Chiral matching of dimension-7 baryon-number-violating operators and application to nucleon decays
This paper classifies dimension-7 baryon-number-violating operators within chiral perturbation theory to systematically analyze nucleon decays, deriving improved lifetime limits for three-body modes from existing two-body constraints and proposing a concrete ultraviolet model that generates these processes.
Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
The universe is built on a set of invisible rules that govern how matter behaves and changes. Among the most fundamental of these rules is the conservation of baryon number, a principle stating that the total amount of ordinary matter, like the protons and neutrons inside an atom, should remain constant over time. For decades, physicists have searched for a crack in this rule, a rare event where a proton or neutron simply vanishes and transforms into lighter particles. Finding such an event would be a monumental discovery, proving that matter is not eternal and offering a glimpse into the physics that existed just after the Big Bang. While the most famous attempts to find this decay focus on the simplest possible changes, a new study suggests that the universe might be hiding its secrets in more complex, three-particle transformations that have been largely overlooked.
A researcher has now turned their attention to these more intricate possibilities, specifically looking for a type of decay where a neutron or proton breaks apart into a lepton and multiple pions, a process that requires a specific shift in the internal symmetry of the particles involved. To do this, they had to bridge the gap between the high-energy world of theoretical particles and the low-energy world of the atoms we can actually observe. They developed a new mathematical framework to translate the behavior of exotic, high-energy forces into the language of nuclear physics. This allowed them to calculate the theoretical rates for these rare, multi-particle decays should the underlying rules of nature permit them. Their work reveals that while these events are incredibly rare, they are theoretically possible, and they provide a new set of targets for the massive detectors currently scanning the depths of the Earth and the ocean.
The researcher began by organizing the theoretical tools used to describe these potential decays. In the standard model of particle physics, forces are often described by operators, which are mathematical descriptions of how particles interact. Most previous searches focused on the simplest interactions, but this researcher realized that a more complex class of interactions, involving an extra layer of motion or change, could be responsible for the specific type of decay they were interested in. They classified these complex interactions into two distinct groups based on how the particles spin and move, creating a clear map of all the possible ways these decays could occur. They then used a technique called chiral perturbation theory to translate these high-level descriptions into concrete predictions for what happens inside a nucleus. This step was crucial because it allowed them to connect abstract theories to the actual particles, like protons, neutrons, and pions, that detectors are built to see.
With this new framework in place, the researcher calculated the expected rates for several specific decay modes. They looked at the decay of a neutron into a charged particle and a single pion, as well as more complex scenarios where a neutron or proton splits into a charged particle and two pions, or a charged particle, a pion, and an eta particle. The results showed that these three-particle decays are significantly suppressed compared to the simpler two-particle versions, meaning they happen much less frequently. However, the researcher found that the rules governing these decays are tightly linked. If the simpler two-particle decays are forbidden or extremely rare, the more complex three-particle versions might still occur, or vice versa. By using the strict experimental limits already established for the simpler two-particle decays, they were able to derive new, much tighter indirect constraints on the three-particle modes.
The implications of these calculations are profound for future experiments. The researcher found that the current experimental limits on the simpler decays already rule out the possibility of the three-particle decays happening at rates that would be easily visible in current detectors. Specifically, by exploiting correlations between the decay modes, they derived indirect lower limits suggesting that the lifetime of a proton or neutron before it undergoes these specific three-particle decays must be longer than to years, depending on the exact mode. To put this in perspective, the universe is only about years old, meaning these events are so rare that a single atom would likely wait longer than the current age of the universe by many orders of magnitude before decaying in this way. Despite this extreme rarity, the study provides a clear roadmap for what future experiments should look for.
To ensure their findings were not just theoretical exercises, the researcher also constructed a concrete model of new physics that could generate these specific interactions. They proposed a scenario involving heavy, undiscovered particles that interact with ordinary matter in a way that violates the conservation of baryon number. In this model, the heavy particles act as intermediaries, facilitating the decay process at a level that matches the researcher's calculations. This model also suggests that the same physics responsible for these decays could be linked to other major mysteries, such as the nature of dark matter and the reason why the universe is made of matter rather than antimatter. By connecting the search for proton decay to these broader questions, the study adds significant weight to the argument for continuing the hunt for these exotic events.
The work ultimately serves as a comprehensive guide for the next generation of neutrino and proton decay experiments. By systematically mapping out the theoretical possibilities and translating them into observable predictions, the researcher has provided the experimental community with a refined set of targets. They have shown that while the universe may be keeping its secrets well hidden, the rules of the game are now clearer than before. The next step is for massive detectors, capable of watching billions of atoms for decades, to see if nature has chosen to reveal one of these rare, three-particle transformations. If it does, it will not only confirm a new way for matter to decay but also open a window into the fundamental forces that shaped the cosmos.
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