Scientists have uncovered another clue in the effort to solve one of the great puzzles of modern physics: why there is more matter than antimatter in the universe.
The discovery relied on observations made with the world's largest machine, the Large Hadron Collider, which helps researchers to probe the fundamental nature of matter.
Everything we see around us is made up of subatomic matter particles such as protons and neutrons, which belong to a category of particles called baryons.
An experiment using the giant particle accelerator, based at CERN in Switzerland, has for the first time seen baryons form more matter than antimatter.
The findings could change our understanding of how small particles interact and help explain the absence of antimatter, said Tom Hadavizadeh, a physicist at Monash University and collaborator on the project.
"We haven't found the new physics yet, but it's given us a new way to look for it," Dr Hadavizadeh said.
The researchers have published their findings in Nature.
The mystery of the missing antimatter
The current leading theory in particle physics — the Standard Model — predicts that for every particle of matter that forms, a corresponding particle of antimatter forms.
Antimatter particles are identical to matter particles, but with their electrical charges reversed.
Scientists have observed similar amounts of matter and antimatter being generated when they create subatomic particles by colliding larger particles at high speed around large underground loops in the Large Hadron Collider.
But antimatter doesn't tend to stick around — if it collides with regular matter, both particles annihilate each other, releasing energy.
If antimatter and matter were truly created in equal amounts, as per the Standard Model, the universe wouldn't exist.
The problem for this theory is that the universe does exist, and it's mostly made of matter, with only tiny amounts of antimatter.
This "matter-antimatter asymmetry" is a major unresolved problem in physics.
"The way that we explain that is that at some point in the early universe, matter should have become slightly favoured over antimatter," Dr Hadavizadeh said.
"There's this little excess that remains once most of the antimatter and matter annihilates away, and that little excess is what we see left over today."
So where did this asymmetry between matter and antimatter come from?
Key particle seen making more matter
Ray Volkas, a physicist at the University of Melbourne who wasn't involved in the research, said that the Standard Model does have a way of explaining some of the matter-antimatter asymmetry.
"It's been known since the early 1960s experimentally that there actually is a subtle difference in the way that matter and antimatter interact [with other particles]," Professor Volkas said.
This subtle difference is called the charge-parity violation, or CP violation, and can help explain why there is less antimatter than matter.
While researchers had observed this asymmetry in some smaller particles, they had not yet observed it in baryons — a type of subatomic particle made from three quarks.
"Almost all of the matter that we come across is baryons," Dr Hadavizadeh said.
The team of more than 1,500 scientists from 20 countries, called the 'Large Hadron Collider beauty' (LHCb) collaboration, used the giant particle accelerator to look for examples of asymmetry in baryons.
They analysed libraries of data from the first few years of the experiment, looking specifically at curiously named "beauty" baryons.
They were able to spot baryons decaying in an asymmetric way — generating more matter than antimatter.
Professor Volkas says it is an "interesting result" but neither he, nor the LHCb researchers, think they've come close to solving the whole matter-antimatter mystery yet.
"The amount of CP violation in the Standard Model is actually not sufficient to explain cosmological matter-antimatter asymmetry," Professor Volkas said.
"It's one of the great mysteries of science."
Are we going to break physics?
Matter-antimatter asymmetry is just one problem with the Standard Model.
While it's beaten all the tests particle physicists have set for it over the decades, the theory has huge gaps in it.
It also can't explain gravity or dark energy, a mysterious phenomenon thought to be behind the acceleration of universe expansion.
"We don't want our theories to be totally wrong — in fact, they can't be because they work too well — but we want them to be incomplete so that we can add things," Professor Volkas said.
He says the LHCb experiment, and similar ones, are getting increasingly thorough at scrutinising the matter-antimatter mystery.
"What they're trying to do is examine this CP violation effect with ever greater precision to try to find if the standard theory continues to be verified, or if it will fail and we'll need to extend or modify the theory."
While this new result is consistent with the Standard Model, the researchers suggest it might point towards places where they can move beyond the theory.
Now that the researchers have measured the asymmetry in baryons, they'll be able to investigate this phenomenon more closely.
The study potentially "unlocks a whole new set of particles" to observe new types of physics, Dr Hadavizadeh said.
