The hunt for dark matter – the mysterious stuff that seems to outnumber regular matter by a ratio of five to one – has so far turned up no direct trace. Now, researchers conducting the Baryon Antibaryon Symmetry Experiment (BASE) at CERN have tried a new approach, using another strange substance – antimatter.
Dark matter and antimatter are at the heart of two of the biggest unsolved mysteries of the universe. According to astronomical observations, there is far more mass out in space than the stuff we see can account for, and this invisible mass is dubbed dark matter. We don’t really know what it’s made of, but there are a whole range of theories attempting to explain it, including electrically-charged particles, dark photons, superheavy gravitinos, or even a “dark fluid” with negative mass.
Antimatter, on the other hand, is very much a real thing that we can make and study directly. In essence, it’s just regular matter that has the opposite charge, but that means if a particle and its equivalent antiparticle meet, they will annihilate each other in a burst of energy. Models suggest that equal amounts of matter and antimatter should have been created in the Big Bang, but in our experience antimatter is extremely rare. The question remains then – where is all the antimatter?
For the new study, the researchers wanted to probe a potential link between the matter-antimatter asymmetry and dark matter. To do so, they set up an experiment similar to many others that have been run in the past. The usual experimental setup for hunting dark matter involves isolating particles, then watching them carefully for any anomalies that might indicate interference from dark matter interactions.
But while past experiments have all used regular matter particles, the new study swaps it out for antimatter. The team took antiprotons created in CERN’s antimatter factory, and confined them in a device called a Penning trap, which prevents them from touching (and annihilating with) any ordinary matter. One by one, they measured and flipped the spin states of these antiprotons, about a thousand times over three months.
The idea is that by taking these measurements over an extended period, the researchers can get a time-averaged frequency of the antiproton’s spin. Then, if anything out of the ordinary happens to this cycle, it could be evidence that dark matter particles are interfering.
In particular, they were looking for a dark matter candidate called an axion. These hypothetical particles are thought to be neutral, very light and flow through the universe like waves, occasionally interacting with regular matter – and antimatter. Since only regular matter has been tested (with no success so far), the researchers tried antimatter to see if anything different occurred.
The team detected no signals from any dark matter-antimatter interaction. That said, a null result isn’t a complete washout – instead, it just means axion-antiproton interactions don’t occur between 0.1 and 0.6 Gigaelectronvolts (GeV), depending on the mass of the axion. Slowly ruling out different combinations allows scientists to zero in on the mysterious stuff.
But, the team says, getting a signal in this experiment was always unlikely. If they did get a signal at this range, it would imply a huge discrepancy between the properties of matter and antimatter, which are generally thought to be very similar – minus the opposite charge. Still, it was worth a look, since a signal would have helped zero in on dark matter and raise new questions about the relationship between matter and antimatter, and antimatter and dark matter.
The research was published in the journal Nature.
Source: CERN