Matter-Antimatter Asymmetry: CERN’s LHCb Experiment Detects Hints Of Charge-Parity Violation In Baryons

Why does the universe we live in exist? After all, our current understanding of the Big Bang — the phenomenon that gave rise to the universe roughly 13.8 billion years ago — tells us that equal quantities of matter and antimatter were created at the time.

The question, then, is — where is all the antimatter?

That is the question that physicists have, for the longest time, tried to find the answer to. So far, though, they have failed to find one.

And why didn’t matter and antimatter particles annihilate each other at the moment of creation, as they are wont to do whenever they come in contact with each other?

The answer, for now, at least, is that we don’t know. What is obvious is that there is a fundamental difference between matter and antimatter — one that gave the former an edge over the latter in the race for survival. Either significantly more matter was created by the Big Bang, or there is an as-of-yet-undiscovered asymmetry between matter particles and their antimatter counterparts.

That is why scientists have been hunting for violations of what is known as charge-parity (CP) symmetry — a central tenet of the Standard Model of particle physics, which states that the laws of physics remain unchanged even if a particle is replaced with its antiparticle, which has the opposite charge, and if its spatial coordinates are inverted.

Here’s where we hit a dead end. So far, the extent of CP violation detected among elementary particles is not significant enough to explain the observed matter-antimatter asymmetry. And, until now, even these tiny hints of CP violations had not been detected in baryons — a class of particles that includes protons and neutrons.

Now, in a study published Monday in the journal Nature Physics, researchers from the LHCb collaboration at the European Organization for Nuclear Research (CERN) describe the first ever detection of CP violation in baryons. Specifically, the scientists detected differences in spatial orientation of decay products of a class of matter and antimatter particles called Lambda-b baryons.

“Overall, the statistical significance – which is how physicists refer to the probability that this result hasn’t occurred by chance – is at the level of 3.3 standard deviations, and a discovery is claimed when this value reaches 5 standard deviations,” CERN said in a statement. “These results ... will soon be updated with the larger data set collected so far during the second run of the LHC. If this earlier evidence for CP violation is seen again with greater significance in the larger sample, the result will be an important milestone in the study of CP violation.”

It is important to note that the extent of CP violation detected in these three-quark bodies is not yet enough to explain the current imbalance between matter and antimatter. However, scientists hope that further measurements may just reveal deviations from the Standard Model and provide the answer to the million-dollar question — why is there something rather than nothing?

“We proved that we are there,” study co-author Nicola Neri, a researcher at Italy’s INFN, told Symmetry magazine. “We have this capability, and we will be able to do even more after the detector is upgraded next year.”