Evrim Yazgin
Cosmos science journalist
In 2012, physicists at CERN’s Large Hadron Collider (LHC) announced the discovery of the Higgs boson, sometimes referred to as the “God particle”.
The Higgs boson was the last piece of the puzzle in the Standard Model of Particle Physics which describes the known phenomena in the universe. The particle was first theorised in 1964 by British physicist Peter Higgs as the particle which lends all other particles their mass.
Since its discovery, the Higgs boson has been investigated in experiments to try and find “new physics”.
Why do we need new physics?
“For people outside of the very closed circle of particle physicists, the expression ‘new physics’ probably makes no sense,” admits particle physicist Csaba Balazs, a professor at Monash University in Melbourne.
“The Standard Model of particle physics explains 99% of all phenomena in the in the solar system. It has been called the most successful scientific theory ever. It works extremely well,” Balazs tells Cosmos. “However, we know that it’s not the final truth because it doesn’t explain certain things.”
In fact, Balazs mentions a litany of the Standard Model’s shortcomings.
“It cannot do anything with dark matter, dark energy, dark radiation, dark forces.”
Balazs, of course, is referring to the fact that observations show 80% of the matter in the universe is not the normal matter we can see, but dark matter which has yet to be directly observed.
Beyond normal matter and dark matter, about 70% of the universe is believed to be made up of dark energy – the mysterious ingredient causing the accelerated expansion of the universe.
Unfortunately for the Standard Model, it’s shortcomings don’t stop there.
“It doesn’t explain why and how we are here,” Balazs says. “The Standard Model cannot tell us the cosmic evolution of how matter evolved, how matter came about.”
These gaps are what physicists refer to as “new physics”.
Finding new physics in a hopeful place
“When I was a student, back decades ago,” Balazs reminisces, “my supervisor told me that I am extremely lucky, because in my lifetime the LHC is going to be operational and it’s going to make amazing discoveries.”
“And he was right. The collider even discovered the Higgs boson.”
But Balazs notes that the Higgs boson was already part of the Standard Model. It was predicted and, therefore, its experimental discovery completed the Standard Model but didn’t go beyond it.
Physicists have been searching for ways to delve deeper into the fundamental makeup of the universe. The obvious path, however, isn’t always the best.
“If you wanted to know more, if you wanted to discover physics beyond the standard model, you just had to build another even bigger collider for even more money,” Balazs says. “That didn’t seem sustainable. So, my generation went on and asked, how can we get an experimental handle on new physics beyond colliders?”
To that end, Balazs co-founded an international collaboration called GAMBIT – the Global and Modular Beyond-the-standard-model Inference Tool. The team of more than 100 physicists attempts to bring together all experimental data to “find traces of new physics”.
This includes data from astrophysical, cosmological, collider, low energy and other types of experiment.
From this data, GAMBIT then performs “sophisticated mathematical inference to find the slightest deviation from the Standard Model,” Balazs says.
Deviations of note
Just how big a deviation from the theoretical expectation is worthy of further investigation? How far off do the numbers have to be before there’s a hint of new physics?
Balazs explains that this is a question of statistics.
“These are measured in statistical significance.” Helpfully, he paints a picture.
“When you flip a coin and then you get 49 times head and 51 times tails, then that’s not statistically significantly different from the expected 50-50,” he says. “Statistically significant would be if you bias the coin and get something in the region of 40-60 or more. And even 40-60 actually can happen 1 in 1,000 or 1 in 10,000 times. So even that is just beginning to be statistically significant, but not very significant.”
“You get into significant regions when you get 10, 20 or 30 out of 100 tails, and the rest is heads.”
Balazs says, however, that, as is often the case with theoretical particle physics and fundamental questions of the nature of the universe, the game is not that simple.
“There are always unknowns, and unknown unknowns, right?” he says. “You cannot 100% trust the experiment.”
Balazs raises an example.
“A collaboration called DAMA claimed discovery of dark matter with high statistical significance,” Balazs says. These controversial claims go back to the late 1990s but have not been replicated in any other experiment.
“This is not being trusted by the physics community because there are doubts about the uncertainty around the very calculations and measurements that that yield this deviation.”
Higgs’s place in new physics
Physicists are looking very hard for new physics to plug the gaps in our understanding of the universe.
“The Higgs boson is one of the most promising avenues toward new physics today, and the simple reason is because it’s the least measured,” Balazs says. “It is also related to this symmetry breaking phenomenon. And the symmetry breaking itself is the least explored, both experimentally and theoretically.”
“Yes, the Higgs boson can do amazing things for you if you are lucky.”
An example of this is in the search for the elusive particles which make up dark matter.
Because the mass of all other particles comes from their interactions with the Higgs field, physicists say the Higgs boson “couples” to everything that has mass. This fact may aid researchers in the quest to find dark matter.
“If dark matter is made of some new, exotic particle that hasn’t been discovered yet, we already know that that particle must have mass, because dark matter exerts gravity,” Balazs explains. “If the Higgs boson couples to everything that has mass, there is no reason to think it does not couple to the dark matter particle.”
“Using the Higgs boson, you can create the dark matter particle in the lab, or using these interactions of Higgs Boson, you can detect dark matter,” he says.
How it’s going
Despite the possibilities, no new physics has been found through investigations of the Higgs boson yet.
In May, physicists from the Polish Academy of Sciences published a paper in the journal Physical Review Letters which shows that LHC data on the Higgs boson does not reveal any new physics.
Balazs is not surprised by the result, saying that at least a dozen similar studies are produced every year and not all are published.
“Basically, what they are saying is we haven’t found anything,” he says. “It gives a glimpse of what’s going on with the Large Hadron Collider.”
But the search for new physics continues.
“The quest of particle physics is to discover physics beyond the standard model, because we are desperate to know the next layer,” Balazs says.
“The amazing thing is that theory went well ahead of experiment. There are dozens, if not hundreds, of new physics models which outline what the next new physics theory will look like. Ironically, we don’t have enough data to differentiate between them. This is basically why we created GAMBIT – to start sorting out the candidates of new physics, try to constrain them and slowly, select the right ones.”
And Balazs notes that the search for new physics is not all head-in-the-clouds work but might directly impact our lives one day.
“Your phone is based on physics which was established 150 years ago,” he says. “If at that time, Maxwell and his colleagues didn’t work out those electromagnetic laws of physics, we would have no computers, we would have none of this.
“So, in 100 years it’s going to be big. It’s going to change the world.”
