Measurements of the 'quark soup' provide glimpse into the mysteries of antimatter
The mass of an atomic nucleus is equal to the mass of its anti-particle, the anti-nucleus. This has recently been proven by tens of millions of unprecedentedly accurate measurements by ALICE, one of the experiments being conducted on CERN's LHC particle accelerator. A tiny difference in mass would be an indication of a violation of fundamental symmetry, which would in turn be a possible clue as to why our universe contains almost no antimatter, despite the fact that an equal amount of matter and antimatter should have been formed during the 'big bang'. "The mystery of the missing antimatter still hasn't been solved, but our results bring us a step further", says FOM workgroup leader Thomas Peitzmann from Utrecht University. The results of their research were published this week in Nature Physics.
Using CERN's Large Hadron Collider, the most powerful particle accelerator in the world, Peitzmann and his colleagues were able to reconstruct the extreme conditions prevalent in the universe just a few microseconds after the big bang. This can best be described as a white-hot 'primordial soup' of elementary particles. In fractions of a second, these elementary particles form an equal number of atomic nuclei and anti-nuclei. "Since we were able to create large quantities of anti-nuclei, we were able to determine the mass of both particles 10 to 100 times more accurately than previous experiments were capable of. With this extremely high degree of accuracy, we were able to prove that atomic nuclei and anti-nuclei have the exact same mass," according to Peitzmann.
Symmetry
According to current knowledge, particles and their anti-particles are the same in every way, except for their opposite charge. This far-reaching symmetry has been predicted by the current theory of elementary particles. With perfect symmetry, the big bang should have created an equal amount of matter and antimatter, but that is not the case. This is one of the reasons for testing this symmetry as accurately as possible.
Experimental challenge
Physicists have developed a number of theories to explain the lack of antimatter in the universe. Some of these theories are based on a small asymmetry between matter and antimatter. Depending on this asymmetry, certain of the characteristics of matter and antimatter could differ from one another. Unfortunately, it is extremely difficult to determine these characteristics, because antiparticles do not occur on earth under normal circumstances. In order to create anti-nuclei, the ALICE experiment collides lead nuclei at near-light speeds to create a situation known as the 'quark soup' with temperatures of 2,000 billion degrees, or 100,000 times hotter than the core of the sun.
A step further
An asymmetry in the mass of atomic nuclei therefore does not seem to explain the missing antimatter. Scientists have known that there is no difference between the mass of electrons and their anti-particles, positrons. "Perhaps the asymmetry has to do with only one of the forces we know about, such as the nuclear strong force. Electrons and positrons do not feel this force, so in order to observe its effects, you have to study atomic nuclei", Peitzmann explains. "It would certainly have been exciting if we had observed a difference, because then we would have to change the Standard Model. But on the other side, we now have a better understanding of the symmetry between matter and antimatter."
This research was funded in part by FOM, NWO, and the ERC.
Reference
Precision measurement of the mass difference between light nuclei and anti-nuclei, Nature Physics, doi:10.1038/nphys3432
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Thomas Peitzmann, +31 (0)30 253 25 12