July 4 will be 10 years since the discovery of the Higgs boson, the long-sought particle that imparts mass to all elementary particles. The elusive particle was the last missing piece in the Standard Model of particle physics, our most complete model of the universe.
In the early summer of 2012, signs of the Higgs boson were detected at the Large Hadron Collider (LHC), the world’s largest particle accelerator, operated by CERN, the European Organization for Nuclear Research. The LHC is designed to smash together billions and billions of protons for the chance to produce the Higgs boson and other particles predicted to be created in the early universe.
Analyzing the products of numerous proton-to-proton collisions, scientists recorded a Higgs-like signal in the accelerator’s two independent detectors, ATLAS and CMS (the Compact Muon Solenoid). In particular, the teams observed signs that a new particle had been created and then decayed to two photons, two Z bosons or two W bosons, and that this new particle was likely the Higgs boson.
The discovery was unveiled within the CMS collaboration, which included more than 3,000 scientists, on June 15, and ATLAS and CMS announced their respective observations to the world on July 4. More than 50 MIT physicists and students contributed to the CMS experiment, including Christoph Paus, a physics professor, who was one of the experiment’s two principal investigators to organize the search for the Higgs boson.
As the LHC prepares to restart “Run 3” on July 5, MIT News spoke with Paus about what physicists have learned about the Higgs boson over the past 10 years, and what they hope to discover with this next deluge of particle data.
Q: Looking back, what do you remember as the key moments leading up to the discovery of the Higgs boson?
A: I remember that by the end of 2011, we had collected a significant amount of data, and there were some initial hints that there could be something, but nothing convincing enough. It was clear to everyone that we were entering the critical phase of a potential discovery. We still wanted to improve our searches and so we decided, what I think was one of the most important decisions we made, that we needed to remove the bias – that is, our knowledge of where the signal might appear. Because it is dangerous as a scientist to say ‘I know the solution’, which can subconsciously influence the result. So we made that decision together in the coordination group and said, we’re going to get rid of this bias by doing what people call a “blind” analysis. This allowed the analyzers to focus on the technical aspects and make sure everything was correct without worrying about being influenced by what they saw.
Then, of course, there had to be the moment where we unblind the data and really look to see if the Higgs is there or not. And about two weeks before the scheduled presentations on July 4 where we finally announced the discovery, there was a meeting on June 15 to show the analysis with its results to the collaboration. The most significant analysis turned out to be the two-photon analysis. One of my students, Joshua Bendavid PhD ’13, led that analysis, and the night before the meeting, only he and one other person on the team were allowed to unblind the data. They worked until 2am when they finally pushed a button to see what it looks like. And they were the first in CMS to have that moment to see that [the Higgs boson] was there. Another student of mine working on this analysis, Mingming Yang PhD ’15, presented the results of that search to the Collaboration at CERN the following afternoon. It was a very exciting moment for all of us. The room was warm and filled with electricity.
The scientific process of the discovery was very well designed and executed, and I think it can serve as a blueprint for how people should do such searches.
Q: What else have scientists learned about the Higgs boson since the particle’s detection?
A: At the time of the discovery, something interesting happened that I didn’t really expect. While we used to talk about the Higgs boson, we got really careful when we saw that “narrow peak.” How could we be sure it was the Higgs boson and not something else? It certainly looked like the Higgs boson, but our vision was kind of blurry. In subsequent years, it might turn out not to be the Higgs boson. But as we now know, with so much more data, everything is completely consistent with what the Higgs boson is expected to look like, so we felt comfortable calling the narrow resonance not just a Higgs-like particle, but just the Higgs boson. particle. And there were a few milestones that really made this the Higgs as we know it.
The first discovery was based on Higgs bosons decaying to two photons, two Z bosons, or two W bosons. That was only a small part of the decline that the Higgs could undergo. There are many more. The amount of decay of the Higgs boson in a given set of particles depends to a large extent on their mass. This characteristic is essential to confirm that we are really dealing with the Higgs boson.
What we have since found is that the Higgs boson decays not only to bosons, but also to fermions, which is not clear because bosons are force-carrying particles, while fermions are matter particles. The first new decay was the decay to tau leptons, the electron’s heavier sibling. The next step was the observation of the Higgs boson decaying to b-quarks, the heaviest quark to which the Higgs can decay. The b quark is the heaviest sibling of the down quark, which is a building block of protons and neutrons and thus of all atomic nuclei around us. These two fermions are among the heaviest generation of fermions in the Standard Model. It was only recently observed that the Higgs boson decays at the expected rate to muons, the second and thus lighter generation charge lepton. Also, the direct coupling with the heaviest top quark was established, which together with the muons span four orders of magnitude in terms of their masses, and the Higgs coupling behaves as expected over this wide range.
Q: As the Large Hadron Collider gears up for its new “Run 3”, what do you hope to discover next?
A very interesting question that Run 3 might give us some first hints about is the self-coupling of the Higgs boson. As the Higgs couples to any solid particle, it can also couple to itself. It’s unlikely there’s enough data to make a discovery, but the first hints of this link would be very exciting to see, and this represents a fundamentally different test than what’s been done so far.
Another interesting aspect that more data will help clarify is whether the Higgs boson could be a portal and decay to invisible particles that could be candidates for explaining the mystery of dark matter in the universe. This is not predicted in our Standard Model and so would reveal the Higgs boson as an impostor.
Of course, we want to double all the measurements we’ve taken so far and see if they continue to meet our expectations.
This includes the upcoming major upgrade of the LHC (running from 2029) to what we call the High Luminosity LHC (HL-LHC). During this program, a factor of 10 more events will be collected, which means for the Higgs boson that we can observe its self-coupling. For the distant future, there are plans for a Future Circular Collider, which could eventually measure the total decay width of the Higgs boson, independent of its decay mode, which would be another important and highly accurate test of whether the Higgs boson is an impostor. .
However, like any good physicist, I hope we can find a crack in the standard model’s armor, which has held up all too well so far. There are some very important observations, for example the nature of dark matter, that cannot be explained with the Standard Model. All of our future studies, from Run 3 on July 5 to the very future FCC, will give us access to completely uncharted territory. New phenomena may emerge and I like to be optimistic.