Phenomenology of strangeness-changing two-body currents in antineutrino scattering

In the subatomic world, particles called antineutrinos are ghostly travelers that rarely interact with matter, yet when they do strike a nucleus, they can transform a neutron into a proton or a proton into a neutron. Sometimes, this interaction is even more dramatic: the antineutrino can change a particle's very identity, turning a neutron into a strange particle known as a hyperon. These hyperons are unstable and decay quickly, but before they vanish, they bounce around inside the atomic nucleus, interacting with other particles. Scientists study these rare events to understand how strange matter behaves within the dense environment of an atomic nucleus, a key piece of the puzzle for understanding the fundamental forces of nature. For years, researchers have focused on the simplest version of this event, where an antineutrino hits a single particle inside the nucleus and creates a hyperon. However, nature is rarely that simple, and there is a possibility that the antineutrino could instead strike a tightly linked pair of particles, creating a hyperon and kicking out a second particle at the same time.

A recent study by physicist J. A. Nowak investigates this overlooked possibility, specifically looking at what happens when an antineutrino hits a pair of nucleons—a proton and a neutron stuck together—and produces a hyperon along with a second nucleon. This process, known as a two-body current, had been largely ignored in computer simulations used to predict what particle detectors would see. Previous models had simply guessed that this rare event might happen about 15 percent as often as the standard single-particle event, but they had no real data to support that number. Nowak's work moves beyond guessing by calculating the actual likelihood of this two-particle interaction and determining how it changes the picture for experiments currently running or planned for the near future.

The researcher built a detailed model to compare the strange, two-particle event against a similar, non-strange event where no hyperon is created. By calculating both processes on the same pair of particles and using established nuclear physics models to scale the results up to a full nucleus, the study determined the true ratio between the two types of events. The findings show that the two-particle interaction is not a constant 15 percent as previously assumed. Instead, its frequency changes depending on the energy of the incoming antineutrino. At lower energies, around 0.6 billion electron volts, the two-particle event occurs about 14 percent as often as the standard single-particle event. As the energy rises to between 2 and 5 billion electron volts, this ratio increases to between 20 and 22 percent. When averaged over the specific energy spectrum of the Booster Neutrino Beam, the rate settles at roughly 15 to 17 percent, which is close to the old guess but with a much more accurate understanding of how it varies with energy.

This discovery has immediate consequences for how scientists interpret data from detectors like SBND and DUNE, which use liquid argon to catch these particles. The study found that these two-particle events leave distinct fingerprints that differ from the standard single-particle hits. For instance, because the energy is shared between two outgoing particles instead of one, the reconstructed energy of the event appears lower than it actually is. In simulations, these events were found to be reconstructed as being 12 to 30 percent lower in energy than they truly are, a significant shift that could skew measurements if not accounted for. Furthermore, these events are more likely to produce a proton at the exact spot where the collision happens, whereas the standard events usually do not. In fact, about half of these two-particle events leave a proton with enough energy to be seen by the detector, compared to only about one-fifth of the standard events.

One of the most surprising and specific findings concerns a particular type of particle called a positive Sigma hyperon. In standard single-particle interactions, it is impossible to create a positive Sigma hyperon without also creating a kaon, a different type of particle. However, the two-particle interaction can create this positive Sigma hyperon directly at the collision point without a kaon. This means that a significant portion of the positive Sigma hyperons seen in detectors without a kaon are not coming from the nuclear interactions scientists thought they were studying, but from these two-particle collisions. The study estimates that these two-particle events provide more than a quarter of the kaon-less positive Sigma hyperons used to study how hyperons change their charge inside the nucleus. This suggests that previous analyses trying to measure charge exchange might have been mixing two different physical processes together.

Despite these clear results, the study also highlights where the science remains uncertain. The biggest unknown is how to perfectly separate the two-particle signal from a standard single-particle event that happens to bounce off a second particle immediately afterward. Depending on how the researchers defined this separation, the estimated rate of two-particle events could vary from 12 to 22 percent at 1 billion electron volts. There is also a possibility that the two processes could interfere with each other, a quantum effect that has not yet been calculated but could be as large as the two-particle rate itself. The study concludes that while the current models are a major step forward, a complete understanding will require more complex calculations that include the interference between these processes and the specific behavior of particles inside the nucleus. For now, the new data provides a much sharper tool for physicists to tune their detectors and ensure they are seeing the true nature of these rare subatomic transformations.