What do shrimp, tennis balls and pulsars all have in common? They are all made from matter.
Admittedly, that answer is a cop-out, but it highlights a big, persistent quandary for scientists: Why is everything made from matter when there is a perfectly good substitute—antimatter?
The European laboratory CERN hosts several experiments to ascertain the properties of antimatter particles, which almost never survive in our matter-dominated world.
Particles (such as the proton and electron) have oppositely charged antimatter doppelgangers (such as the antiproton and antielectron). Because they are opposite but equal, a matter particle and its antimatter partner annihilate when they meet.
Antimatter wasn’t always rare. Theoretical and experimental research suggests that there was an equal amount of matter and antimatter right after the birth of our universe. But 13.8 billion years later, only matter-made structures remain in the visible universe.
Scientists have found small differences between the behavior of matter and antimatter particles, but not enough to explain the imbalance that led antimatter to disappear while matter perseveres. Experiments at CERN are working to solve that riddle using three different strategies.
Antimatter under the microscope
It’s well known that CERN is home to Large Hadron Collider, the world’s highest-energy particle accelerator. Less known is that CERN also hosts the world’s most powerful particle decelerator—a machine that slows down antiparticles to a near standstill.
The antiproton decelerator is fed by CERN’s accelerator complex. A beam of energetic protons is diverted from CERN’s Proton Synchrotron and into a metal wall, spawning a multitude of new particles, including some antiprotons. The antiprotons are focused into a particle beam and slowed by electric fields inside the antiproton decelerator. From here they are fed into various antimatter experiments, which trap the antiprotons inside powerful magnetic fields.
“All these experiments are trying to find differences between matter and antimatter that are not predicted by theory,” says Will Bertsche, a researcher at University of Manchester, who works in CERN’s antimatter factory. “We’re all trying to address the big question: Why is the universe made up of matter these days and not antimatter?”
By cooling and trapping antimatter, scientists can intimately examine its properties without worrying that their particles will spontaneously encounter a matter companion and disappear. Some of the traps can preserve antiprotons for more than a year. Scientists can also combine antiprotons with positrons (antielectrons) to make antihydrogen.
“Antihydrogen is fascinating because it lets us see how antimatter interacts with itself,” Bertsche says. “We’re getting a glimpse at how a mirror antimatter universe would behave.”
Scientists in CERN’s antimatter factory have measured the mass, charge, light spectrum, and magnetic properties of antiprotons and antihydrogen to high precision. They also look at how antihydrogen atoms are affected by gravity; that is, do the anti-atoms fall up or down? One experiment is even trying to make an assortment of matter-antimatter hybrids, such as a helium atom in which one of the electrons is replaced with an orbiting antiproton.
So far, all their measurements of trapped antimatter match the theory: Except for the opposite charge and spin, antimatter appears completely identical to matter. But these affirmative results don’t deter Bertsche from looking for antimatter surprises. There must be unpredicted disparities between these particle twins that can explain why matter won its battle with antimatter in the early universe.
“There’s something missing in this model,” Bertsche says. “And nobody is sure what that is.”
Antimatter in motion
The LHCb experiment wants to answer this same question, but they are looking at antimatter particles that are not trapped. Instead, LHCb scientists study how free-range antimatter particles behave as they travel and transform inside the detector.
“We’re recording how unstable matter and antimatter particles decay into showers of particles and the patterns they leave behind when they do,” says Sheldon Stone, a professor at Syracuse University working on the LHCb Experiment. “We can’t make these measurements if the particles aren’t moving.”
The particles-in-motion experiments have already observed some small differences between matter and antimatter particles. In 1964 scientists at Brookhaven National Laboratory noticed that neutral kaons (a particle containing a strange and down quark) decay into matter and antimatter particles at slightly different rates, an observation that won them the Nobel Prize in 1980.
The LHCb experiment continues this legacy, looking for even more discrepancies between the metamorphoses of matter and antimatter particles. They recently observed that the daughter particles of certain antimatter baryons (particles containing three quarks) have a slightly different spatial orientation than their matter contemporaries.
But even with the success of uncovering these discrepancies, scientists are still very far from understanding why antimatter all but disappeared.
“Theory tells us that we’re still off by nine orders of magnitude,” Stone says, “so we’re left asking, where is it? What is antimatter’s Achilles heel that precipitated its disappearance?”
Antimatter in space
Most antimatter experiments based at CERN produce antiparticles by accelerating and colliding protons. But one experiment is looking for feral antimatter freely roaming through outer space.
The Alpha Magnetic Spectrometer is an international experiment supported by the US Department of Energy and NASA. This particle detector was assembled at CERN and is now installed on the International Space Station, where it orbits Earth 400 kilometers above the surface. It records the momentum and trajectory of roughly a billion vagabond particles every month, including a million antimatter particles.
Nomadic antimatter nuclei could be lonely relics from the Big Bang or the rambling residue of nuclear fusion in antimatter stars.
But AMS searches for phenomena not explained by our current models of the cosmos. One of its missions is to look for antimatter that is so complex and robust, there is no way it could have been produced through normal particle collisions in space.
“Most scientists accept that antimatter disappeared from our universe because it is somehow less resilient than matter,” says Mike Capell, a researcher at MIT and a deputy spokesperson of the AMS experiment. “But we’re asking, what if all the antimatter never disappeared? What if it’s still out there?”
If an antimatter kingdom exists, astronomers expect that they would observe mass particle-annihilation fizzing and shimmering at its boundary with our matter-dominated space—which they don’t. Not yet, at least. Because our universe is so immense (and still expanding), researchers on AMS hypothesize that maybe these intersections are too dim or distant for our telescopes.
“We already have trouble seeing deep into our universe,” Capell says. “Because we’ve never seen a domain where matter meets antimatter, we don’t know what it would look like.”
AMS has been collecting data for six years. From about 100 billion cosmic rays, they’ve identified a few strange events with characteristics of antihelium. Because the sample is so tiny, it’s impossible to say whether these anomalous events are the first messengers from an antimatter galaxy or simply part of the chaotic background.
“It’s an exciting result,” Capell says. “However, we remain skeptical. We need data from many more cosmic rays before we can determine the identities of these anomalous particles.”
