What have we learned in the ten years after the discovery of the Higgs boson?
Ten years ago, on 4 July 2012, the ATLAS and CMS collaborations cautiously announced the discovery of a particle “consistent with the Higgs boson” at CERN. In the end it turned out to *be* the Higgs boson, the particle that had been predicted by theorists nearly forty years earlier. What was it like to witness the announcement of one of the discoveries of the century? And what have we learned about the mysterious Higgs in the ten years since?
When a big announcement is made in the world of particle physics, there’s a special buzz in the air. In the days leading up to the announcement of the discovery of a new particle at CERN, this buzz became a crackle. “It was an extremely exciting time full of rumours and speculations”, remembers Lea Caminada, now professor at PSI and University of Zurich. “I had just finished my PhD and was doing research in the US. Of course we knew what our experiment, CMS, had seen, but we had no idea whether our results would be consistent with those of our friendly competitor ATLAS.”
Ben Kilminster, now also a professor at University of Zurich, was based at Fermilab in the US at the time and had a special kind of advanced knowledge. A member of the CDF collaboration at Fermilab’s Tevatron collider, he had also recently joined the CMS experiment at the LHC. He led CDF’s Higgs research group in 2010 and 2011, which reported possible sightings of a Higgs-like particle at 125 GeV at the Tevatron – however, with too little data to actually pass as a true observation of something new.
“We were (unsuccessfully) trying to snatch the Higgs discovery from the LHC, but I was lucky enough to actually know both sides – what the Tevatron had and what the LHC had,” he recounts. “It really was very exciting.” The announcement was made in Europe with a video link to a conference in Australia, so it was the middle of the night into the 4th of July national holiday for scientists in the US. Kilminster drove to Fermilab in the wee hours to watch the live stream there in a conference hall packed with people, some of whom in their pyjamas.
Hans Peter Beck from University of Bern can claim to have actually been in the same room with Peter Higgs and Francois Englert, theoretical “fathers” of the Higgs boson, on the day of the announcement. He was in CERN’s Council Chamber taking questions from media during the press conference following the announcement – as chair of one of the ATLAS experiment’s editorial boards on Higgs searches he had known about the result for quite some time. “It was my job to ask the right questions in order to make sure that what we will publish is sound, robust and will pass peer review in a flawless process.” Soundness was established from all sides – and both experiments published their individual Higgs discovery papers timely together on 4 July 2012. They have since published 348 Higgs papers unveiling more and more with ever increasing precision of what makes out this newly found particle, which is indeed in all aspects the Higgs predicted by Peter Higgs and others in 1964.“Still, there is room for unknowns and exotic decays or slight deviations from its predicted couplings are not ruled out,” Beck points out. “Finding new physics in and around this found Higgs boson is pursued with high pressure!”. So what do we know about the Higgs particle?
What is this Higgs thing anyway?
First of all, a bit of background. The Standard Model of Particle Physics needed the Higgs particle to explain why particles have mass at all, and why some are heavy, some light and some massless. It – or rather the field that it is associated with and that permeates all of space – was postulated by Robert Brout, François Englert and Peter Higgs in the mid-sixties. The Higgs boson is the manifestation of this field: the stronger a particle interacts with this field, the more Higgs bosons are exchanged, the heavier the passing particle is.
Scientists were pretty certain about many Higgs properties, including the particle combinations it is most likely to decay into. However, they had no idea about its own mass, which made the race between different colliders like the Tevatron and the LHC even more exciting. Given that the Higgs boson only appears in about one in a billion LHC collisions and that the traces it leaves are similar to the decays of other particles that emerge in proton-proton collisions, its discovery is a major achievement. Special signatures in the detectors of the Higgs decaying either into two photons, into four leptons – for example two electrons and two muons – or two leptons, and two undetectable neutrinos, manifesting themselves as missing transverse energy, finally led to the famous quote by the then-CERN DG Rolf Heuer “I think we have it.”
What have we learned since 2012?
“We’ve come a long way since then,” Ben Kilminster summarises the efforts of the past ten years. Scientists at ATLAS and CMS have not only established the main production and decay modes predicted by the Standard Model, but have also found other, less well-known paths. The most recent measurement is that of the Higgs decaying into tau leptons, the heaviest known leptons, which ties in with a number of open questions in particles physics. Measurements up to now show that the Higgs behaves as expected. “So far everything is consistent with Standard Model predictions,” Kilminster says. “But new physics beyond this model is subtle. It wouldn’t jump out at you. Are we really seeing everything? With more data, we’ll be able to do more precise checks.”
“It’s a fascinating particle,” Lea Caminada agrees. “First of all, it is unique because it is the only scalar boson, i.e. a boson without spin, a property that all other bosons have. Its decay modes as well as its production modes can tell us a lot about how the world works. We have taken a lot of data and have made a lot of progress in understanding the Higgs. So far, everything fits with the Standard Model.”
She points out, though, that ATLAS and CMS scientists are still in the process of analysing the data taken during the LHC’s last run. Her group has picked an especially tricky analysis: they want to find out if and how the Higgs boson couples to charm quarks. Because its mass is lower than that of the bottom and top quarks, its coupling to the Higgs are also much rarer, which makes it hard to measure and pick out of the huge amount of information coming from the LHC collisions. They are searching for events in which the Higgs boson is produced together with a charm quark. But they still need more data from the next LHC run to proof that charm and Higgs interact with each other. “There are still so many things that we want to look at,” Caminada says, “not only for the Higgs – does it couple to itself, for example? – but for a whole range of other searches as well.”
Similarly, Hans Peter Beck isn’t worried either that anybody will get bored any time soon. With twice the amount of data that exists from Runs 1 and 2 expected until the end of Run 3 in 2025, there’s enough for everyone. “Some 500 analyses based on Run-2 data are ongoing, so who knows, maybe further information about the Higgs is still hiding in there? It may even be that the Higgs offers a direct way to find dark matter at the LHC, as it could manifest when the Higgs would also decay to dark matter particles, as is predicted in some theorised extensions to the Standard Model.”
What might the future hold?
And scientists hope that it’s not only the Higgs that the next round of data from the LHC will reveal more about. More data and higher energies mean that hitherto unseen or very rare processes gain in visibility. “Many items on my top-ten list of interesting things to discover at the LHC are still waiting to be ticked off. Dark matter, for example, or “invisible Higgs” decays, or the explanation for why neutrinos have mass.” Ben Kilminster hopes that Run 3, the High-luminosity LHC or a future facility could add a few ticked boxes. “Maybe there’s also more than one Higgs boson? Maybe there are other ways for particles to gain mass? The Higgs we know is the simplest manifestation of this process, but it doesn’t have to be…”
Behind all of this is the quest to solve some of the biggest mysteries of humankind, Hans Peter Beck points out. “Let’s be clear, we’re not doing all of this to find new particles. When we find them, great. But the drive behind all this amazing science is to understand the Universe, the Big Bang, all of nature and how it works at ever increasing precision and insights.”
Author: Barbara Warmbein
Swiss Institute of Particle Physics (CHIPP)
c/o Prof. Dr. Ben Kilminster
Department of Physics