the incredibly dense remnants of collapsed stars – are some of the most energetic events in the universe, producing a variety of signals that can be observed on Earth. New simulations of neutron star mergers by a team from Penn State and the University of Tennessee Knoxville reveal that the mixing and changing of tiny particles called neutrinos that can travel astronomical distances undisturbed impacts how the merger unfolds, as well as the resulting emissions. The findings have implications for longstanding questions about the origins of metals and rare earth elements as well as understanding physics in extreme environments, the researchers said.
The paper, published in the journal Physical Review Letters, is the first to simulate the transformation of neutrino “flavors” in neutron star mergers. Neutrinos are fundamental particles that interact weakly with other matter, and come in three flavors, named for the other particles they associate with: electron, muon and tau. Under specific conditions, including the inside of a neutron star, neutrinos can theoretically change flavors, which can change the types of particles with which they interact.
“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.”
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.”
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.
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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