Physicists have stated that their search for evidence of a "light sterile neutrino," a hypothetical particle that could provide answers to cosmic mysteries including the formation of dark matter and why the Universe is composed of matter at all, has come up empty-handed.
The Fermilab MicroBooNE experiment was supposed to follow up on prior evidence of neutrinos acting strangely, but the negative result puts the hypothesis of a new elementary particle to rest.
Neutrinos are enigmatic subatomic particles that come in three different kinds, according to current theory. The Liquid Scintillator Neutrino Detector (LSND) experiment in Los Alamos, however, discovered more of one sort than anyone expected in 1995.
Most explanations hypothesized the presence of a fourth type of neutrino, known as a sterile neutrino, which scarcely interacts with normal matter at all.
More recent experiments have shown data that are broadly consistent with the sterile neutrino hypothesis, but the MicroBooNE finding calls the theory into question.
What is the definition of a sterile neutrino?
Neutrinos are subatomic particles proposed in 1930 by Austrian physicist Wolfgang Pauli to explain how some radioactive atoms fire forth electrons.
It wasn't until 1956 that Americans Clyde Cowan and Frederick Reines witnessed brief flashes of light caused by neutrinos slamming into atoms in a tank of water that their existence was established.
Neutrinos are now an important feature of particle physics' Standard Model. This is our best particle theory, describing the 17 known elementary particles and how they interact through three fundamental forces (electromagnetism and the strong and weak forces).
The 17 particles in the Standard Model are divided into two groups: 12 fermions, which make up matter, and five bosons, which carry the forces.
All forces interact with fermions, however, not all fermions interact with all forces. Neutrinos, for example, are solely impacted by the weak force (as well as gravity, which isn't included in the Standard Model).
The electron, muon, and tau neutrinos are the three families of fermions, each of which has a neutrino.
In terms of the weak force, all of these neutrinos are "left-handed." It's difficult to put into words exactly what that entails, but suffice to say that left- and right-handed particles are mirror images of one another, and the weak force affects them differently.
All other fermions are available in both left- and right-handed forms. This leads us to believe that right-handed neutrinos exist in nature as well.
In search of sterile neutrinos
How would we know if sterile neutrinos exist? One method is to exploit a phenomenon known as neutrino oscillation, which allows the three types of neutrinos to convert into each other.
Experiments that measure these oscillations usually look at how many of a certain type of neutrino appear or disappear in a specific circumstance.
MicroBooNE (which yielded the new negative result) and its predecessor MiniBooNE were both "appearance" experiments, as was the LSND experiment that first inspired the sterile neutrino idea.
They fire a muon-neutrino beam over a short distance (between 30 and 500 meters) and count the number of electron neutrinos discovered at the other end.
They observed more electron-neutrinos than expected at LSND and MiniBooNE. Other studies have shown that muon-neutrinos cannot directly oscillate into electron-neutrinos at such long distances.
However, if some muon-neutrinos decay into very light sterile neutrinos, which subsequently decay into electron-neutrinos, it may explain how those extra electron-neutrinos appeared.
if there are neutrinos that are sterile?
There's a strong possibility that if experiments prove the existence of a light sterile neutrino, heavier sterile neutrinos exist as well.
The nature of the "dark matter" that appears to make up the majority of the Universe, why neutrinos have any mass at all, and why the Universe contains so much more matter than antimatter could all be answered by these heavier relatives.
There is only one issue. For cosmologists, the light sterile neutrino we started with is a headache.
We should be able to see indications of sterile neutrinos created shortly after the Big Bang if it exists.
Recent surveys of the cosmic microwave background radiation, as well as the distribution of galaxies and light elements in the space between them, have found no evidence of these sterile neutrinos.
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