SN1987A constraints on the neutrino-dark-fermion interaction from resonant scattering with C$\nu$B

Neutrinos are the ghostly particles of the universe, trillions of them passing through your body every second without ever touching a single atom. They are so light and so shy that for decades they remained one of the most mysterious pieces of the cosmic puzzle. While we know they exist, we do not fully understand how they get their tiny mass or if they interact with each other in ways we cannot yet see. This uncertainty leaves a gap in our understanding of the universe, particularly regarding why the cosmos is expanding at the rate it is and how matter clumped together to form galaxies. To solve this, physicists have proposed that neutrinos might talk to one another through a new, invisible force carried by a light particle, or perhaps even transform into a type of dark matter that we cannot detect directly. If such interactions exist, they would leave a subtle fingerprint on the neutrinos traveling across the universe, potentially changing the number of particles that reach our detectors.

A team of researchers has now used a historic cosmic event to test these ideas, looking for signs that neutrinos are interacting with a sea of ancient, invisible particles left over from the Big Bang. This ancient sea, known as the cosmic neutrino background, is everywhere, filling the universe just as the cosmic microwave background fills it with light, but it has never been directly observed. The researchers focused on a specific event: a massive star that exploded in a nearby galaxy in 1987, sending a burst of neutrinos toward Earth. By analyzing the data from that explosion, they asked a simple question: did the neutrinos arrive in the numbers and with the energies we expected, or did some of them vanish along the way? Their investigation reveals that the answer depends heavily on how we model the explosion and how the neutrinos change their identity as they travel.

The study centers on a scenario where a neutrino from the exploding star collides with one of the ancient, slow-moving neutrinos from the Big Bang. If the energy of this collision is just right, the two particles can merge to form a short-lived, invisible particle that immediately decays into dark matter. This process acts like a cosmic filter, removing some of the neutrinos from the stream before they reach Earth. The researchers calculated how many neutrinos would be lost in this way and compared those predictions against the actual data recorded by detectors in 1987. They found that the number of neutrinos that survived the journey depends on two main factors: the specific way the star exploded and the nature of the force connecting the neutrinos.

To test this, the team used two different models to describe the explosion. The first model is a simplified, mathematical description that assumes the explosion happens in two distinct stages, with a brief, bright burst of energy followed by a longer, cooling phase. The second model is a complex computer simulation that recreates the physics of a collapsing star in great detail, showing how different types of neutrinos are produced at different times. When they applied their theory of invisible interactions to these models, they discovered a crucial difference. In the simplified model, the neutrinos that vanished were mostly the ones that started as electron neutrinos, and the data from 1987 allowed the researchers to set a strict limit on how strong the invisible force could be. However, in the detailed simulation, the story changed. That simulation predicted a large amount of heavy, non-electron neutrinos being produced very early in the explosion. Because these heavy neutrinos interact differently with the invisible force, they did not vanish as easily. They arrived at Earth and filled in the gaps left by the missing electron neutrinos, effectively hiding the signal of the interaction.

This finding means that the limits scientists can place on these new interactions are not absolute; they depend entirely on which model of the explosion is correct. If the detailed simulation is right, the data from 1987 is not sensitive enough to rule out the existence of these interactions, because the "missing" neutrinos were replaced by others that looked just like the ones we expected to see. The researchers also tested two different types of invisible forces. One type affects all neutrinos equally, while the other affects heavier neutrinos more strongly. They found that if the force is stronger for heavier neutrinos, the detailed simulation actually shows a clear signal of missing particles, because the heavy neutrinos that arrive at Earth are the ones that get filtered out the most. In this specific case, the data from 1987 does provide a meaningful limit, but only because the heavy neutrinos are the ones being removed.

The study concludes that with the limited number of neutrinos detected in 1987, it is impossible to draw a single, definitive conclusion about these invisible interactions without knowing exactly how the star exploded and how the neutrinos changed their identity during the journey. The small amount of data acts like a blurry photograph; depending on how you interpret the background, the subject looks different. However, the researchers are optimistic about the future. They calculated that if a similar explosion were to happen in our own galaxy today, a next-generation detector called Hyper-Kamiokande would be able to capture thousands of neutrinos instead of just a few dozen. With that much more data, the blurriness would disappear. The detector would be able to see the subtle differences in the neutrino stream clearly enough to determine whether these invisible interactions exist, regardless of which explosion model is correct. This future observation would finally allow scientists to close the door on this specific type of new physics or, perhaps more excitingly, open a window to a hidden sector of the universe that has remained invisible until now.