The transformation of neutrinos in a neutron star merger could be crucial to shaping the way events of the collision unfold.
For the first time, a team of physicists has simulated how neutrinos change their flavors during a merger event. Their results show that tweaking the parameters and outcomes of these transformations also alters the outcomes of the merger, including the heavy r-process elements such as gold and platinum produced in the violent kilonova explosion.
In fact, when neutrino transformations are removed from the simulation entirely, heavy element production is lower by up to an entire order of magnitude.
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"Previous simulations of binary neutron star mergers have not included the transformation of neutrino flavor," says physicist Yi Qiu of The Pennsylvania State University.
"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 fall 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."
Named 'ghost particles' thanks to their incredibly low masses and limited interactions with other particles, neutrinos come in three "flavors" corresponding to particles with which they are associated: electron, muon, and tau.
Quantum effects cause the tiny particles to oscillate between each flavor as they travel, with their final form affecting their interactions with any particles they meet. Under extreme conditions, this flavor-swap could make a big difference.
Neutron star collisions certainly qualify as extreme, involving some of the densest objects in the Universe. Qiu and colleagues simulated neutrino transformations during neutron star collisions, tweaking various parameters, including the transformations involved.
They particularly focused on the electron-to-muon neutrino conversion, which is the most relevant transformation within the merger environment.
Neutron star collisions are known to be factories in which heavy elements are produced. The fusion that takes place inside stellar cores can only produce elements up to iron; the r-process, or rapid-neutron-capture process, is how certain others are made, such as gold, uranium, and strontium.
"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 an 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," explains physicist David Radice of Penn State.
"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 transformations could also increase the brightness of the post-merger gravitational waves by up to 20 percent; however, there's still a lot that remains unknown. For example, the researchers are unsure exactly how and when the transformations take place in neutron star mergers. Refined simulations could help answer these questions.
"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," Qiu says.
The research has been published in Physical Review Letters.