Leptogenesis

Shortly after the Big Bang, the universe contained matter and antimatter in nearly equal amounts. When the two meet, they annihilate each other and leave only radiation behind. If there really were exactly equal amounts of both, the universe today would be empty — no stars, no planets, no life. Instead, matter obviously exists. Leptogenesis is a model that explains how a small excess of matter could have arisen. Specifically, it describes a process in which very heavy, unstable particles called neutrinos — a kind of electrically neutral, almost massless elementary particle — decay early in the universe. In doing so, slightly more matter particles are produced than antimatter particles. This minimal lead — estimated at one matter particle per billion pairs — is enough to leave behind a small amount of matter after mutual annihilation. Everything we see today is made up of this leftover matter.

The significance for our understanding of the universe

The question of why matter outweighed antimatter is considered one of the greatest unsolved problems in physics. Without an answer to it, it remains unclear why anything exists at all. Leptogenesis is one of the few candidates that can offer a complete explanation.

The model also has a second advantage: it is connected to the question of why ordinary neutrinos are so unusually light. It predicts that, alongside the known, extremely light neutrinos, there were also extremely heavy relatives. These heavy neutrinos decayed long ago, but their former properties would explain why today’s neutrinos have so little mass. Physicists call this the seesaw mechanism: the heavier one type is, the lighter the other becomes. Leptogenesis thus links two different puzzles of physics together.

The course of leptogenesis

In the leptogenesis model, very heavy, right-handed neutrinos — a hypothetical particle type that does not occur in the Standard Model of particle physics — decay shortly after the Big Bang in two different ways. One decay produces a light neutrino and a particular other particle. The other decay instead produces their antiparticles. Crucially, these two pathways do not occur with equal frequency. There is thus a slight imbalance: nature does not treat matter and antimatter identically in this process. Physicists call this property CP violation.

This imbalance initially creates an excess on the side of a particular class of particles, the so-called leptons — which include neutrinos and electrons. Subsequently, other processes in the hot, early universe convert this lepton excess into a baryon excess. Baryons are particles such as protons and neutrons, i.e. the building blocks of atomic nuclei. The end result is a small but permanent excess of ordinary matter.

Leptogenesis in research and the news

Leptogenesis usually appears in the news when experiments deliver new data about neutrinos. Neutrino observatories such as the IceCube detector array in Antarctica or Japanese facilities like Super-Kamiokande measure the properties of these particles with growing precision. Every new measurement can provide clues as to whether the theory of leptogenesis is correct — or whether it must be ruled out.

Of particular importance is the question of whether neutrinos are their own antiparticles. If they were, this would have direct consequences for leptogenesis. Experiments such as LEGEND or nEXO are therefore searching for a very rare radioactive decay, neutrinoless double beta decay, which would answer exactly this question. Although leptogenesis has not yet been directly observed or proven, it is considered in theoretical physics to be one of the most serious explanatory approaches for the matter-antimatter asymmetry in the universe.