Speaker
Description
We are searching for dark matter and with the help of neutrinos in these days. There are three known types or flavors of neutrinos, and they have some rather strange properties. One of these strange properties is their helicity. Elementary particles have spin, and when they travel the spin is either oriented along their direction of motion (right-handed helicity) or opposite to their motion (left-handed helicity). Most particles can have either helicity depending on the interaction, but the helicity of neutrinos is always left-handed. We aren’t entirely sure why, but we do know that if right-handed neutrinos exist they wouldn’t interact with regular matter through the electroweak force. They would only interact with matter gravitationally, so they are known as sterile neutrinos.
If sterile neutrinos exist, and they are just regular neutrinos with right-handed helicity, then they would be hot dark matter and not the cold dark matter we’re looking for. But there are some theories where sterile neutrinos are much more massive than regular neutrinos. These heavy sterile neutrinos could comprise dark matter. That is if they exist.
If there are heavy sterile neutrinos out there, they could be discovered by their radioactive decay. Heavy particles can decay into lighter particles over time, so it’s possible that sterile neutrinos can decay to their lighter counterparts, emitting x-ray photons in the process. In an effort to discover these x-ray emissions, a team combed through data from the Chandra X-Ray Observatory. They didn’t find any evidence of sterile neutrinos. Their results weren’t strong enough to entirely rule out the idea, but it does narrow down the theoretical candidates a bit. Specifically, the study places a hard limit on how sterile neutrinos can decay if they exist.
We still don’t know what dark matter is. Studies like this might seem disappointing, but they play an important role. By narrowing down our options, they force us to focus on more viable dark matter candidates. We’ve learned something more, but for now, we are still in the dark.
We are searching for dark matter and with the help of neutrinos in these days. There are three known types or flavors of neutrinos, and they have some rather strange properties. One of these strange properties is their helicity. Elementary particles have spin, and when they travel the spin is either oriented along their direction of motion (right-handed helicity) or opposite to their motion (left-handed helicity). Most particles can have either helicity depending on the interaction, but the helicity of neutrinos is always left-handed. We aren’t entirely sure why, but we do know that if right-handed neutrinos exist they wouldn’t interact with regular matter through the electroweak force. They would only interact with matter gravitationally, so they are known as sterile neutrinos.
If sterile neutrinos exist, and they are just regular neutrinos with right-handed helicity, then they would be hot dark matter and not the cold dark matter we’re looking for. But there are some theories where sterile neutrinos are much more massive than regular neutrinos. These heavy sterile neutrinos could comprise dark matter. That is if they exist.
If there are heavy sterile neutrinos out there, they could be discovered by their radioactive decay. Heavy particles can decay into lighter particles over time, so it’s possible that sterile neutrinos can decay to their lighter counterparts, emitting x-ray photons in the process. In an effort to discover these x-ray emissions, a team combed through data from the Chandra X-Ray Observatory. They didn’t find any evidence of sterile neutrinos. Their results weren’t strong enough to entirely rule out the idea, but it does narrow down the theoretical candidates a bit. Specifically, the study places a hard limit on how sterile neutrinos can decay if they exist.
We still don’t know what dark matter is. Studies like this might seem disappointing, but they play an important role. By narrowing down our options, they force us to focus on more viable dark matter candidates. We’ve learned something more, but for now, we are still in the dark.
We review sterile neutrinos as possible Dark Matter candidates. After a short summary on the role of neutrinos in cosmology and particle physics, we give a comprehensive overview of the current status of the research on sterile neutrino Dark Matter. First we discuss the motivation and limits obtained through astrophysical observations. Second, we review different mechanisms of how sterile neutrino Dark Matter could have been produced in the early universe. Finally, we outline a selection of future laboratory searches for keV-scale sterile neutrinos, highlighting their experimental challenges and discovery potential.
Sterile neutrinos are, as we say in the US, a whole new ball game. Unlike standard model neutrinos, we don’t know if they are real. And unlike the neutrinos we know, they seem to only interact through gravity. Sounds boring, you might think. Why bother? First of all, everybody knows that I like a good dark matter candidate. I am especially fond of one that I can argue should exist anyway, regardless of our missing, invisible matter problem. Sterile neutrinos share two of my favourite qualities for a hypothetical particle: they are well-motivated and they happen to be interesting dark matter candidates.
We think sterile neutrinos should exist thanks to a property of standard model neutrinos: handedness. Specifically, the neutrinos we know are lefties (and antimatter antineutrinos are all righties). Though I am referring to this as handedness, this property – formally known as chirality – isn’t quite like everyday life because it isn’t classical. Like particle spin, it is a quantum feature.
Every known particle can come in both left and right-handed forms – apart from neutrinos. They come only as left-handed particles. Naturally, over the years, physicists have wondered whether there are right-handed neutrinos (and left-handed antineutrinos).
The sterile neutrino is that hypothetical right-handed neutrino. It is named “sterile” because it only interacts through gravity. While this property makes sterile neutrinos different from other neutrinos, they do have a mass and aren’t electrically charged, just like standard model neutrinos. This means they could be dark matter and, unlike standard model neutrinos, they potentially have sufficient mass to explain the apparent gravitational impact of dark matter’s presence.
“Detecting ordinary neutrinos is difficult enough. That work is even more complicated with sterile neutrinos”
Those of us who are theorists get the exciting work of figuring out how the idea that sterile neutrinos could be dark matter would work mathematically. Experimentalists get the joy – and incredible challenge – of going out and looking for physical evidence.
One of these searches recently caused some headlines by finding a null result: no sterile neutrinos. Detecting ordinary neutrinos is difficult enough. That work is even more complicated with sterile neutrinos, which can only be “seen” through their interactions with quantum fluctuations of their standard model counterparts. To find sterile neutrinos, you have to look for a specific type of behaviour in everyday neutrinos.
The experiment that recently announced results, MicroBooNE, is located at Fermilab, not far from Chicago. It consists of a large container of argon attached to a beamline where neutrinos are produced by colliding protons together. It is easier to follow the trajectory of neutrino events in argon, due to its high density and sensitivity to the charged particles that are produced in the collisions.
MicroBooNE’s primary task is to better understand how neutrinos interact with argon and to try to replicate the hints seen in earlier experiments that sterile neutrinos are real. Two experiments, MiniBooNE and LSND, saw an excess of muon neutrinos oscillating into electron neutrinos over distances that didn’t physically make sense. This oddity could be explained if the muon neutrinos were first becoming sterile neutrinos, before changing into electron neutrinos.
Sadly for some, the MicroBooNE team announced recently that it hadn’t, so far, seen the same electron neutrino excess. This is consistent with data from other experiments, leaving us with quite the mystery. Why are different experiments getting different results? We don’t know.
But even if nothing turns up once we have explored every place this hypothetical particle could be hiding, that will still be valuable. If sterile neutrinos turn out to only be a figment of the particle theorist’s imagination, we will know it is time to move on.
| Session | Neutrino Physics |
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