Particle physics experiments

CERN experiments investigate whether antimatter falls up or down

Physicists at CERN have discovered that antimatter falls. Sure, that sounds obvious, but scientists haven’t yet been able to confirm that it responds to gravity in exactly the same way as ordinary matter. A new experiment provides the best answer yet.

Antimatter is very similar to the matter that makes up everything around us, with one important difference: its particles have the opposite electrical charge. And this simple difference has major implications – every time a particle and its antiparticle meet, they annihilate.

Luckily for us matter-based beings, antimatter is extremely rare in the universe, but no one really knows why. The Big Bang should have produced equal amounts of matter and antimatter, which would have ended up wiping out everything in the universe billions of years ago. The fact that we are here today to ask the question shows that an unknown person has created an imbalance.

Physicists are therefore studying antimatter closely to see if there are other differences between it and ordinary matter, besides charge, that could explain the imbalance. The standard model says there is should not be any other difference, so if scientists find anything, it could open up a whole new world of physics.

This means going back to basics to examine antimatter. For example, each element absorbs and emits different wavelengths of light, producing a unique fingerprint called an emission spectrum. Antimatter should have the same spectrum as its matter counterpart, but it wasn’t until 2016 that scientists at CERN finally checked. Indeed, antihydrogen was found to have the same spectrum as hydrogen.

How antimatter responds to gravity is another seemingly simple question that has taken years of study. This might sound like something we should already know, but most of the time antimatter is suspended in electromagnetic traps to keep it from annihilating with containers. Antimatter is expected to respond to gravity in the same way as ordinary matter – but there’s a small chance that it won’t and will actually fall to the top rather.

To test the idea, the team placed antiprotons and negatively charged hydrogen ions in an electromagnetic device called a Penning trap. Once inside, the particles follow a cyclic path and, by measuring their frequency, scientists can calculate their charge-to-mass ratio. This ratio should be the same for matter and antimatter particles, but any difference would be attributed to variations in their interactions with gravity.

And sure enough, the team discovered that matter and antimatter respond to gravity in the same way. At least, within the limits of the uncertainty of the experiment, which is less than 97% of the gravitational acceleration experienced by the particles. This is four times more accurate than previous experiments, according to the team.

However, this still leaves room for new physics. Other experiments test antimatter’s interactions with gravity through what seems like a much simpler approach – dropping antimatter particles and seeing where they go. If these teams find results that differ from the current experiment, it could hint at physics beyond the Standard Model.

The research was published in the journal Nature.

Source: RIKEN via Eurekalert