The First Observed Difference Between Matter and Antimatter in Baryons

At the Big Bang, equal parts of matter and antimatter was formed, but today there is almost no antimatter left. To understand what happened when antimatter disappeared, particle physicists all over the world first try to show that there are differences between matter and antimatter. The next step is to explain why the deviations occur theoretically.

The discovery was made at the world’s largest particle accelerator, Large Hadron Collider, LHC, at CERN at one of the four detector complexes, the LHCb experiment, which is designed to study very small differences between matter and antimatter.

As early as the mid-60s, it was possible to measure differences between matter and antimatter experimentally based on the Standard Model, in the form of so-called CP-symmetry violations. But this one has just been possible for mesons, subatomic particles consisting of a quark — antiquark pair.

For decades, researchers have also searched for differences between baryons, consisting of three quarks, and antibaryons, consisting of three antiquarks, but have not been able to confirm any difference until now.

The researchers have now, for the first time, observed differences between matter and antimatter in the Λ⁰_b-baryon. This “beauty baryon” has, just like the proton, a down quark and an up quark, but the proton’s second up quark has been replaced by the much heavier beauty quark.

The Universe’s visible matter consists mainly of protons, neutrons and electrons. Both protons and neutrons are built by three quarks, why it is particularly important that differences for baryons has been observed. Since these heavier baryons have similar properties as protons and neutrons, they may give new insights and clues to what has happened to the missing antimatter.

“The first step is to find these differences, which we have now achieved. The next step is to find even more differences between baryons and antibaryons. In this way, theoretical physicists can lay a complicated puzzle, answering the question if all observations put together can be explained within the rules laid down by the Standard Model, or if the model needs to be supplemented with new physics”, says Patrik Adlarson, member of the LHCb experiment, and researcher in nuclear physics at the Department of Physics and Astronomy.

Because of the mass of the beauty quark, the examined baryon has a mass which is five to six times larger than the proton mass. It is also relatively short lived and decays to more stable particle configurations that the experiment measures. If the CP-symmetry is preserved, the decay chain for matter and antimatter must be equally frequent. But the researchers have now discovered that it differs how it decays compared to its corresponding anti-particle, “anti-Lambda-b”, and thus shows that matter and antimatter behave somewhat differently.