Low scale leptogenesis and $TM_1$ mixing in neutrinophillic two Higgs doublet model($\nu$2HDM) with S4 flavor symmetry

The universe is filled with matter, from the stars above to the atoms in our own bodies, yet it is a profound mystery why this matter exists at all. According to the most fundamental laws of physics, the Big Bang should have produced equal amounts of matter and antimatter, which would have instantly annihilated each other, leaving behind a cosmos of pure energy and nothing else. The fact that we are here, surrounded by a universe of stars and planets, implies that a tiny, unexplained imbalance tipped the scales in favor of matter billions of years ago. Physicists have long sought the mechanism behind this survival, looking for a process that could generate a slight excess of matter over antimatter in the early universe. At the same time, another deep puzzle has emerged: neutrinos, ghostly particles that stream through everything, were long thought to be massless, but experiments have proven they possess a tiny, elusive mass. Understanding how these particles acquire mass and how they might have driven the creation of matter are two sides of the same coin, and solving one could unlock the secrets of the other.

A team of researchers has proposed a new theoretical framework that attempts to solve both of these mysteries simultaneously. They have constructed a modified version of the Standard Model, the rulebook that describes all known particles and forces, by adding a few specific ingredients. The core of their idea involves introducing a second type of Higgs field, a field that gives particles their mass, which interacts very weakly with ordinary matter but strongly with a new type of heavy neutrino. This setup, known as a neutrinophilic two Higgs doublet model, allows the heavy neutrinos to exist at a much lower energy scale than previously thought possible, bringing them within the reach of current and future particle experiments. To make the model predictive and to explain the specific patterns in which neutrinos change their identity as they travel, the researchers imposed a specific mathematical symmetry on the system. This symmetry acts like a strict set of rules that dictates how the particles can interact, forcing the model to produce a very specific pattern of neutrino mixing known as Trimaximal mixing.

By running detailed calculations within this framework, the researchers found that their model successfully reproduces the observed properties of neutrinos, including their masses and the angles at which they mix. The model predicts that the lightest neutrino has a mass of nearly zero, while the other two have masses that fit perfectly within the range measured by global experiments. A particularly striking prediction concerns the effective mass of the neutrino, a value that determines how likely the particle is to undergo a rare process called neutrinoless double-beta decay. The model suggests this value is extremely small, falling between 4 and 5 millionths of a billionth of a gram. This is significantly lower than the sensitivity limits of current experiments, meaning that even our most advanced detectors are not yet powerful enough to see this signal, though future, larger experiments may eventually be able to probe this tiny range.

The study also investigated whether this framework could explain the origin of the matter-antimatter imbalance. In the early universe, the heavy neutrinos introduced in the model would have decayed, and due to the specific interactions allowed by the new symmetry, these decays could have produced a slight excess of leptons, a family of particles that includes electrons and neutrinos. This lepton excess would then have been converted into the excess of matter we see today. The researchers demonstrated that this process, known as leptogenesis, can work effectively even with the heavy neutrinos existing at a relatively low energy scale of around 10 TeV, which is far more accessible than the trillions of electron volts required by older theories. They showed that by carefully balancing the masses of the new particles and the strength of their interactions, the model can generate exactly the amount of matter asymmetry observed in the universe today.

However, the model is not without its constraints. The researchers had to ensure that the new particles and forces they introduced did not violate known experimental limits, particularly those concerning rare processes where one type of charged lepton transforms into another, such as a muon turning into an electron and a photon. Their calculations showed that the strength of the interactions in their model is weak enough to satisfy these strict experimental bounds, which are currently set by the MEG collaboration. The study concludes that this specific arrangement of particles and symmetries offers a coherent and testable explanation for both the nature of neutrino mass and the existence of matter in the universe. It provides a clear path forward, linking theoretical predictions with experimental possibilities, and suggests that the answers to these cosmic questions may lie just beyond the reach of our current instruments, waiting to be discovered in the next generation of particle physics experiments.