Disclaimer: I write a first version of this article as part of my guest blogging in the Particle Physics People blog at Interactions.org. Given the intense discussions about the future of high energy physics that are taking place in the last months, including the update of the European Strategy, I though it would be nice to extend a bit this entry and post it this time in my new blog. Hopefully this will also allow testing the comments functionalities!
It is a well known fact within the community that high energy Physics finds itself at a crossroads. Paradoxically, the main reason for this state of affairs is none other than the extreme success of both our theoretical framework and our experimental programs. Indeed, our current understanding of elementary particles, as encapsulated by the Standard Model, has so far been confirmed with exquisite precision by countless experiments, except for a handful of anomalies that might or might not lead to something deeper. Even then, there are plenty enough urgent fundamental questions that are so far left unanswered! Indeed, while the discovery of the Higgs boson in 2012 by the ATLAS and CMS collaborations implies that there are no obvious targets of where the next layer of complexity in our understanding of physical reality can be identified, we should not be idle and keep pushing our efforts towards addressing these pressing questions.
To begin with, the Standard Model (SM) does not provide a candidate for dark matter, the mysterious non-luminous form of matter five times more abundant than normal matter and whose existence we infer from astronomical observations. It does not provide either a microscopic mechanism for the dark energy accelerating the expansion of the universe. Neither does the SM explain how the observed asymmetry between matter and antimatter was generated in the early universe, nor the fact that neutrinos have non-zero masses. While some of these puzzles have different levels of relevance (for example, it is not complicated to augment the Standard Model to include neutrino masses), all of them are truly fundamental questions which we should work hard to answer if our goal is to understand Nature at its deepest levels.
In addition to these ’observational’ conundrums, the SM also contains several puzzles of a more theoretical nature. To begin with, we still don’t know for sure if the scalar boson observed at the LHC is really the SM Higgs boson, or if it is instead a more complicated creature. For example, it could very well be that the Higgs is a composite particle itself, in the same way as how protons are not fundamental but rather composed by quarks and gluons. In addition, in the SM the mass of the Higgs boson is not protected by any symmetry, and for this reason it will tend to grow up to the highest energies at which the theory is valid. In this respect, we do not really understand the unbearable lightness of the Higgs particle. We also have no clue whatsoever of the origin of the flavour structure in the SM, for instance why there are three generations and not 27, and what mechanism determines the observed values of the masses of the SM particles. Moreover, the Higgs boson interactions are mediated by a force completely different from any of the forces we know (such as electromagnetism, drive by the gauge principle) so if there are any hidden sectors beyond the SM the Higgs offers a unique portal to access therm. So there is definitely no lack of fascinating problems to be tackled! Whether these theoretical puzzles are real conceptual issues or not is hotly debated within the community. For example, perhaps the flavour puzzle is merely a consequence of anthropic selection, as speculative as the latter is. In any case is clear that there is still a lot to learn about fundamental physics at the higher energies and smallest distances.
Going even deeper into the foundations of high-energy physics, we don’t know how to marry the two most arguably successful physical theories ever formulated, quantum mechanics and general relativity. Indeed, the ongoing quest for quantum gravity has turned out to be a formidable challenge attacked without success by some of the most brilliant physicists of the last decades. The fact that the experimental signatures of quantum gravity are in most cases orders of magnitude beyond our foreseeable experimental reach does for sure not help in this context. Quantum gravity has been so far the playground of mostly theoretical speculations, though there are hopes that its effects can be probed experimentally in the near future either from cosmological observations or from ultra-high precision measurements of quantum systems. Formulating a theory of quantum gravity would be a massive breakthrough, comparable or even more important than the formulation of quantum mechanics and general relativity themselves.
I encourage the interested reader to take an interactive look at the various mysteries of the Standard Model and the various “Theories of Everything” that have been proposed in this infographic by Quanta Magazine.
In addition, particle physics is much, but really much, broader than just searching for new theories beyond the Standard Model or trying to formulate the theory of quantum gravity. Particle physics is also about achieving an improved understanding of pressing questions within the strong interaction, including how the masses and spin of the hadrons are generated in terms of its constituents, whether or not heavy quarks are part of the proton wave function, the possible onset of extreme dynamical regimes of gluon-dominated matter, or pinning down the properties of the Quark-Gluon Plasma, the hot and dense medium created in the collisions between heavy ions. Anyone who reduces high energy physics to model building is deliberately misleading, and leaving out crucial (and thriving) areas of our field.
