New evidence from neutrinos points to one of the many theories about why the universe is made of matter and not antimatter.
In the beginning, there was matter and antimatter; then, only matter remained. Why? This is one of the greatest mysteries in physics. For decades, theoretical physicists have proposed possible solutions, most involving the existence of additional particles beyond the known types in the universe. Last week, scientists announced exciting results that point to a possible solution, but the data wasn’t sufficient for a definitive discovery. Regardless of the final answer, resolving this question could allow us to learn more than just why we exist in a universe made of matter; it could unlock secrets from the early ages of the universe or even connect us to the elusive dark matter that continues to baffle scientists.
Most theories explaining how matter came to dominate antimatter fall into two main categories. The first, known as the weak-interacting baryon theory, posits the existence of extra copies of the Higgs boson, the particle responsible for giving everything else its mass. If particles similar to the Higgs boson existed, they may have contributed to a sudden transition, such as water changing from a liquid to a gas, in the early universe, potentially leading to an abundance of matter at the expense of antimatter. When matter and antimatter interact, they annihilate each other completely, so it’s likely that most of the matter in the early universe was destroyed, leaving behind a small surplus of matter to form galaxies, stars, and planets around our own.
Another leading theory, known as the lepton formation theory, relies on neutrinos, particles much lighter than quarks, that travel through the aether and rarely stop interacting with anything. According to this scenario, in addition to the ordinary neutrinos we know, there are very heavy neutrinos, so heavy that they may have been formed only from the immense energies and temperatures that existed immediately after the Big Bang, when the universe was extremely hot and dense. When these particles decayed into smaller, more stable types, the proponents of this theory suggest they may have produced slightly more matter than antimatter as byproducts, leading to the pattern we observe today.
Two puzzles for the price of one
A recent statement from scientists working on the Tokai to Kamioka (T2K) experiment in Japan offers promising evidence for the concept of leptons’ composition. The experiment detects neutrinos traveling 300 kilometers beneath the Earth’s surface, changing between three distinct types or “flavors.” This unique ability of neutrinos is known as oscillation. Researchers at Tokai to Kamioka discovered that neutrinos oscillate more than antineutrinos, suggesting that they don’t simply behave as mirror images of each other, but rather behave quite differently. This difference between a particle and its antimatter counterpart is known as “normal charge symmetry violation,” a powerful clue that could be useful in understanding how antimatter preceded particles after the Big Bang. “We don’t consider this a discovery yet,” says Chang-Ki Yong of Stony Brook University, a member of the Tokai to Kamioka team. The current experiment ruled out the possibility of a neutrino breach inconsistent with normal charge, with a 95% confidence level, and shows evidence that the particles may exhibit the maximum possible normal charge symmetry. However, more data, and possibly future experiments, will be needed to precisely measure how much neutrinos and antineutrinos differ.
Even if physicists were to make a definitive discovery that violates the normal charge symmetry of neutrinos, they would not have fundamentally solved the cosmic question of antimatter. Seda Ipek, a theoretical physicist at the University of California, Irvine, says such a discovery would be “necessary, but not sufficient” to prove the formation of leptospheres. The second condition for the theory is that neutrinos and antineutrinos must be the same thing. How could this apparent contradiction be possible? Matter and antimatter are thought to be identical except that they carry opposite charges. Could neutrinos be both matter and antimatter simultaneously without carrying charges? If this were true, it might also explain why neutrinos are so light, perhaps less than the mass of one electron in six millionths of a mass. And if neutrinos and antineutrinos are the same thing, they might acquire mass not through interaction with the Higgs field (associated with the Higgs boson), as most particles do, but through another process called oscillation. Their tiny masses may be inversely proportional to the heavy neutrinos that appeared at the beginning of the universe. The Institute of Biochemistry explains, “When one has a high mass, the other has a low mass, just like a swing mechanism.”
“Lepto formation is a very clever way to illustrate things,” says Jessica Turner, a theoretical physicist at Fermilab in Batavia, Illinois. “Evidence that neutrinos are antimatter can be obtained from experiments looking for a hypothetical interaction called non-neutrino beta decay doubling, which can only occur if neutrinos are capable of self-destruction like matter and antimatter do when they interact.” But even this finding wouldn’t provide conclusive proof of lepto formation. “If we can see the extreme violation of symmetry, measure the normal charge, and observe that the neutrino itself was an antiparticle, then we can say that’s circumstantial evidence, not direct evidence,” Turner says.
Link to the dark sector
Physicists suggest that another theoretical option, weakly interacting baryons, might be easier to search for. Marcella Carina, head of the Theoretical Physics Division at Fermi National Accelerator Laboratory, says that while the formation of heavy Higgs bosons, which are involved in lepton generation, would exceed the capabilities of particle accelerators, the additional Higgs boson predicted by this theory might only appear in the Large Hadron Collider’s hadrons. Even if the machine doesn’t produce them directly, these Higgs-like particles could secretly interact with the conventional Higgs bosons it does produce, but this interaction would still be detectable.
Baryons with electromagnetic interactions also require a violation of the normal charge symmetry of the universe, but not necessarily in neutrinos. In fact, normal charge symmetry has been observed in quarks, albeit in very small amounts, making it difficult to explain the imbalance between matter and antimatter. One place where normal charge symmetry violation might be hiding is the dark sector, the realm of unseen dark matter thought to make up the bulk of matter in space. Dark matter may behave differently from antimatter, and this difference could explain the universe as we know it. “My field of work focuses on trying to connect the matter-antimatter imbalance in the universe with the idea that we need something we haven’t seen yet to explain dark matter,” says Karina.
Evidence for the existence of baryons can be assessed through electromagnetic interaction not only by detecting additional Higgs particles, but also through numerous experiments searching for dark matter and the dark ribbon. Furthermore, if a cosmic phase transition occurred shortly after the Big Bang, as theory suggests, it may have produced gravitational waves that could be detected by future experiments, such as the LISA antenna, a space-based gravitational wave detector that was planned for launch in the 1930s.
Meanwhile, precise measurements of charge symmetry violations in neutrinos will be available, at least in the near future. Upcoming projects such as the Underground Neutrino Experiment (DUNE) and the Hyper Kamioka experiment, the successor to Tokai to Kamioka, must possess the sensitivity necessary for accurate calculations. Ed Blacher, a spokesperson for DUNE at the University of Chicago, says, “The data from Tokai to Kamioka look very interesting, and that makes me very excited that something interesting is being studied in the next generation of experiments.”
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