The Higgs discovery at 10

At the 2012 symposium, Peter Higgs and François Englert sat side by side in the CERN auditorium. A year later, they would share the Nobel Prize in Physics – Englert’s co-author, Robert Brout, having passed away just one year before the discovery. Higgs famously wiped a tear from his eye as the announcement was made, and went on to say that it was remarkable that the particle had been found in his lifetime. It was a sentiment that many of the older ones of us in the room could identify with: for physicists of my generation, the Brout-Englert-Higgs (BEH) mechanism has been a constant presence, and its experimental unveiling undoubtedly a highlight of my career. I was still a teenager when Brout, Englert, and, independently, Higgs published their papers in 1964, followed shortly by that of Gerry Guralnik, Carl Hagen and Tom Kibble. The symposium was also a very special moment for all young scientists in the room whose excitement one could almost touch: not everybody can experience such a moment.

I have to confess that those papers made little impression until much later! However, by the time it came to doing physics at the Large Electron Positron collider, LEP, Brout, Englert and Higgs were centre stage. LEP was built to put the Standard Model to a rigorous test through precision studies of the weak vector bosons, W and Z, but since the mass of the Higgs boson was largely unconstrained, we all held out hopes that it might make its first appearance in our detectors.

By the turn of the century, when LEP was due to switch off, a raft of precision measurements of the W and Z, along with many other electroweak parameters, had put the Standard Model on solid experimental ground. The LEP experiments had also helped to put limits on the possible mass range for the Higgs boson, indicating that it might still be in reach of LEP. They even went so far as to suggest that the most likely place for it to be was just above the highest energy that had already been explored. We had no choice but to push LEP’s collision energy as high as it would go. It was an exciting time, but nothing conclusive appeared. LEP was retired, and in 2001, the focus of the Higgs search crossed the Atlantic as Fermilab’s Tevatron began its Run II, further constraining the mass range available for the Higgs. The lower bound, however, remained 114 GeV as set by LEP, right up to the moment the discovery was announced by ATLAS and CMS. With a mass of 125 GeV, the Higgs is surprisingly light, but it was nevertheless out of reach of both LEP and the Tevatron. We did not know at the time, of course, but the Higgs was always going to be the LHC’s to discover.

Before moving on to look at what has happened in the 10 years since July 2012, let me first take a short look back on the months leading up to this seminal discovery. It is important to have two independent experiments to crosscheck each other and to confirm results. Many fluctuations often appear when starting data taking and analysis. Although they usually disappear with increasing statistics, on this occasion, some remained, and with slightly increasing significance, although they were not always in the same places for both experiments. This was the situation at the summer conference in 2011. Eyebrows were raised, but none of the fluctuations seen by either experiment came close to a signal that could be interpreted as evidence. Despite a nascent sense of anticipation, we had to explain to an expectant media that we needed more data.

As time went on, we raised the energy of the LHC from 7 to 8 TeV, and increased the collision rate. The experiments duly collected more data. Over the following months most of the fluctuations did indeed disappear, but one developed into a real signal in both experiments, and it was at the same mass value. It was clear that we were nearing a discovery. The reaction from the scientists and from the media and from society as a whole was overwhelming for a discovery of this nature. To find on the front page of so many of the world’s respected daily newspapers an informed discussion of the importance of five standard deviations was certainly a first for me.

Ten years on

Ten years on from the discovery, on 4 July this year, CERN organised a full-day symposium to take stock of what we have learned, and look forward to what the future of Higgs research holds. I had the honour of opening that symposium, and it is a sign of the times that I did so while self-isolating in a Turkish hotel room as I recovered from COVID. That did not, however, prevent me from enjoying a range of fascinating talks.

In the decade since the discovery, we have not only confirmed that it is indeed the particle predicted by Brout, Englert and Higgs in 1964, but we have also defined its mass to high precision, and measured many of its couplings to other Standard Model particles. As we gather more data, and measure Higgs parameters in ever finer detail, the Higgs remains one of the best laboratories we have to open the door to physics beyond the Standard Model. A recently-published Fermilab measurement of the W boson’s mass demonstrates the importance of tying down every free parameter in the Standard Model as precisely as possible. The better each parameter is measured experimentally, the tighter the others are constrained and the greater the chance of precision measurements leading to new physics. As the LHC’s Run 3 gets underway, with higher beam energies than ever before opening up potential new windows on discovery, I am excited by the prospect of new insights into the Higgs. With the High-Luminosity LHC to follow, the best years of Higgs physics at the LHC are still ahead of us.