One of the main hopes to unravel the next layer of the physical reality is that the thorough exploration of the Higgs boson properties can shed some light on the SM mysteries. For instance, we are now only starting to scratch the surface of the Higgs particle, and current and future measurements at the LHC will tell us more about its underlying nature. Indeed, one of the main goals of the High-Luminosity upgrade of the LHC (HL-LHC), which will deliver up to a factor 10 more collisions, is the accurate profiling of the properties of the Higgs boson, where any deviation with respect to the tightly fixed properties of the SM would represent a “smoking gun” for new physics beyond it. Crucial in this context is the measurement of its self-coupling: not only we have never observed a fundamental scalar particle interacting with itself, it could play a role to explain the matter-antimatter asymmetry in the universe by means of electroweak baryogenesis.
While the HEP community is certainly together in its support for the full exploitation of the physics potential of the HL-LHC as a major priority, it’s less clear what should come next. Should we build yet a bigger particle collider? A different type of collider? Perhaps the key is in the intensity, high-precision frontier? Should we focus on completely different types of experiments, perhaps more weighted towards astrophysics and cosmology? Something else that no one has even thought of before?
In this context, one particularly attractive proposal goes under the name of Future Circular Collider (FCC). The FCC would be a gargantuan particle collider with a radius of around 100 kilometers, dwarfing the already pretty huge LHC. This collider could accelerate protons up to the extreme energies of 100 TeV, about 7 times more powerful than those available at the LHC. In addition, this machine could also accommodate the collisions between electrons and positrons at high energy and luminosity, which would make extremely high precision characterization of SM particles possible, such as the Higgs boson, the W and Z gauge bosons, and the top quarks. Similar machines are under active study by the Chinese HEP community. Another proposal for the next collider is the International Linear Collider (ILC), a high energy linear accelerator of electrons and positrons, to be hosted by Japan.
While it would be amazing if we had machines like those at our disposal, they will come with a hefty price tag, and it is obviously not a decision that can be taken lightly, and the science case in each option must the weighted carefully. One particularly challenging aspect of the current situation for high-energy physics is that there is no machine that can guarantee discoveries, such as new particles or novel fundamental interactions. This was not the case in the past: at the LHC for instance there was a “no-lose” theorem guaranteeing that it would either discover the Higgs boson or instead an altogether novel force of nature. It is worth emphasizing that this is true also for many other fields, such as cosmology, where there is no current or planned experiment that can lead to guaranteed breakthroughs such as evidence for inflation or pinning down the nature of dark energy. The pros and cons of the various proposals for future collider are now being discussed in detail within the community, see for example the recent EPPSU open meeting in Granada.
The bottom line of all this lengthy disquisition is that future progress in HEP should be driven by exploration, rather than by theoretical prejudice, see also Nima Arkani-Hamed’s reflections. For many years (better said, decades) HEP was driven by theoretical efforts, with experiments successfully confirming prediction after prediction. But now our field is experiencing a U-turn, where we should think outside the box and be ready for the unexpected. A nice example of the latter is provided by the recent anomalous in the b-quark sector presented by LHCb. These anomalies seem to indicate the violation of one of the cornerstones of the Standard Model, namely the symmetry telling us that leptons of different families (say muons and electrons) interact with other particles in exactly the same way. Only time will tell the fate of these anomalies, but if confirmed they would represent an arguably more important discovery than that of the Higgs itself!
With the same motivation, and in order to make sure that no stone is left unturned, it is healthy for our field to develop a varied program of experiments that are not limited to high-energy colliders. For instance, CERN has recently set up a Working Group focusing on the potential of ’Physics Beyond Colliders’ (PBC). The idea underlying this approach is that high-precision measurements of specific properties of known particles can reveal the presence of new, heavy particles beyond the direct reach of future colliders. This is possible by means of quantum effects, where heavy virtual particles pop up from the vacuum for a fleeting moment, leaving a measurable imprint in the SM particles.
A prime example of this precision program is shown above: the muon storage ring at Fermilab. There the “muon g-2” experiment aims to measure with exquisite precision the internal magnet of the muons, its so-called magnetic moment. The hope is to resolve a long-standing discrepancy between similar measurements and the SM predictions, which could unveil new physics beyond the SM.
Can we now summarise what the best option is for the future of HEP? Well, not really, this is precisely the million dollar question! Every member of the HEP community, including several popular bloggers within the field, has something important to say there. I think that irrespective of the exact path that our field chooses for the next years, the future is certainly bright for particle physics and everyone should certainly stay tuned for news from the high-energy frontier!