The ALICE experiment has released some interesting new data today, and while there are no great new discoveries in these latest results, they have improved our knowledge of a basic symmetry of nature. 

Nearly a century ago physicists were excited by the early success of quantum mechanics and were trying to construct a relativistic theory of quantum mechanics. In the midst of this work, British physicist Paul Dirac developed a new equation that described the properties of atoms and smaller particles, and which still carries his name, but his theory had an apparent flaw. It predicted that for each of the known matter particles, there would exist a second particle with the same mass and the opposite electric charge. In particular experimentalists had been studying electrons, but had never found any form of antielectron. For many years people tried to remove these particles, or to explain why they might gain a 2000 fold increase in mass to be a proton. Then the positron was discovered and identified as the anti-electron, leading to a Nobel Prize for Dirac, based on what he initially considered to be an error in his theory.

Since that time scientists have generated large numbers of positrons and anti-protons, and studied them extensively. Many of the biggest experiments of the last fifty years have been based on the production and annihilation of anti-particles, and they have been used to generate many of the more exotic and short-lived particles.

However until very recently, anti-atoms did not exist. Since anti-protons and anti-electrons have similar properties to ordinary protons and electrons, one would expect to find them in bound states forms anti-hydrogen or even anti-helium or anti-lithium. (In fact, the laws of physics and our best models of the Universe predict large clouds of anti-atoms and entire anti-stars and anti-planets. And yet none have ever been observed in nature, leading to one of the biggest unsolved puzzles in modern physics. But that is another story...)

Using the Large Hadron Collider in Switzerland, ALICE managed to generate a number of anti-nuclei and compared their mass-to-charge ratio to their ordinary counterparts. If the Standard Model of particle physics is correct, then these anti-nuclei should have the exact same properties (aside from electric charge) as regular nuclei. The LHC has already produced anti-hydrogen, and the results of that study are still being analyzed. Now they have also produced heavier nuclei.

The result, which they published today in Nature Physics, has increased the evidence the matter-antimatter symmetry. Specifically they generated large numbers of both anti-deuterons and anti-helium nuclei, and found no difference in the masses and charges (aside from the expected opposite signs of the charge on the constituent particles). In fact the limits they have now placed on breaking matter-antimatter symmetry are now at least an order of magnitude better than any previous results.

Speaking as a theorist, it might have been more fun if they had found some form of symmetry breaking. This result simply increases our confidence in existing theories, but provides nothing new to study and debate. However even this increased proof of matter-antimatter symmetry provides an interesting discussion for cosmologists, as it throws further mystery on the question of where all the anti-matter in the Universe has gone. But that is a story for another day...