The LHC has a research programme mapped out until around 2040, with a major luminosity upgrade scheduled to come on stream in 2029. The Higgs will provide a rich seam of physics for many years to come, and if we are to reveal all of its secrets, we are likely to need a more powerful machine to follow the LHC. The Higgs self-coupling, for example, will need higher energies for a complete measurement, and the question of Higgs compositeness might not be addressed at the LHC alone.

In the meantime, the study of rare processes, often a good way to unveil new physics, is growing in importance as the data accumulates. The LHCb experiment has shown that it can measure processes so rare that they happen only a handful of times in a billion particle decays. In doing so, they have made measurements that appear to be slightly at odds with the Standard Model – leaving particle physicists cautiously excited and hungry for more data.

The Higgs is not the only discovery the LHC has chalked up to date. A raft of new exotic mesons, for example, have been found, as well as multi-quark states whose discovery required as much patience as the Higgs itself. QCD, the theory of strong interactions, allows for bound states of four and five quarks, as well as the familiar baryons and mesons, made up of three and two, respectively. LHCb discovered the first pentaquark in 2015, and although tetraquark candidates had been seen before, the experiment’s discovery of several new ones has strengthened QCD.

When the LHC started up, proponents of a theory known as supersymmetry, SUSY for short, were hoping that evidence for it would be the first significant result the LHC experiments would reveal, but that was not to be. One of the things that makes many SUSY models so exciting is that SUSY particles remaining in the universe today would be stable, making them a strong candidate for Dark Matter.

Physics beyond the Standard Model is proving elusive, however. LHC results show that the simplest SUSY models are not valid, and are in fact incompatible with the mass of the Higgs boson as measured. SUSY is not yet down and out, but it is on the ropes. In the light of what we have learned already from the LHC it is looking increasingly tenuous, and many new theoretical ideas are emerging to fill the gap. There is no shortage of candidates for the kind of facility that may be used to probe these new theories and take particle physics beyond the LHC. In the US, Fermilab is transforming itself into a neutrino research laboratory, and in exchange for US contributions to the LHC’s high luminosity upgrade, CERN is building neutrino detectors for Fermilab. Transatlantic collaboration has always been important, and is as much so today as ever.

For the longer term, there are plans afoot that could lead to new colliders being built in China and Japan, while in Europe, CERN is involved in several ongoing accelerator R&D projects: the Compact Linear Collider, CLIC; the Future Circular Collider, FCC; a muon collider; and a very long-term R&D programme for a proton-driven plasma wakefield accelerator, AWAKE.

Particle physics is an internationally-coordinated field of research. Europe’s rolling strategy for particle physics is developed in consultation with physicists from around the world. The most recent European strategy cycle concluded in 2020 with a recommendation to the CERN Council that the High-Luminosity LHC should be fully exploited as a first priority, while at the same time investigating the feasibility of a potential future energy-frontier machine with a circumference of around 100 km. Now underway, the FCC feasibility study will examine environmental, technical, geological and financial issues before delivering its conclusions to the CERN Council in 2025.

I am often asked why it took so long to go from theory to discovery. The answer is a lesson in the scientific method, and what I like to call the virtuous circle of basic research, applied research and innovation. In 1964 three theorists proposed a solution to a problem: why the range of the electromagnetic interaction is so different to that of the weak interaction. They drew on Bardeen, Cooper and Schrieffer’s work on superconductivity, and their efforts received little attention – not only from the teenage me! It took a decade for the theoretical framework to evolve, and for the Standard Model to mature. When it did, the Brout-Englert-Higgs mechanism found favour, and searching for the boson became an important priority. Then we needed the technology. The theory did not predict the boson’s mass, so we had no idea what kind of accelerator we would need to build. As time went on basic research fed into applied research, and new tools became available, through innovation, for particle physicists to use. By the time we got to the LHC, virtue had turned full circle. Superconductivity had developed to the point at which it would allow us to build a machine guaranteed to put the 1964 theory to the test: either the boson would be found, or something else would have to appear in its place.

As so often happens in science one generation’s fundamental advances give rise to the next generation’s innovation, which in turn provides tools for future fundamental research. It is the virtuous circle at the core of human advancement. The Higgs celebration at CERN on 4 July 2022 gave tantalising hints of where the next step on our voyage of discovery might lie, and I cannot wait to see where it takes us next.