Could It Be the End of the Particle Era? Rethinking Discoveries in High Energy Physics

In the realm of high energy physics, a significant shift may be on the horizon as concrete experimental evidence of new physics beyond the Standard Model remains elusive. New particles may never be produced again in particle colliders, prompting a profound reevaluation of the notion of discovery.
Published in Physics
Could It Be the End of the Particle Era? Rethinking Discoveries in High Energy Physics
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From a modern point of view, the discovery of a new particle requires its “on-shell” production in particle collisions, if only for a brief moment. Its energy E and its 3-momentum p must combine with its rest mass m as E2 - p2c2 = m2c4, where c represents the speed of light.  In this frame, particle colliders have played a pivotal role in the field of particle physics over the past seven decades, enabling the pursuit of higher energies and the localization of particle interactions within small regions of space. In 2012, a significant milestone was reached at CERN with the long-awaited observation of the Higgs boson. This groundbreaking achievement marked the discovery of the final piece in completing the puzzle of the Standard Model (Fig. 1), which currently stands as our best theory for understanding elementary particles and their interactions.

Fig 1. Standard Model of Elementary Particles.
Fermilab, Office of Science, United States Department of Energy,
Particle Data Group (Wikimedia Commons, license: CC BY 3.0).

But soon, excitement gave way to uncertainty. As the Standard Model do not pretend to be the final theory—in particular, it fails to explain gravitation—it was hoped that the LHC would pave the way for revolutionary discoveries in the search for a more complete and exhaustive theory. However, the absence of experimental evidence for physics beyond the Standard Model has since left physicists contemplating a vast and uncertain landscape, reminiscent of a desert with only little hope to find an unexpected oasis. The new challenges are undeniable, all the more so as on-shell discoveries are likely to be elusive due to the potentially too heavy nature of undiscovered particles, far beyond our current and future technological capabilities (Fig. 2). Indeed, as the paper I co-authored in the European Physical Journal H underlines, there is currently no compelling reason to anticipate new particles in the accessible energy range.

Figure 2: Time evolution of the energy reach of particle colliders.
During the first decades of their development, the energy reach of particle colliders grew exponentially, doubling about every six years. Nevertheless, CERN's Large Hadron Collider is already lagging behind this growth. The trend towards saturation would continue with the planned design of future projects, underlying limitations in the quest for higher energies. Updated version by Jordan Nash [1] of a plot produced by the NLC ZDR Design Group and NLC Physics Working Group at SLAC [2].

This observation does not signify the end of progress in fundamental physics; rather, it serves as a clarion call for a profound shift in scientific approaches. The quest to understand higher energy phenomena may now call for innovative thinking and novel methodologies to explore uncharted territories in our pursuit to unravel the mysteries of the universe. As a historian of science working at RWTH Aachen University, I teamed up with Robert Harlander, a theoretical physicist from the same institution, and Gregor Schiemann, a philosopher of science at the University of Wuppertal, to address such issues from an interdisciplinary perspective. Our collaboration emerged from the Research Unit Epistemology of the Large Hadron Collider, which delves into the philosophical, historical and sociological implications of activities at the world’s largest research machine at CERN.

The Evolving Particle Concept

In the realm of natural philosophy and physics, the particle concept has held a significant role over time, serving as a powerful heuristic, illustrative, and operational tool. Despite being frequently challenged, its persistence over the past century has demonstrated its remarkable adaptability. Initially established at the turn of the 19th and 20th centuries with the observation of elementary particles such as electrons in cathode rays (Fig. 3), the concept of “classical” particles underwent a first significant expansion with the advent of quantum theory. From mere building blocks of matter, particles evolved to encompass constituents of radiation and mediators for interactions. Subsequently, the realization of their mutability during the early developments of quantum field theory in the 1930s, including creation, annihilation, and decay processes, became vital for interpreting experimental tracks in cloud chambers. The observation of new particles through their decay products even became the norm later, when high energy physicists switched from the study of cosmic rays to that of artificially produced particles in accelerators. This transition led to a focus on the search for “resonances” or “peaks” in scattering cross-sections, a process which remains today fundamental in particle discovery.

Figure 3: Joseph John Thomson, generally considered the discoverer of the electron in 1897, working with a cathode-ray tube (License: CC BY-SA 2.0).

Exploring the particle era in all its breadth, it became obvious to us that the expansion of the particle concept and our understanding of the particle properties relevant to their observation or discovery were evolving hand in hand. Theoretical and experimental advancements broadened both observational and conceptual perspectives, offering new interpretations of events in terms of particles. So, as we argue in the paper, the potential end of on-shell particle discoveries and the need for new means of observation may herald a new chapter in the history of the particle concept, if not the opening of a whole new book.

Challenges and Opportunities

The present situation in high energy physics is already driving physicists to broaden their horizons and explore new avenues of research. Collaborative efforts between different fields and interdisciplinary approaches may hold the key to unlocking new physics. Cosmological studies, astrophysics, and other fields combined with current and future particle collider experiments can contribute valuable insights and potential experimental signatures that may lead to the discovery of new particles or even entirely novel physics beyond the Standard Model. In our paper we argue that one consequence of the decline of on-shell discoveries already is a shift of focus, from the search for individual particles to the exploration of broader theoretical frameworks, in particular in the form of effective field theories. In this way, research is directed toward comprehending the properties of quantum fields and the virtual effects they generate, rather than their quantum excitations, which we call particles. In this frame, theoretical predictions and precise measurements will likely play a crucial role in guiding future research. In fact, this is not totally new: the mass of the top quark was successfully predicted—one could talk of an indirect measurement through virtual effects—in the early 1990s, before its on-shell discovery (see, e.g., [3]). But in the future, such effects could play a much more substantial role in the search for the ultimate theory of nature.

Whatever the final form, we defend that physicists will have to embrace new perspectives and methodologies to explore the uncharted territories of fundamental physics. The field has experienced such transformations in the past. And now it may find itself on the edge of another radical shift as the end of on-shell discoveries could open the door to potential breakthroughs that will redefine the nature of discoveries in high energy physics. Embracing this new era would require creativity, innovation, and a willingness to challenge established practices. As our investigation underlines, the fate of the concept of particle will certainly depend on this process.

 

[1] Nash, J. (2010). Current and Future Developments in Accelerator Facilities. Talk at the 2010 IOP Meeting. Slides available at: https://www.hep.ucl.ac.uk/iop2010/talks/14.pdf
[2] Kuhlman, S. et al. (1996). Physics and technology of the Next Linear Collider: A Report submitted to Snowmass ‘96.
[3] Bernabéu, J., Pich A., and Santamaria, A. (1991). Top quark mass from radiative corrections to the  decay. Nuclear Physics B 363, 326–344.

Poster image credits: View of the LHC tunnel in sector 3-4 / Maximilien Brice © 2009-2023 CERN (License: CC-BY-SA-4.0)

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