Weblog
5 November 2025
David Radice analysis group / Penn State
The fabric of a neutron star is not like something we’ve on Earth. Reasonably than atoms and molecules, neutron stars are a dense sea of nuclear materials, held collectively by gravity so intense that electrons and protons could be squeezed collectively to turn out to be neutrons. So there’s a lot in regards to the physics of neutron stars we nonetheless don’t perceive. It’s sophisticated, as they are saying, and issues get even worse when neutron stars collide.
There are two approaches to the research of neutron star collision. The primary is observing them. For the reason that collision of neutron stars can result in supernova explosions, we will take a look at the assorted parts that type within the supernova remnant to get an concept of how neutron stars work together. We are able to additionally now take a look at the gravitational waves produced by the collision for much more information. All that, mixed with observations of pulsar neutron stars, provides us a reasonably good concept of the general construction of neutron stars. However there are vital questions that stay, comparable to whether or not nucleons throughout the core of neutron stars break all the way down to turn out to be a sea of quarks.
To reply these questions, astronomers look towards the second method, which is laptop simulations. From particle physics we all know how nuclear materials interacts on a neighborhood scale, and so we will extrapolate these rules to create laptop fashions. However collisions between a couple of dozen nucleons are very completely different from collisions between neutron stars, notably relating to neutrinos. Now a brand new mannequin takes this under consideration.
In particle accelerators, neutrinos solely play a minor function. Since they solely work together weakly with different matter, as soon as two nuclei collide, the neutrinos are free to depart. They take a little bit of mass-energy with them, however not a lot else. However in neutron stars the neutrinos can’t escape simply. The fabric is so dense that neutrinos produced by one nuclei collision could be absorbed or scattered by different close by nucleons. This not solely impacts how nucleons work together, it additionally adjustments the neutrinos themselves due to a course of often called neutrino mixing.
Neutrinos are available three completely different flavors. Every taste corresponds to a special sort of lepton (electrons, muons, and tauons). Not like different particles, the flavour of neutrinos isn’t particular. I imply, an electron is an electron. It might probably’t simply spontaneously change right into a muon and tauon. However an electron neutrino can swap—physicists name it oscillating—into muon or tauon neutrinos. Technically, neutrinos exist in an indefinite quantum superposition of all three, which you may name a blended state. Since every neutrino taste interacts with different matter in barely other ways, this mixing of neutrino flavors can change the result of neutron star collisions.
For this new research, the workforce created the primary laptop simulation mannequin that takes neutrino taste mixing and their interactions with different materials under consideration. They discovered that neutrino mixing can have a big influence. For instance, since solely electron neutrinos can collide with a neutron to create a proton and electron, taste mixing will increase the variety of neutrons accessible for the manufacturing of recent parts. This could enhance the quantity of heavy parts, comparable to gold and platinum, produced by neutron star collisions.
The blending might additionally have an effect on issues comparable to the quantity of X-rays produced and even the form of gravitational waves created by the collision. Sooner or later we’d be capable to examine optical and gravitational observations of neutron star collisions with fashions like this and at last perceive how neutrinos get into the combination.
