I knew we had it

As we celebrate the ten-year anniversary of the discovery of the Higgs boson, CERN’s Director-General at that time reminisces about the years leading up to this milestone.


I knew we had it
As we celebrate the ten-year anniversary of the discovery of the Higgs boson, CERN's Director-General at that time reminisces about the years leading up to this milestone.

Rolf-Dieter Heuer
T en years ago, on the morning of the 4 July 2012 -and after almost two hours of painstakingly careful presentations from the spokespersons of the CMS and ATLAS experiments 1 , Joe Incandela and Fabiola Gianotti -I said to the audience in the fully packed auditorium at CERN, "I think we have it". With admirable scientific caution, the two experiments at the Large Hadron Collider (LHC) had announced the observation of a new particle 2,3 that looked like the long-awaited Higgs boson (Fig. 1). Although we were only calling it Higgs-like, there was never really any doubt that after almost 50 years the wait for its discovery was finally over. Deep in my heart, I knew we had it.
The discovery of the Higgs boson was undoubtedly one of the greatest highlights of my career. The Brout-Englert-Higgs mechanism 4-6 and its associated scalar boson had been a constant presence in my professional life for as long as I can remember. When Robert Brout together with François Englert, and, independently, Peter Higgs published their papers 4,5 , which were soon to be followed by that of Gerry Guralnik, Carl Hagen and Tom Kibble 6 , I was just a teenager.
I'm often asked why it took so long to go from the original papers of Robert Brout and François Englert and of Peter Higgs to the discovery. The answer is an object lesson in how research works. In 1964, when they proposed a potential solution to the question of why the range of the electromagnetic interaction was so different from that of the weak interaction, they were drawing on work from superconductivity. Initially, their work received limited attention and it took a whole decade for the theoretical framework of electroweak physics, the unified description of electromagnetic and weak interactions, to evolve and for the standard model to mature. When it did, the Brout-Englert-Higgs mechanism found favour in the community, and searching for the Higgs boson became a priority.
However, the theory did not predict the boson's mass. Patience became the order of the day, as technology evolved to eventually enable the construction of a sufficiently powerful machine. The LHC is the world's largest superconducting installation, which -in a peculiar way -brings the Higgs boson's experimental discovery back to its theoretical origins. It is often this way in science: one generation's advances in fundamental science give rise to the next generation's innovation, which in turn provides tools for future fundamental research. It's a virtuous circle, and it's at the core of human advancement.
Back in the auditorium in 2012, seated next to François Englert, Peter Higgs wiped a tear from his eye, and said that the most remarkable thing about his eponymous particle was that it had been discovered in his lifetime. After that, the soon-to-be Nobel Prize winners stepped out of the spotlight, telling the assembled journalists that this was a day for the experimentalists to enjoy their success -there would be plenty of opportunities to talk to the theorists later.
A milestone on the journey to success was achieved at the Large Electron-Positron Collider (LEP), which played a central part in my career and in that of many experimental physicists of my generation. LEP -as well as the SLAC Large Detector (SLD) at the Stanford Linear Collider -was The associated cyan-coloured band represents the estimate of the theoretical uncertainty as discussed in ref. 7 . The vertical yellow band shows the 95% confidence level exclusion limit on m H of 114.4 GeV derived from the direct search at LEP-II 8 . The dashed curves are the results obtained using different determinations of the total contribution of the five light quark flavours to the hadronic vacuum polarization, ∆α (5) had . Q 2 denotes the momentum transfer. Credit: reproduced with permission from ref. 7 , Elsevier comment built to put the standard model of particle physics to a rigorous test through precision studies of the gauge bosons of the weak interaction, the W and Z bosons. It began operations in 1989. By the turn of the century, its job was done. A raft of precision measurements of the W and Z bosons, along with other parameters of the electroweak interaction had not only put the standard model on solid experimental ground 7 , but had also helped to put limits on the possible mass range for the Higgs boson. Crucially, the analysis showed that the Higgs boson could have a relatively low mass, which might even have been in the reach of LEP. In fact, the most likely place to look for the Higgs boson was just above the energy range that had already been explored 7,8 , as shown in Fig. 2. The incentive to push particle colliders to higher energies was huge.
