In physics, symmetry is a big deal, specifically with regards to matter and antimatter. If both were created in equal quantities in the Big Bang 13.8 billion years ago, why has matter prevailed, but antimatter (which has the same mass but opposite charge) has not?
This is a continuing problem in physics, and while several models have been proposed, none have solved it. This latest research adds another piece to the puzzle.
Using the ALICE (A Large Ion Collider Experiment) instrument at the LHC (Large Hadron Collider) at CERN, a team of scientists has confirmed the prediction that matter and antimatter are an exact mirror of each other. In particular, the mass difference of light nuclei and their antinuclei were found to be almost identical. In short: They should completely annihilate each other. Another study, using CERN’s BASE (Baryon Antibaryon Symmetry Experiment) instrument, came to the same conclusion last week.
While the result, published in Nature Physics, is welcome for providing further proof that matter and antimatter are opposites, it complicates things when it comes to understanding the cosmos.
Physicists have been trying to work out if all physical laws show a type of symmetry called CPT (charge, parity and time reversal) symmetry, or invariance. The three tenets of this are that a particle has a reversed charge, is replaced by a mirror image and undergoes a reversal of time in its antimatter form. “The key here is all fundamental forces known today must fulfil CPT invariance,” Constantinos Loizides of the Lawrence Berkeley National Laboratory (LBNL) in California, who works on the ALICE instrument and was involved in the research, told IFLScience. “They are the same when applied to an antiparticle instead of a particle, traversing in a mirrored space backwards instead of forwards in time.”
The video above explains CPT symmetry in detail. Research Square/ALICE Collaboration/CERN.
In this particular research, CPT invariance was found to hold true for deuterons and antideuterons, and helium-3 and antihelium-3 nuclei – within the uncertainties of the measurement. These light nuclei were formed in the ALICE instrument by smashing lead nuclei together at high energies. By measuring how long it took for the resulting light nuclei to travel to a detector, their mass-to-charge ratios could be worked out, which were found to be the same for the nuclei and antinuclei. In addition, the binding energies holding together all the nuclei were deduced to be the same across the matter/antimatter pairs
These results, which are up to 100 times more precise than previous similar measurements, confirmed CPT invariance. Had CPT not been true for these lighter nuclei, it would have clearly hinted at unknown physics beyond the Standard Model – which explains most of subatomic physics as we know it to date.
The Standard Model is limited – it cannot explain dark matter, for example – so physicists have been trying to create new models to explain these more exotic phenomena. While not unexpected, this measurement constricts the possible theories that could explain what’s going on in the universe beyond the Standard Model.
And such measurements may have also implications for the Big Bang. “How is it possible, when every law in nature is almost equal between particles and anti-particles, that matter was left from the Big Bang?” said Loizides.
But, as Loizides notes, this is just one small piece in the much larger cosmic puzzle. “It will take a lot of measurements to understand better what made the universe as we see it today,” he said. "So, it's not expected that a single measurement can make an incredible difference."