Editor’s Note: Don Lincoln is a physics researcher. He is the author of “The Large Hadron Collider: The Extraordinary Story of the Higgs Boson and Other Stuff That Will Blow Your Mind” and he produces a series of science education videos. Follow him on Facebook. The opinions expressed in this commentary are his.
Scientific understanding proceeds by fits and starts, with an occasional detour down a wrong path. However, today the world’s physicists have a real advance to celebrate. They have observed the most massive known fundamental subatomic particle directly interacting with an energy field that gives mass to the building blocks of the universe.
This has never been done before and it gives increased insight into a phenomenon that was only discovered just a few short years ago. This energy field is important because, without it, atoms couldn’t exist.
The discovery was announced at the Large Hadron Collider Physics (LHCP) 2018 conference in Bologna. Two independent experiments (ATLAS and CMS) searched for this phenomenon and both found clear evidence that it occurs. The two experiments have made their results available to the public and the scientific community, with the ATLAS paper being submitted and the CMS paper being published.
Our understanding of the origins of the mass of fundamental (e.g. containing no structure within them) subatomic particles is incomplete. In 1964, British physicist Peter Higgs and Belgian physicist Francois Englert independently developed ideas leading to what we now call the Higgs field, an energy field that permeates the universe and gives fundamental subatomic particles their mass. Mass is related to weight, and also to why it’s hard to move heavy objects in outer space, where there is no weight. Without this field, these particles would have no mass at all.
This field is kind of like water through which particles can move. Some particles have difficulty moving through the field, like a sumo wrestler slogging his way across the pool. These particles are the heavy ones, made massive by their interactions with the field.
Other particles interact much less, like a barracuda, which can dart through water very quickly. These streamlined fish are analogous to particles with very low mass.
Higgs and Englert’s ideas lay unconfirmed for nearly 50 years, until 2012, when scientists proved that they were correct. If the Higgs field was real, then a particle called the Higgs boson was predicted to exist. And indeed, this particle was discovered using the Large Hadron Collider (LHC), the world’s most powerful atom smasher located at the CERN laboratory in Europe. For their successful prediction, Higgs and Englert shared the 2013 Nobel Prize in Physics.
But Monday’s announcement isn’t about the Higgs boson itself, it’s about seeing the Higgs boson interact with the heaviest known denizen of the subatomic world. The most massive fundamental subatomic particle known is the top quark, discovered in 1995 at Fermilab, located outside Chicago. The top quark is unstable and can only be created and studied inside powerful particle accelerators.
Today’s discovery was made possible when scientists used the LHC to collide together beams of protons traveling at nearly the speed of light. In a handful of these collisions, both Higgs bosons and top quarks were made.
Because the Higgs boson gives particles mass, it interacts most strongly with top quarks. And, because of the strength of this interaction, the results reported Monday are an ideal laboratory in which to study the detailed nature of the origins of mass.
While the discovery of the Higgs boson in 2012 and subsequent measurements suggest that Higgs and Englert’s ideas published in 1964 were largely correct, there remain many mysteries in our understanding of the way the mass is generated.
The first is that their theory predicts a mass for the Higgs boson that is in striking disagreement with the value we currently measure. While scientists don’t have an answer for that embarrassing prediction, it is expected that studying how Higgs bosons and top quarks interact could give some important clues to answering the question.
Another mystery stems from the fact that the Higgs theory doesn’t arise from a deeper and more underlying theory. It’s simply a band-aid added to the theoretical edifice. This has long been known and is intellectually discomfiting. Studying the very strong interaction between Higgs bosons and the ultra-heavy top quarks could provide hints on what is going on.
Because the Higgs boson interacts most strongly with the top quark, measurements like the ones announced Monday have the potential to answer these mysteries. And, of course, particle physics is an exploratory and discovery-focused field of science. The high energy collisions like the ones reported today are prime candidates for possible discoveries of unexpected phenomena – after all, the LHC is an instrument of scientific exploration.
While today’s result is scientifically fascinating, it is also one that is personally exciting for me. Like many of my age cohort of particle physics researchers, I collaborated in the discovery of the top quark in 1995, the Higgs boson in 2012, and now today’s announcement. That’s not so unusual. Modern particle physics experiments involve hundreds or thousands of researchers and many in the Goldilocks cohort can make the same claim – not too old and not too young, but just right.
The first two measurements were difficult and barely possible when they were announced, but are now relatively straightforward. Monday’s measurement is a difficult one, both because of its rarity and the complexity of the data, but it, too, will eventually become common.
The Large Hadron Collider is the largest and most powerful tool for studying the laws of nature ever built. It is performing breathtakingly well and it’s just getting started. At the end of this year, scientists will have collected only about 3% of the data that is expected over the lifetime of the facility. At the end of 2018, the LHC will shut down for two years for refurbishment and upgrades and then return with a vengeance, running through 2030 and making who knows what kind of discoveries. I am looking forward to seeing what else we uncover.