Mixed leptogenesis sources and low-energy neutrino portals

The universe began in a state of extreme heat and density, a moment when the fundamental forces of nature were unified and the particles we know today were just beginning to take shape. In the earliest fractions of a second, a process known as baryogenesis is believed to have tipped the scales, creating a slight excess of matter over antimatter. Without this imbalance, the matter and antimatter created in the Big Bang would have annihilated each other completely, leaving behind a cosmos filled only with light. To understand how this happened, physicists look to the behavior of neutrinos, ghostly particles that rarely interact with anything. These particles may hold the key to the missing antimatter, particularly if they possess a hidden property that allows them to decay into lighter, unseen particles. This decay could have generated the necessary imbalance in the early universe, setting the stage for the existence of stars, planets, and life.

A recent study by Yingnan Xu investigates a specific scenario where these heavy, unseen particles decay in a way that links two distinct phenomena: the creation of the matter-antimatter imbalance and the behavior of light neutrinos today. The research focuses on a theoretical model involving two heavy particles that are slightly different in mass. These particles are thought to have decayed shortly after the Big Bang, producing a surplus of matter. The study explores a unique connection where the same mathematical rules that govern this ancient decay also dictate how light neutrinos interact with a mysterious, ultra-light scalar particle. This scalar particle is a type of matter that carries a specific charge, allowing it to exist alongside neutrinos without immediately destroying them. The researcher asked a precise question: if the heavy particles created the matter imbalance we see today, what does that force them to believe about the strength of the interaction between light neutrinos and this new scalar particle?

The author found that the answer is not a single number but a tightly constrained range. They discovered that the strength of the interaction between the light neutrinos and the scalar particle cannot be arbitrarily small if the heavy particles are to produce the observed amount of matter. If the interaction were too weak, the heavy particles would not have been able to generate the necessary imbalance. Conversely, if the interaction were too strong, it would have disrupted the delicate balance required for the heavy particles to decay in the specific way needed. This creates a "forbidden zone" where the interaction strength simply cannot exist. The study calculates that for the heavy particles to work as a source of matter, the interaction strength must fall within a specific window. This window is determined by the masses of the heavy particles and the specific way they decay, effectively locking the properties of the light neutrinos to the conditions of the early universe.

One of the most striking findings concerns the behavior of the light neutrinos themselves. The researcher identified a special configuration where the usual sources of matter imbalance vanish, leaving only the new, mixed interaction to do the work. In this specific case, the two heavier neutrinos that result from the decay must have exactly the same strength of interaction with the scalar particle. This equality is not a guess but a mathematical necessity derived from the symmetry of the system. However, the study also shows that this equality is just one point in a continuous family of possibilities. As the interaction strength changes, the balance between the two neutrinos shifts, but the total mass of the neutrinos remains consistent with what we observe in experiments today. This means that while the universe could have evolved with different interaction strengths, the mass of the neutrinos we measure now acts as a fixed anchor, limiting how much the interaction can vary.

The paper also looks at what happens after the initial decay, during a period known as reheating, when the energy from the decaying heavy particles is transferred to the rest of the universe. This process fills the cosmos with a "bath" of particles, including the new scalar particles. The researcher calculated how many of these scalar particles are produced directly from the heavy decays versus how many are created later by the hot soup of ordinary matter. They found that while the particles created directly from the heavy decays carry almost all of the energy, the particles created later from the hot bath are far more numerous in terms of their momentum spread. In fact, the particles from the hot bath contribute more than eleven times the variance, or spread, of the primary particles, even though they carry less than one ten-thousandth of the energy. This distinction is crucial because the spread of momentum affects how these particles move and interact later on, influencing the structure of the universe.

Using these calculations, the study predicts the lifetime of the heavy neutrinos in a vacuum. For the specific parameters that fit the observed amount of matter in the universe, the heavier of the two neutrinos would survive for approximately 2.80 × 10^11 seconds before decaying, while the lighter one would last about 1.62 × 10^12 seconds. These are vast timescales, far longer than the current age of the universe, suggesting that if these particles exist, they are incredibly stable. The study also explores how the mass of the scalar particle affects these lifetimes. If the scalar particle has a tiny mass, it can block certain decay paths, effectively turning off the decay for the lighter neutrino while leaving the heavier one able to decay. This sensitivity means that the mass of the scalar particle acts as a switch, determining which neutrinos can decay and which cannot.

The research further examines how the presence of these particles changes the way they move through space. The interaction between the neutrinos and the scalar particles creates a feedback loop where the density of the particles affects their own mass. The study shows that this effect is significant enough to alter the speed at which the particles decay, particularly for the lighter neutrinos. The researcher derived equations that describe how the mass of the scalar particle changes as the density of the universe changes, showing that the particles behave differently in a crowded environment compared to an empty one. This dynamic mass is a key factor in understanding how these particles would have influenced the evolution of the early universe, potentially leaving a signature in the cosmic background radiation or the distribution of galaxies.

Ultimately, this work ties together the high-energy physics of the early universe with the low-energy physics we can observe today. It demonstrates that the rules governing the creation of matter are inextricably linked to the properties of the lightest particles in the universe. By constraining the possible values of the interaction strength, the study narrows down the search for new physics. It suggests that if we can measure the decay of neutrinos or the properties of the scalar particle with enough precision, we might be able to confirm or rule out this specific mechanism for the origin of matter. The findings provide a clear roadmap for future experiments, pointing to specific ranges of interaction strength and particle mass that should be targeted. The universe, it seems, has left a set of clues in the behavior of neutrinos, and this study helps us read them, revealing a hidden connection between the birth of the cosmos and the particles that drift through it today.