Discovering the hidden particles of the universe

Concept art of colliding neutron stars

During binary neutron star collisions, hot neutrinos can be briefly trapped at the interface, remaining out of equilibrium with the cool cores of the merged stars for 2 to 3 milliseconds. This interaction helps guide the particles toward equilibrium and provides new insights into the physics of such couplings. Credit:

New simulations show that neutrinos were created during these cataclysms neutron star the collisions are briefly out of thermodynamic equilibrium with the cool cores of the merging stars.

Recent simulations by Penn State physicists have shown that in binary neutron star mergers, hot neutrinos can be trapped for a short time and remain out of equilibrium, providing a new understanding of these cosmic events. This research emphasizes the role of simulations in the study of phenomena that cannot be replicated experimentally.

What happens when neutron stars collide?

When stars collapse, they often leave behind extremely dense but relatively small and cool remnants called neutron stars. If two stars collapse in close proximity, the remaining binary neutron stars spin up and eventually collide, heating the collision point to extreme temperatures.

New simulations of these events show the hot neutrinos—tiny, essentially massless particles that rarely interact with other matter—that are created during the collision can be briefly trapped at these interfaces and remain out of equilibrium with the cold cores of the stars. together for 2 to 3 milliseconds. During this time, simulations show that neutrinos can interact weakly with the star’s matter, helping to push the particles toward equilibrium—and providing new insight into the physics of these powerful events.

Innovative simulations of neutron star mergers

A paper describing the simulations, by a research team led by Penn State physicists, was recently published in the journal Physical review papers.

“For the first time in 2017, we observed here on Earth signals of various types, including gravitational wavesfrom a binary neutron star merger,” said Pedro Luis Espino, a postdoctoral researcher at Penn State and University of California, Berkeley, who led the research. “This led to a huge increase in interest in the astrophysics of binary neutron stars. There is no way to reproduce these events in a laboratory to study them experimentally, so the best window we have into understanding what happens during a binary neutron star merger is through mathematically based simulations arising from Einstein’s theory of general relativity.

Simulation of binary neutron star mergers

Volume density translation in a simulation of a neutron star binary merger. The new research shows that neutrinos created at the hot interface between merging stars can be trapped for a short time and remain out of equilibrium with the cool cores of the merging stars for 2 to 3 milliseconds. Credit: David Radice, Penn State

Neutron Star Composition and Collision Dynamics

Neutron stars get their name because they are thought to be composed almost entirely of neutrons, the uncharged particles that, along with positively charged protons and negatively charged electrons, make up atoms. Their incredible density – only black holes are smaller and denser – is thought to squeeze protons and electrons together, fusing them into neutrons. A typical neutron star is only tens of kilometers across, but has about one and a half times the mass of our Sun, which is about 1.4 million kilometers across. A teaspoon of neutron star material can weigh as much as a mountain, tens or hundreds of millions of tons.

“Pre-merger neutron stars are effectively cold, while they can be billions of degrees Kelvin, their incredible density means that this heat contributes very little to the energy of the system,” said David Radice, assistant professor of physics and astronomy and astrophysics. in Penn State’s Eberly College of Science and a research team leader. “As they collide, they can get really hot, the interface of colliding stars can heat up to temperatures in the trillions of degrees Kelvin. However, they are so dense that photons cannot escape to dissipate heat; Instead, we think they cool by emitting neutrinos.”

Insights from neutrino behavior in star mergers

According to the researchers, neutrinos are created during collisions as neutrons in stars collide with each other and explode into protons, electrons and neutrinos. What happens next in those first moments after a collision has been an open question in astrophysics.

To answer this question, the research team created simulations that require massive amounts of computing power that model the merger of binary neutron stars and all the associated physics. The simulations showed for the first time that, however briefly, even neutrinos can be trapped by the heat and density of the merger. Hot neutrinos are out of equilibrium with the still-cool cores of stars and can interact with stellar matter.

“These extreme events stretch the limits of our understanding of physics, and studying them allows us to learn new things,” Radice said. “The period when the merging stars are out of balance is only 2 to 3 milliseconds, but just like temperature, time is relative here, the orbital period of the two stars before the merger can be as little as 1 millisecond. This brief out-of-equilibrium phase is when the most interesting physics happens, once the system returns to equilibrium, the physics is better understood.”

The researchers explained that the precise physical interactions that occur during the merger can affect the types of signals that can be observed on Earth from binary star mergers.

“How neutrinos interact with the star’s matter and are eventually emitted can affect the oscillations of the merging remnants of the two stars, which in turn can affect what the merger’s electromagnetic and gravitational wave signals look like when they reach us. here. on Earth,” said Espino. “Next generation gravitational wave detectors can be designed to look for these kinds of signal differences. In this way, these simulations play a crucial role in allowing us to gain insight into these extreme events informing future experiments and observations in a kind of feedback loop.”

Reference: “Neutrino trapping and out-of-equilibrium effects in neutron star merger binary remnants” by Pedro Luis Espino, Peter Hammond, David Radice, Sebastiano Bernuzzi, Rossella Gamba, Francesco Zappa, Luís Felipe Longo Micchi and Albino Perego, Albino20 2024 , Physical review papers.
DOI: 10.1103/PhysRevLett.132.211001

In addition to Espino and Radice, the research team includes postdoctoral researchers Peter Hammond and Rossella Gamba at Penn State; Sebastiano Bernuzzi, Francesco Zappa and Luís Felipe Longo Micchi at Friedrich-Schiller-Universität Jena in Germany; and Albino Perego at the Università di Trento in Italy.

Funding from the US National Science Foundation; US Department of Energy (DOE), Office of Science, Division of Nuclear Physics; Deutsche Forschungsgemeinschaft; and the European Union’s Horizon 2020 and Europa Horizon initiatives supported this research. The simulations were performed on the Bridges2, Expanse, Frontera and Perlmutter supercomputers. The research used the resources of the National Energy Research Center for Scientific Computing, a DOE Office of Science User Facility supported by the US Department of Energy’s Office of Science. The authors acknowledge the Gauss Center for the eV Supercomputer

for funding this project by providing computing time on the GCS SuperMUC-NG Supercomputer at the Leibniz Supercomputing Center.

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