Into the Mix

David Radice research group / Penn State

The material of a neutron star is unlike anything we have on Earth. Rather than atoms and molecules, neutron stars are a dense sea of nuclear material, held together by gravity so intense that electrons and protons can be squeezed together to become neutrons. So there’s much about the physics of neutron stars we still don’t understand. It’s complicated, as they say, and things get even worse when neutron stars collide.

There are two approaches to the study of neutron star collision. The first is observing them. Since the collision of neutron stars can lead to supernova explosions, we can look at the various elements that form in the supernova remnant to get an idea of how neutron stars interact. We can also now look at the gravitational waves produced by the collision for even more data. All that, combined with observations of pulsar neutron stars, gives us a pretty good idea of the overall structure of neutron stars. But there are significant questions that remain, such as whether nucleons within the core of neutron stars break down to become a sea of quarks.

To answer those questions, astronomers look toward the second approach, which is computer simulations. From particle physics we know how nuclear material interacts on a local scale, and so we can extrapolate those principles to create computer models. But collisions between a few dozen nucleons are very different from collisions between neutron stars, particularly when it comes to neutrinos. Now a new model takes this into account.¹

In particle accelerators, neutrinos only play a minor role. Since they only interact weakly with other matter, once two nuclei collide, the neutrinos are free to leave. They take a bit of mass-energy with them, but not much else. But in neutron stars the neutrinos can’t escape easily. The material is so dense that neutrinos produced by one nuclei collision can be absorbed or scattered by other nearby nucleons. This not only affects how nucleons interact, it also changes the neutrinos themselves thanks to a process known as neutrino mixing.

Neutrinos come in three different flavors. Each flavor corresponds to a different type of lepton (electrons, muons, and tauons). Unlike other particles, the flavor of neutrinos isn’t definite. I mean, an electron is an electron. It can’t just spontaneously change into a muon and tauon. But an electron neutrino can switch—physicists call it oscillating—into muon or tauon neutrinos. Technically, neutrinos exist in an indefinite quantum superposition of all three, which you could call a mixed state. Since each neutrino flavor interacts with other matter in slightly different ways, this mixing of neutrino flavors can change the outcome of neutron star collisions.

For this new study, the team created the first computer simulation model that takes neutrino flavor mixing and their interactions with other material into account. They found that neutrino mixing can have a significant impact. For example, since only electron neutrinos can collide with a neutron to create a proton and electron, flavor mixing increases the number of neutrons available for the production of new elements. This can increase the amount of heavy elements, such as gold and platinum, produced by neutron star collisions.

The mixing could also affect things such as the amount of X-rays produced and even the shape of gravitational waves created by the collision. In the future we might be able to compare optical and gravitational observations of neutron star collisions with models like this and finally understand how neutrinos get into the mix.