Two types of particles play a leading role: electrons and muons. The latter were discovered in 1936 and were cause for surprise at the time. “Who ordered that?” physicist Isidor Rabi wondered. The muon is now known as “the heavy sister” of the electrons, because it behaves identically, says Marcel Merk, UM professor of Particle Physics, and the national representative of the Netherlands in the so-called LHCb experiment in Geneva. LHCb stands for Large Hadron Collider in CERN.
“The Standard Model, the umbrella theory that describes the behaviour of particles and forces of nature, also states that both particles are the same, except for their mass. We know the muon really well and are constantly carrying out measurements with it. By the way, it also rains right through us from the cosmos all the time; indeed, through our bodies. It is part of the natural background radiation.”
Analyses by (among others) Jacco de Vries and Silvia Ferreres, both working for the UM and Nikhef, showed last week that the theory may have shortcomings: muons do not always behave the same as electrons. “We demonstrated that in a process in which certain kinds of particles – quarks – shatter. You would expect them to turn equally often into electrons as into muons, but that appears not to be the case. We saw a deviation of 15 per cent. And that is, given the measuring uncertainty, such that we formally refer to it as ‘evidence’, or an ‘indication’.”
What does that mean? If the indication is upheld in new experiments, you could conclude that there may be a new force of power, says Merk. “And that provides a new outlook for questions for which the 50-year-old Standard Model has no answers. Things like the mystery of antimatter. In collisions in the particle accelerator, we see that every particle has a counterpart with an opposite electrical charge. So, for every proton there is an antiproton, for every electron there is a positron. But the big question is: why do we not see any of that in the universe? Where did all that antimatter go?”
In the meantime, researchers are analysing data from comparable experiments, in which they use other processes to look at the behaviour of the muons and electrons. The results will be published in August or March next year. “If these are even more convincing, we can say it is an ‘observation’ and that means that the findings are correct. But our interest goes further. We want to know exactly what this force of nature consists of and how it manifested itself in the early universe.”
Merk is the first to dampen the enthusiasm and would rather speak of careful excitement. “With an indication, you are in a kind of in-between phase. It is not nothing, but no hard evidence either. We shall see which way it goes.”