Quo Vadis, High Energy Physics?

High Energy Physics finds itself at a crossroads, a fact commonly recognized within the scientific community. 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. Even then, there are still enough urgent fundamental questions that are so far left unanswered!

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.

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 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. So there is definitely no lack of fascinating problems to be tackled!

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.

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.

As I was saying, one of the main hopes for our field 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.

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 this at our disposal, they will come with a hefty price tag of several billion euros at the very least. 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.

For instance, despite frequent claims of the contrary, there is no guarantee that a high precision study of the properties of the Higgs boson will unveil new physics beyond the Standard Model. Of course, it could lead to the discovery of new physics, which would of course be awesome, and it is thus an extremely interesting and important experimental program. But we should make sure that we do not oversell our field and that we avoid making promises that we cannot fulfil.

The bottom line of all this lengthy disquisition is that future progress in HEP should be driven by exploration, rather than by theoretical prejudice. 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’. 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 of course famous bloggers, 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 bright for particle physics and everyone should certainly stay tuned for news from the high-energy frontier.