New simulations show that neutrino flavor transformations change both the composition and the signals left behind after neutron star collisions.
When two neutron stars collide and merge, the result is one of the most energetic events in the universe. These cataclysms generate multiple kinds of signals that can be detected from Earth.
New simulations from researchers at Penn State and the University of Tennessee Knoxville show that the way tiny particles called neutrinos mix and change affects both how the merger progresses and what it emits. Neutrinos can travel across vast cosmic distances with almost no disturbance, and their behavior shapes the outcome of the collision.
According to the team, the results speak to longstanding questions about where metals and rare earth elements come from, and they also help scientists probe physics in extreme environments.
The study, published in Physical Review Letters, is the first to model the transformation of neutrino “flavors” during neutron star mergers. Neutrinos are fundamental particles that interact only weakly with matter and appear in three flavors named for the particles they accompany: electron, muon and tau. Under particular conditions, including those inside a neutron star, neutrinos can theoretically switch flavors, which in turn changes the kinds of particles they interact with.
“Previous simulations of binary neutron star mergers have not included the transformation of neutrino flavor,” said Yi Qiu, graduate student in physics in the Penn State Eberly College of Science and first author of the paper. “This is partly because this process happens on a nanosecond timescale and is very difficult to capture and partly because, until recently, we didn’t know enough about the theoretical physics underlying these transformations, which falls outside of the standard model of physics. In our new simulations, we found that the extent and location of neutrinos mixing and transforming impacts the matter that is ejected from the merger, the structure and composition of what remains after the merger — the remnant — as well as the material around it.”
Building Advanced Simulations
The researchers built a computer simulation of a neutron star merger from the ground up, incorporating a variety of physical processes, including gravity, general relativity, hydrodynamics and the neutrino mixing. They also accounted for the transformation of electron flavor neutrinos to muon flavor, which the researchers said is the most relevant neutrino transformation in this environment. They modeled several scenarios, varying the timing and location of the mixing as well as the density of the surrounding material.
The researchers found that all of these factors influenced the composition and structure of the merger remnant, including the type and quantities of elements created during the merger. During a collision, the neutrons in a neutron star can be launched at other atoms in the debris, which can capture the neutrons and ultimately decay into heavier elements, such as heavy metals like gold and platinum as well as rare earth elements that are used on Earth in smart phones, electric vehicle batteries and other devices.
“A neutrino’s flavor changes how it interacts with other matter,” said David Radice, Knerr Early Career Professor of Physics and associate professor astronomy and astrophysics in the Penn State Eberly College of Science and an author of the paper. “Electron type neutrinos can take a neutron, one of the three basic parts of an atom, and transform it into the other two, a proton and electron. But muon type neutrinos cannot do this. So, the conversion of neutrino flavors can alter how many neutrons are available in the system, which directly impacts the creation of heavy metals and rare earth elements. There are still many lingering questions about the cosmic origin of these important elements, and we found that accounting for neutrino mixing could increase element production by as much as a factor of 10.”
Detectable Emissions from Earth
Neutrino mixing during the merger also influenced the amount and composition of matter ejected from the merger, which the researchers said could alter the emissions detectable from Earth. These emissions typically include gravitational waves — ripples in space time — as well as electromagnetic radiation like X-rays or gamma rays.
“In our simulations, neutrino mixing impacted the electromagnetic emissions from neutron star mergers and possibly the gravitational waves as well,” Radice said. “With cutting-edge detectors like LIGO, Virgo and KAGRA and their next generation counterparts, such as the proposed Cosmic Explorer observatory that could start operations in the 2030s, astronomers are poised to detect gravitational waves more often than we have before. Better understanding how these emissions are created from neutron star mergers will help us interpret future observations.”
The researchers said modeling the mixing processes was similar to a pendulum being turned upside down. Initially, many changes occurred on an incredibly rapid timescale, but eventually the pendulum settles to a stable equilibrium. But much of this, they said, is an assumption.
“There’s still a lot we don’t know about the theoretical physics of these neutrino transformations,” Qiu said. “As theoretical particle physics continues to advance, we can greatly improve our simulations. What remains uncertain is where and how these transformations occur in neutron star mergers. Our current understanding suggests they are very likely, and our simulations show that, if they take place, they can have major effects, making it important to include them in future models and analyses.”
Now that the infrastructure for these complex simulations has been created, the researchers said they expect other groups will use the technology to continue to explore the impacts of neutrino mixing.
“Neutron star mergers function like cosmic laboratories, providing important insights into extreme physics that we can’t replicate safely on Earth,” Radice said.
Reference: “Neutrino Flavor Transformation in Neutron Star Mergers” by Yi Qiu, David Radice, Sherwood Richers and Maitraya Bhattacharyya, 26 August 2025, Physical Review Letters.
DOI: 10.1103/h2q7-kn3v
In addition to Qiu and Radice, the research team includes Maitraya Bhattacharyya, postdoctoral scholar in the Penn State Institute for Gravitation and the Cosmos, and Sherwood Richers at the University of Tennessee, Knoxville. Funding from the U.S. Department of Energy, the Sloan Foundation and the U.S. National Science Foundation supported this work.
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