20 June 2022
I’m a PhD student working on the ATLAS experiment from Vancouver, Canada. For my PhD I’ve studied the way the Higgs boson interacts with W bosons, which are the type of fundamental particle that is involved in radioactive decay. I feel like I’ve spent much of the past five years as a PhD student getting to know the Higgs boson, which has joined me on many sleepless nights and hours at the computer. But five years before that, I could have barely described what a particle is (actually, I might still struggle…). The Higgs boson discovery was a fact by the time I joined the world of particle physics at all. As a result, I feel envious of those who were tuned in enough to feel a thrill when the Higgs boson was discovered! From my perspective, the Higgs boson has been a unique tool to use in our pursuit of the answers to some pretty big questions.
We know that the Standard Model, our prevailing theory of fundamental physics, is not perfect. It doesn’t include gravity or explain dark matter, among many other shortcomings. I became interested in particle physics because I loved the idea of understanding the underlying structure of our universe, and I remain interested in it because of the promise of replacing the Standard Model with a more complete theory of everything.
Halfway through the lifetime of the Large Hadron Collider (LHC), though, we haven’t made any definite observation of physics that falls outside of the Standard Model’s predictions. (There may be some hints, but they remain to be investigated.) As a result, the path toward a theory of everything – or at least a more complete theory than the Standard Model – isn’t any clearer than it was before the Higgs boson discovery. On top of this, all the particles that the Standard Model predicts have been found. Of course, both of these facts also represent triumphs of modern physics: we’ve developed what is arguably the most successful scientific theory of all time, and we’ve managed to find every particle it predicts despite the vastly different energy ranges required to do so. But taken together, they also make for what sounds like a fairly boring period of particle physics. I find this particularly disheartening when I consider how much it differs from the situation just a few decades ago. The first quarks were only discovered in the 1960s and 70s, the W and Z bosons in the 1980s, and that still left a couple more particles (the top quark and tau neutrino) for the 1990s. The physicists of the early aughts and 2010s had the Higgs boson to look forward to. And now, as I launch my career, it can feel like there’s nothing left within reach – no particles predicted by the Standard Model, and apparently no new ones either. No matter how much I love the day-to-day aspects of particle physics research with ATLAS, I also want to believe that my hours of work are moving us closer to being able to answer the big questions that got me interested in this field in the first place.
Beyond this current lack of particle physics drama, though, there’s a lot of reason to remain hopeful. I find it helpful to remember just how dramatic and recent the Higgs boson discovery really was. It marked the first observation of an entirely new particle whose significance to the Standard Model is enormous. It also provided access to the study of an entirely new force, the Yukawa interaction, which occurs between Higgs bosons and fermions (particles like electrons and quarks) and gives them mass. One decade is not nearly long enough to have learned everything there is to know about a new particle and a new interaction. We have much more work to do before we can be satisfied that we’ve left no stone unturned in our pursuit of new physics related to the Higgs boson.
And there’s good reason to think that some sign of physics beyond the Standard Model may still be discovered by looking closely at the Higgs boson. Many variations on the Standard Model have been theorized, each aimed at addressing some of its shortcomings. Some of the theories claim that the Higgs boson would interact more or less strongly with other particles than the Standard Model predicts; in other words, they modify the predicted coupling strengths of the Higgs boson. If we can measure these coupling strengths, we can check if they agree with the predictions of the Standard Model, or of a different theory. Since the coupling strengths are directly related to how often a Higgs boson is produced or decayed, we can count the number of Higgs bosons that are produced and decay via certain pathways and extract a measure of the coupling strengths from that number. In practice, this is a complicated process. We need to efficiently identify Higgs bosons while excluding the many other less-interesting processes that occur much more frequently at the LHC. In the years between 2015 and 2018, there were 16 million billion recorded proton-proton collisions, but only about one in a billion of those produced a Higgs boson. For this reason, these sorts of measurements take a long time to perfect. As we learn more and more about our detector, and take advantage of techniques like machine learning to enhance our discrimination of Higgs boson processes from everything else, we are now able to measure certain coupling strengths with a precision of about 3%. This is on the order of the size of deviation predicted by some important theories of new physics. In other words, we are only now closing in on the experimental precision required to check if the Standard Model’s predictions really do best describe the Higgs boson’s behaviour, or if there is instead some new theory that seems to do a better job.
The Higgs boson discovery launched us into a new era of physics. Having found every particle predicted by the Standard Model, we’re now forced to take more creative routes toward the discovery of new physics. Above all, I find it encouraging to remember that the Standard Model is definitely not the final answer. There has to be new physics hiding out there somewhere. In a way, this post-Higgs-discovery era has all the excitement that characterized the years leading up to 2012: something new will be found, at some point, and at that point we’ll surely learn something fundamental about our universe.