Two of the most mysterious and elusive particles in the cosmos, neutrinos and dark matter, might be tied together by a weak force that permeates galaxies and gives neutrinos mass.
Most fundamental particles get their mass from the Higgs field, which permeates the entire universe like a pool of molasses, slowing down all particles that interact with it and making them heavy. But neutrinos are a million times lighter than the next lightest particle, the electron, so some have hypothesised that they may get their mass from some other, weaker force.
at Brookhaven National Laboratory in New York, and his colleagues have come up with an idea for what that force might be. They calculated that if dark matter particles and neutrinos were able to interact, the dark matter could weigh down neutrinos.
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“This model is sort of like the Higgs mechanism, but it’s not the Higgs mechanism,” says at Vanderbilt University in Nashville, Tennessee. The more dark matter the neutrinos travel through, the heavier they would get. This would result in neutrinos with masses under 0.1 electron volts, consistent with our observations.
Particle patchwork
This suggested force is repellent, but not very strong. So, to get through a clump of dark matter, a neutrino must move very quickly. It would slow slightly as it travelled. After gaining mass through this interaction, neutrinos encountering more dark matter would eventually be repelled once they gained enough mass and lost enough speed.
Most of the neutrinos we’ve seen in our solar system are moving quite fast, which allows them to hurtle through the dark matter pervading our local area and get to us. The force isn’t strong enough to noticeably slow down these speedy particles, but its effects would be more noticeable for neutrinos that were moving slowly to begin with.
“If you go to other areas in space where there is a higher concentration of dark matter you would find a larger neutrino mass, and if you go somewhere where dark matter is very sparse, you would find that neutrinos have very little mass,” says Davoudiasl. The universe would be a patchwork of heavy and near-massless neutrinos.
Weiler says the model is “plausible, but fine-tuned”, meaning that the researchers had to essentially invent values to plug into their models for how this hypothetical dark force would work. “That doesn’t mean that nature didn’t follow that path, but it becomes less plausible the more fine-tuned it is,” he says.
If this force does exist, it will be nearly impossible to detect. Our galaxy has roughly the same density of dark matter everywhere, so neutrino mass should be about the same everywhere that we can observe.
Big bang leftovers
Even if we were able to create dark matter in a laboratory, we’d never be able to make enough to measurably affect neutrino mass, says Davoudiasl.
One way we could detect this force’s effects is by looking for the oldest neutrinos in the universe. Relic neutrinos are leftovers from the big bang expected to permeate everything – and because they’ve been around for so long, they should be moving relatively slowly.
That low energy means that the dark matter force would repel them. “They may come near these regions of space where their mass starts to turn on, but if that mass starts to get too big they’d get turned around and propelled back,” says Davoudiasl.
So if proposed experiments to look for relic neutrinos actually find them, that would mean that there is no dark matter force giving neutrinos mass. But if they don’t, it could imply that these two types of matter that pass through almost everything without affecting it are actually interacting with one another.
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Read more: The sun’s energy could speed up dark matter so we can detect it
