is another important question for Snowmass.

Beyond these questions, we don’t know what the next big energy ladder will be. Looking up, to a few TeVs and beyond, we search for new forces and new types of particles; we seek evidence of space-time dimensions beyond the familiar

$3+1$

; and we wonder what separates the electroweak scale from a unification scale or the Planck scale. There is also a low energy frontier. Searches for axions, dark matter particles and other weakly interacting particles push us to explore very small scales of energy with new acuity.

The physics of flavors, the relationship between the different quarks and leptons, is extremely rich. The problem of identity – what makes an electron an electron and a top quark a top quark – is so simple to state, but we have yet to find a promising theoretical approach to the diverse character of the constituents of matter. Neutrinos, in particular, are full of mysteries: what is the origin of their mass, are they their own antiparticles, can we detect the cosmic background of neutrinos?

Other topics – quantum chromodynamics and strong interactions, the special status of the top quark as the heaviest constituent, unified theories of strong, weak and electromagnetic interactions, precision measurements using physics techniques atomic and molecular – present many opportunities. Links with astroparticle physics, cosmology and gravitational physics are also gaining in intensity. This abundance of science topics calls for a national agenda that is diverse both in scope—engaging a wide variety of questions—and in breadth—conducting experiments from modest to gargantuan. How to achieve an optimal mix?

This brief overview provides an overview of the scientific issues to be considered, without sampling the instruments and technologies. Let me conclude by mentioning topics that are not strictly technical but deserve the attention of a community study. How can we best welcome and nurture the next generation of physicists, reflecting the diversity of the human family? How can we offer young physicists the opportunity to make their own decisions, even their own mistakes? How do you provide secure career paths for colleagues who design, build and operate accelerators and detectors or who create computer simulations? How to effectively feed both theoretical research which engages with experimentation and exploratory theory which does not yet speak with experimentation? How can we make common cause with other disciplines, within physics and beyond, to strengthen the whole scientific enterprise and secure the place of science in our society?

I also hope that Snowmass attendees will take the time, with our colleagues around the world, to reflect on the role of American particle physics in the world. How could our relationship with CERN evolve for the benefit of all? How can we improve our position as a reliable partner in international collaborations based abroad as well as in the United States? How can we most effectively encourage promising international students to come to our institutions and contribute to US-based research programs? Can our community once again muster the courage to offer the world a compelling new accelerator project suited to the scientific imperatives and commensurate with our nation’s history of groundbreaking research? Do not make small projects!

## The references

1. F. Gianotti and GF Giudice, “A roadmap for the future”, Nat. Phys. 16997 (2020).
2. C. Quigg, “Dream Machines”, Rev. Accel. Science. Technology. ten3 (2019).
3. C. Quigg, Perspectives and Questions: Meditations on the Future of Particle Physics.

Chris Quigg is Senior Scientist Emeritus at the Fermi National Accelerator Laboratory. His research covers many topics in particle physics, from heavy quarks to cosmic neutrinos. His work on electroweak symmetry breaking and supercollider physics, awarded the American Physical Society’s 2011 JJ Sakurai Prize for Outstanding Achievement in Particle Theory, paved the way for exploration at Fermilab’s Tevatron and at CERN’s Large Hadron Collider. His current research focuses on experiments at the LHC. He is finishing a book on particle physics for curious readers. Quigg served as co-chair of Snowmass 2001.

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