Neutrino Transformations Play Pivotal Role in Neutron Star Mergers

The intricate dance of neutrinos during a neutron star merger holds significant importance in determining how these astronomical events unfold. A breakthrough study led by a team of physicists has for the first time simulated the transformation of neutrino flavors during such a cataclysmic merger, revealing that these transformations can significantly influence the outcomes of the collision, particularly in the production of heavy r-process elements like gold and platinum, observed in the explosive phenomenon known as a kilonova.

The simulations indicated that by excluding neutrino flavor transformations from the models, the yield of heavy elements could drop by an entire order of magnitude, demonstrating the critical role these particles play. As reported, the discovery of how these elements form emphasizes the necessity for further understanding in astrophysics.

Related: A Heavy Element Detected Forming in a Neutron Star Merger

“Historically, previous simulations of binary neutron star mergers have omitted the flavor transformations of neutrinos,” explains physicist Yi Qiu of The Pennsylvania State University. “This omission stems from the challenges of capturing events that occur on a nanosecond timescale and the limited theoretical knowledge surrounding these transformations, which extend beyond the standard model of physics.”

In their recent simulations, Qiu and colleagues found that the degree and location of neutrino mixing and transformation affect not only the ejected matter from the merger but also the remnant structure and composition, as well as the surrounding materials.

Commonly referred to as “ghost particles” due to their extraordinarily low mass and minimal interaction with other matter, neutrinos come in three varieties: electron, muon, and tau. Quantum mechanics allows these tiny particles to oscillate between different flavors during their travel. The flavor that remains upon contact with other particles can significantly alter their interactions, potentially leading to major consequences in extreme environments like neutron star mergers.

In these violent cosmic collisions, which feature some of the most dense objects in the universe, Qiu and his team focused on simulating neutrino transformations, specifically the conversion of electron-type to muon-type neutrinos – a process essential to the merger’s environment.

Neutron star collisions are critical sites for producing heavy elements. The nuclear fusion processes within stars can generate elements up to iron, while heavier elements such as gold and uranium originate from the rapid-neutron-capture process, known as the r-process.

“Electron-type neutrinos can convert a neutron – one of an atom’s fundamental components – into a proton and an electron, whereas muon-type neutrinos lack this capability. Therefore, the flavor conversion of neutrinos directly affects the neutron availability in the system, which has implications for heavy metal and rare earth element synthesis,” highlights physicist David Radice from Penn State.

Notably, Qiu and colleagues found that accounting for neutrino mixing in their simulations may boost element production by a factor of ten. Their findings also suggest potential increases in the brightness of post-merger gravitational waves by as much as 20 percent due to these transformations. Nevertheless, considerable uncertainties linger, including aspects of how and when these flavor transitions occur during neutron star mergers. More refined simulations are needed to clarify these processes.

“Current theories indicate that neutrino transformations are likely, and our simulations show that, should they occur, they can have substantial ramifications that make their inclusion essential in future models and analyses,” concludes Qiu.

This significant research has been documented in Physical Review Letters.