In the beginning, there was matter and antimatter, and then there was only matter. Why? This question is one of the defining mysteries of physics. For decades theorists have come up with potential solutions, most involving the existence of extra particles beyond the known species in the universe. Last week scientists announced tantalizing findings that point toward one possible solution, but the data fall short of a definitive discovery. Whatever the final answer is, resolving the question may tell us more than just why we live in a universe of matter—it could expose secrets from the earliest epochs of the cosmos or even connect us to the invisible dark matter that eludes scientists.
Most of the theories about how matter got the upper hand over antimatter fall into two main camps. One, called electroweak baryogenesis, posits extra versions of the Higgs boson—the particle related to how everything else gets mass. If these Higgs cousins exist, they could have helped set off an abrupt phase transition, akin to the shift when water goes from liquid to gas, early in the universe that might have led to slightly more matter than antimatter in space. When matter and antimatter come into contact, they annihilate each other, so most of the stuff in the young universe would have been destroyed, leaving behind just a small surplus of matter to make the galaxies and stars and planets around us.
The other leading theory, called leptogenesis, stems instead from neutrinos. These particles are much, much lighter than quarks and pass through the cosmos ethereally, rarely stopping to interact with anything at all. According to this scenario, in addition to the regular neutrinos we know of, there are extremely heavy neutrinos that are so gargantuan that they could have been forged only from the tremendous energies and temperatures present just after the big bang, when the universe was very hot and dense. When these particles inevitably broke down into smaller, more stable species, the thinking goes, they might have produced slightly more matter than antimatter by-products, leading to the arrangement we see today.
Two Mysteries for the Price of One
The recent announcement, which was made by scientists at the T2K (Tokai to Kamioka) experiment in Japan, offers hopeful signs for the leptogenesis concept. The experiment observes neutrinos as they travel through 300 kilometers underground and change between three types, or flavors—a peculiar ability of neutrinos called oscillation. The T2K researchers detected more oscillations in neutrinos than in antineutrinos, suggesting the two do not just act as mirror images of each other but, in fact, behave differently. Such a difference between a particle and its antimatter counterpart is termed CP violation, and it is a strong clue in the quest to understand how matter outran antimatter after the universe was born. “We don’t call it a discovery yet,” says T2K team member Chang Kee Jung of Stony Brook University. The experiment has now ruled out the possibility that neutrinos have zero CP violation with 95 percent confidence, and it shows hints that the particles might display the maximum possible amount of CP violation allowed. Yet more data, and probably future experiments, will be needed to precisely measure just how much neutrinos and antineutrinos differ.
Even if physicists make a definitive discovery of CP violation in neutrinos, they will not have completely solved the cosmic antimatter question. Such a finding would be “necessary but not sufficient” to prove leptogenesis, says Seyda Ipek, a theoretical physicist at the University of California, Irvine. Another requirement of the theory is that neutrinos and antineutrinos turn out to be the same thing. How is that seeming contradiction possible? Matter and antimatter are thought to be identical except for a reversed electrical charge. Neutrinos, having no charge, could be both at the same time.
If this possibility is the case, it could also explain why neutrinos are so light—perhaps less than one six-millionth of the mass of the electron. If neutrinos and antineutrinos are the same, they might gain mass not by interacting with the Higgs field (which is associated with the Higgs boson), as most particles do, but through another process called the seesaw mechanism. Their puny masses would be inversely proportional to those of the heavy neutrinos that arose in the early universe. “When one is up, the other is down, like a seesaw,” Ipek says.
“Leptogenesis is a very elegant way of explaining things,” says Jessica Turner, a theoretical physicist at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill. “Firstly, you answer why there’s more matter than antimatter. And second, you explain why neutrinos have such small masses.” Evidence that neutrinos are their own antimatter counterparts could come from experiments searching for a theorized reaction called neutrinoless double beta decay, which could only occur if neutrinos were able to annihilate themselves as matter and antimatter do on contact. Even this finding, though, would not fully prove leptogenesis took place. “If you measure the most possible CP violation we can see, and if you observed that neutrinos were their own antiparticles, we would say that’s circumstantial evidence, not direct evidence,” Turner says.
Connecting to the Dark Sector
The other theoretical option on the table, electroweak baryogenesis, might be easier to investigate, physicists say. Whereas the creation of heavy neutrinos involved in leptogenesis would most likely be beyond the capabilities of particle accelerators, the extra Higgs bosons predicted by this theory just might show up at the Large Hadron Collider, says Marcela Carena, head of Fermilab’s theoretical physics department. Even if the machine does not make them directly, these Higgs relatives could subtly but detectably interact with the traditional Higgs bosons it produces.
Electroweak baryogenesis also requires additional CP violation in the universe but not specifically in neutrinos. In fact, CP violation has already been discovered in quarks, though in such small amounts that it does not explain the matter-antimatter imbalance. One place this theory’s missing CP violation might be hiding is the so-called dark sector—the realm of the invisible dark matter that is thought to make up most of the matter in space. Perhaps dark matter and dark antimatter behave differently, and this difference can explain our universe as we know it. “My line of work has been trying to connect the matter-antimatter imbalance in the universe to the idea that we know we need something we haven’t seen so far in order to explain dark matter,” Carena says.
Evidence for electroweak baryogenesis could come not just through detecting extra Higgs particles but also via the numerous experiments hunting for dark matter and the dark sector. Furthermore, if a cosmological phase transition occurred shortly after the big bang, as the theory supposes, it might have produced gravitational waves that could be found by future experiments, such as the Laser Interferometer Space Antenna (LISA), a space-based gravitational-wave detector due to launch in the 2030s.
Ultimately, though, the universe could surprise us. Perhaps neither leptogenesis nor electroweak baryogenesis occurred. “Those are not the only two options—the theory realm is very vast,” Ipek says. She recently worked on a model involving CP violation in the strong interaction of the quarks inside protons and neutrons, for instance, and theorists are looking into many other ideas as well. “I think we need to let ourselves explore all possibilities,” Turner says. “Nature unravels as it does; we can’t control that. We just try our best to understand it.”
In the meantime, a definitive measurement of CP violation in neutrinos, at least, is within sight. Upcoming projects such as the Deep Underground Neutrino Experiment (DUNE) and T2K’s successor Hyper-Kamiokande (Hyper-K) should have the sensitivity required for a precise accounting. “The T2K data look as interesting as they could look,” says DUNE co-spokesperson Ed Blucher of the University of Chicago. “It makes me very excited that there’ll be something interesting to study in the next generation of experiments that are coming.”
