Matter and antimatter should have completely wiped each other out eons ago, leaving the Universe a very empty place.
Obviously that didn’t happen. Experiments at the Large Hadron Collider (LHC) may have uncovered new clues as to how we avoided this apocalypse, hinting at a surprising difference in the decays of particles called baryons and their antimatter twin.
Antimatter should be essentially identical to regular matter, except that its antiparticles have the opposite charge to their corresponding particles. That tiny difference has major consequences though – if ever the two shall meet, they annihilate each other in a burst of energy.
Models indicate that the Big Bang should have created matter and antimatter in equal amounts, but that implies that the total sum of particles formed in those early moments would have cancelled out long before stars, planets, and life could form.
Since we’re here to ponder the puzzle in the first place, it’s clear that something intervened. Through some unknown mechanism, it seems the cosmos has been left with a fraction more matter than antimatter.
CERN physicists have now analyzed LHC data to discover compelling evidence that there are other differences in how matter and antimatter behave, contributing to this imbalance to which we owe our very existence.
In theory, all particles should be subject to what’s known as charge-parity (CP) symmetry. Basically, if you flipped the charge of all particles in the Universe, and inverted their spatial coordinates, this mirror-Universe should still follow all the same laws of physics as our own.
But it turns out that some interactions violate this symmetry. A landmark 1964 experiment found that particles called K2 mesons could occasionally decay into products that they wouldn’t be able to without violating CP symmetry. It was very rare – about 2 in every 1,000 decay events – but it was enough to upset accepted views of physics at the time.
Many experiments in later decades found similar violations in a range of other particles, but only ever in other types of mesons. That would not have been enough to account for the rarity of antimatter. CP violations had not yet been observed in baryons, the other major class of particles that makes up the majority of observable matter in the Universe.
The new study has now finally identified CP violations in baryons, using an experimental setup that’s similar to the 1964 study – albeit on a much bigger scale. Instead of K2 mesons, the team focused on particles called beauty-lambda baryons (Λb) and their antiparticles.
If CP symmetry is at play, then Λb and anti-Λb particles should decay at the same rate. If there’s a significant difference between the two, however, that’s a sign of CP violation.
Researchers on the LHCb collaboration analyzed tens of thousands of decays captured during the first two runs of the LHC, between 2009 and 2018. Intriguingly, they found a difference of around 2.45 percent between matter and antimatter decays. That’s 5.2 standard deviations from zero, making it a large enough discrepancy to confirm an observation of CP violation.
“The reason why it took longer to observe CP violation in baryons than in mesons is down to the size of the effect and the available data,” says Vincenzo Vagnoni, spokesperson for the LHCb collaboration.
“We needed a machine like the LHC capable of producing a large enough number of beauty baryons and their antimatter counterparts, and we needed an experiment at that machine capable of pinpointing their decay products.
“It took over 80,000 baryon decays for us to see matter-antimatter asymmetry with this class of particles for the first time.”
This major breakthrough could provide clues to brand new forces and particles, which could help solve the enigma of why antimatter didn’t annihilate the entire contents of the Universe.
“The more systems in which we observe CP violations and the more precise the measurements are, the more opportunities we have to test the Standard Model and to look for physics beyond it,” says Vagnoni.
The research has been submitted to the journal Nature, and the pre-peer-reviewed version is currently available on arXiv.
