When the news came on July 4, 2012, some scientists were moved to tears. Others jumped up and cheered. After decades of anticipation, physicists had finally discovered the Higgs boson.
In the years since that first detection, physicists have become increasingly familiar with this fundamental force-carrying particle produced by the invisible field that gives particles mass. They have improved the measurements of the mass, latitude, spin, couplings with different particles and other characteristics of the Higgs boson. They have been given more accurate readings than they expected to be able to make.
Yet there is still much to learn. Most Higgs measurements have not yet reached the precision scientists need to distinguish between models that could lead to new insights and discoveries. Some aspects of the Higgs boson haven’t even been explored yet.
“It took 60 years to first detect the Higgs boson, and in the last 10 years we’ve gotten to know it pretty well,” said Rebeca Gonzalez Suarez, a CERN physicist, the education and outreach coordinator for the ATLAS collaboration and an associate professor at Uppsala University in Sweden. “So far it looks very normal – very much like the expectations we have of it from the standard model. But there are still opportunities to surprise us.”
Today, physicists continue to refine their measurements — even developing ideas for future colliders — to fully reveal the mysteries of the Higgs boson and its place in the universe.
“By measuring the Higgs very accurately, we can gain a better understanding of physics beyond the Standard Model and perhaps find a portal to a new sector beyond the Standard Model,” said Kétévi Assamagan, a physicist at Brookhaven National from the United States Department of Energy. Laboratory in New York.
As physicists try to get an increasingly accurate understanding of the Higgs, here are four questions they hope to answer.
1. Does the Higgs boson interact with itself?
One of the biggest questions about the Higgs is how it might interact or mate with itself.
“I think this is the most important question about the Higgs right now,” said Caterina Vernieri, an assistant professor and a Panofsky fellow at SLAC National Accelerator Laboratory. “It’s really an unknown cornerstone in our understanding of the Higgs.”
Experiments have shown that the Higgs pairs with other particles, including a menagerie of fundamental particles such as the W and Z bosons, quarks, taus and muons. According to the standard model, it is also expected to pair with itself. Uncovering the exact details of how this happens could help physicists further refine the Standard Model and even shed light on the evolution of the early universe and the imbalance between matter and antimatter.
If physicists discover that the Higgs boson does not interact with itself in the way predicted by the Standard Model, it could change their understanding of the particle and suggest that the universe is not in the energy state that physicists predict, which would set the rules. influence how matter interacts.
To find out if the Higgs is pairing itself, physicists look to particle collisions for hints of Higgs boson pairs, or even rarer Higgs boson triplets, which would only be created if the Higgs pairs itself.
So far, data from experiments at CERN’s Large Hadron Collider haven’t seen a pair of Higgs bosons, but they haven’t ruled out the possibility either — there’s just not enough data yet. According to Standard Model predictions, the self-coupling should produce pairs of Higgs bosons infrequently in collision experiments — more than 1,000 times less often than a single Higgs boson is produced.
Physicists hope future runs will help mitigate this, as the LHC turns out to be more Higgs boson-producing events.
2. How does it link Higgs to other particles?
While physicists don’t yet know if the Higgs pairs with itself, they do know that it pairs with other particles. In some cases – such as with the top quark, the heaviest of the Standard Model particles – the linkage is quite well understood. But physicists are just beginning to get a handle on how much other particles, such as the relatively lighter muon, interact with Higgs bosons.
How much a particular particle will couple with a Higgs is predicted by the Standard Model and is related to the mass of the particle: the more massive the particle, the greater the coupling. So far, coupling measurements are consistent with these predictions. But the accuracy of these measurements is not yet great enough to see whether there could be deviations from the Standard Model. By knowing exactly how the Higgs pairs can help scientists understand how particles get their mass.
“If we see discrepancies when we do precision measurements of the coupling of the Higgs boson with other particles, that could tell us if there is new physics,” Vernieri says.