When LEP started up for the last time in the year 2000, the decision was taken to push its collision energy as high as it could possibly go. At first, the experiments saw nothing, but in early summer, the ALEPH Collaboration reported some Higgs boson candidates around a mass of 114 GeV and later, the DELPHI Collaboration reported another candidate. Although no candidates were observed by the L3 and OPAL experiments, this was enough for the operation of LEP to be extended by a month.
As time passed, no additional candidates appeared. CERN's management had a difficult decision to take: either keep the collider running in the hope that with more data the signatures initially observed by the ALEPH and DELPHI experiments would turn out to be real, or switch it off to make room for the next collider, the LHC, which was to be built in the LEP tunnel. In the end, they chose the latter.
In 2001, the focus of the Higgs search moved to the other side of the Atlantic, where Fermilab's Tevatron collider began its second operating period, and further constrained the mass range available for the Higgs boson, as shown in Fig. 3. The lower bound remained at a mass of 114 GeV set by LEP, and would stay there right up to the moment when the Higgs boson discovery was announced by the ATLAS and CMS Collaborations. With a mass of 125 GeV, the Higgs boson is surprisingly light, but it was nevertheless out of reach of both LEP and the Tevatron. This brings us back to 2012, my fourth year as Director-General of CERN. When I began my mandate, a major discovery just around the corner seemed to be the stuff of fantasy because the LHC had suffered a serious setback, requiring a significant re-design of the electrical interconnects between magnets, which delayed the schedule by a year. Nevertheless, by 2010, the research programme got underwaycautiously at first -and at a lower collision energy than the LHC was designed for. There was a great deal of expectation when the LHC experiments began re-measuring known standard model processes, and then moved on to search for new physics. At the time, many expected that supersymmetry would be the LHC's first major discovery: the theory was so compelling, and much of the parameter space predicted for supersymmetry would be accessible with the LHC. But that was not to be.
It was not long, however, before the ATLAS and CMS experiments began to see something new at around 125 GeV in the two-photon mass spectra. As Director-General, I had perhaps the most privileged vantage point as I got to see the analyses from both experiments, which were otherwise carefully guarded secrets in order to avoid any possible bias. As the statistics slowly accumulated, I could see that despite the inevitable ebb and flow of the candidates, both experiments seemed to be detecting the same thing.
As 4 July approached, I was already convinced that taken together, the two experiments' results would constitute a discovery, which -in particle physics -requires a statistical significance of five standard deviations or more. I was nevertheless very pleased that their individual analyses enabled them each to present results at that level on the day. It must have been a great time to be a young But the excitement was not confined to particle physicists: the discovery was reported around the world, and to me, Jeffrey Kluger captured the moment perfectly: 9 "Despite our fleeting attention span, we stopped for a moment to contemplate something far, far bigger than ourselves. And when that happened, faith and physics -which don't often shake hands -shared an embrace. " In the decade since 4 July 2012, we have learned a great deal about the Higgs boson, but still there remains much to be understood. As we gather more data and measure the Higgs parameters in ever finer detail, exemplified in Fig. 4 for its couplings, the Higgs boson remains one of the best to search for physics beyond the standard model. A recently published measurement of the W boson's mass from the CDF Collaboration at the Tevatron 10 demonstrates the importance of pinning 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 is the chance of precision measurements leading to a discovery of physics beyond the standard model.
As we celebrate the ten-year anniversary of the discovery of the Higgs boson, the LHC is about to embark on its third operating period with a higher collision energy than ever before, thus opening up potential new windows for discovery. I'm excited by these prospects and those of the High-Luminosity LHC that is to follow this period. Despite the significance of the discovery of the Higgs boson, I'm prepared to stick my neck out and say that the best is yet to come. ❐