3. Are there other Higgs particles?
So far, physicists have only found one Higgs boson, which the Standard Model predicts. But some alternative theories that extend the Standard Model call for many more types of Higgs particles.
“There’s no reason there shouldn’t be more,” said Sally Dawson, a theoretical particle physicist at Brookhaven National Laboratory. “There’s a whole host of possibilities on what that could look like.”
Some models suggest that there is a version of the Higgs that has different properties than the boson we know. The Higgs boson, discovered in 2012, has no spin and no electrical charge, but other Higgs particles may have different characteristics. Other models propose that there is one type of Higgs that interacts with heavy particles and another that interacts with lighter particles. Or maybe the Higgs boson we see is actually a composite of several different particles.
“Any additional Higgs we can discover would indicate that there must be new physics,” Assamagan says. “It might help us explain some things that don’t necessarily fit in the standard model.”
Some phenomena that can be explained by additional Higgs particles are dark matter, neutrino oscillations, the mystery of neutrino masses and why there is an imbalance between matter and antimatter in the universe. If there are other Higgs particles, physicists hope to see their footprints in collision experiments.
4. Is the Higgs connected with dark matter or other unusual particles?
Because the Higgs boson helps explain where mass comes from, many scientists believe it must interact with dark matter: the mysterious substance that appears to be connected to everyday matter only by gravity.
“The Higgs could be the portal between us and this dark sector that could be hiding dark matter,” said Gonzalez Suarez.
Certain theories predict that dark matter interacts with normal matter by interchanging Higgs bosons. If this is the case, a collision that produces Higgs particles could also create dark matter particles.
“The Higgs in the Standard Model doesn’t decay into dark matter, but some models suggest there is an interaction,” Dawson says. “It’s very possible that measuring Higgs properties can tell you something about dark matter.”
In other scenarios, when the Higgs decays, it can produce other completely new, invisible particles that physicists haven’t even thought of. No unusual particles have been seen in collision experiments — where their existence would be inferred from missing energy in the aftermath of a collision — but physicists aren’t done looking.
How will physicists answer these questions?
Physicists are studying the Higgs at the LHC, which is just rising again after a three-year hiatus following upgrades to the experiments and accelerator complex and pandemic delays. These upgrades are intended to allow physicists to make more precise measurements of the Higgs boson. However, unless there are very large deviations, this precision is probably not sufficient to see if there are deviations from the Standard Model.
After its current run, which is scheduled to last until the end of 2025, the LHC will receive another upgrade that will transform the throttle into the next-generation High-Luminosity LHC, which is scheduled to run until approximately 2040. This will allow physicists to measure how the Higgs couples to other particles to an uncertainty of about 5%. While physicists expect to produce more Higgs bosons during this high-brightness phase, measuring self-coupling will still be a challenge.
In the long run, scientists are thinking about ways to study Higgs bosons outside of the LHC, which is designed to study a myriad of phenomena through proton-proton collisions. Protons collide in a wide variety of ways, giving scientists a lot of ground to explore if they’re not sure where to look. But they are messy, which can make it difficult to locate specific types of particles and events.
That’s why some scientists have proposed a future “Higgs factory,” which they could specifically tune to produce many Higgs bosons. Instead of colliding protons, a Higgs factory would collide matter and antimatter pairs, such as electrons and positrons. These particles would annihilate each other, eliminating much of the messiness of the collisions observed at the LHC and giving scientists a closer look at the Higgs bosons produced. Such an instrument should enable scientists to achieve 1% accuracy for precision measurements of most couplings and to investigate theoretical predictions for Higgs self-coupling.
Meanwhile, physicists aren’t out of hopes that something unexpected will pop up in ongoing experiments. With every upgrade to the LHC, there is a chance that physicists may see new particles or compounds with new, hidden sectors. Or perhaps unexpected factors could cause pairs of Higgs, for example, to be produced in greater quantities than expected, Gonzalez Suarez says.
“You never know with experimental science,” Dawson says. “It’s always exciting because there are so many possibilities, and we don’t know which one is the right one.”