Photograph of the inside of the MiniBooNE neutrino detector.
Image credit-Fred Ullrich
For some decades now physicists have known that particles called neutrinos come in three types namely electron, muon and tau but during the mid 1990s the Liquid Scintillator Neutrino Detector (LSND),which was an experiment at Los Alamos National Laboratory in New Mexico made a quite abnormal discovery as it detected evidence of a new particle referred to as sterile neutrino which was capable of passing through matter without interacting with it. Further experiments were conducted but none could detect any hidden particle separate from the ones that were known so the results from the LSND experiment were put aside. Various experiments have been conducted to see if the hidden particle detected before could once again be observed but all have so far given results that this particle does not exist. Actually all but one which is MiniBooNE a follow-up experiment at Fermi National Accelerator Laboratory (Fermilab), located near Chicago which has picked up the hidden particle's scent again.
Before we go into more details on this MiniBooNE experiment I will like to take us to the basics and explain what neutrinos and sterile neutrinos are.
Image credit- Eli Perelman
What is a Neutrino?
A neutrino is a tiny and almost weightless subatomic particle that is similar to an electron but possesses no electrical charge unlike said electrons. Neutrinos are very abundant in the universe but are extremely difficult to detect because they don't interact normally with matter and literally cannot be seen because they don't interact electromagnetically that is they don't interact with light. Detecting a neutrino is like trying to catch a bullet with a simple butterfly net. Neutrinos only interact with gravity and electromagnetic decay.
Detecting a neutrino
One common way to detect a neutrino is using a water tank. In this process you fill a big tank with water, light is then shined through the water from one side of the tank and we already know that the velocity of slight slows down in water so if a neutrino with enough energy happens to knock into an electron, the electron will zip through the water faster than the light does. When this happens the electron gives off a weak glow which is like a sonic boom for light and allows us detect the neutrino. The world's largest neutrino detector happens to be a balloon over the south pole which makes use of the whole antarctic ice shield as it's water tank.
Neutrinos tells us that the universe is not the same as it's mirror image. Gravity, electromagnetism and strong nuclear force are the same if you take their mirror images, if you switch left or right, clockwise or counterclockwise their properties will remain the same. However for neutrinos they are all left handed and their mirror images does not exist.Neutrinos are special because they do not behave the way we expect particles to behave according to the standard model.
Neutrinos are produced by many different sources from all the radioactive elements to all the radioactive sources on the earth, nuclear reactors and stars.
Neutrino detector exhibit in the National Museum of Nature and Science, Tokyo, Japan.
Sterile neutrinos
Sterile neutrino are hypothetical particles that do not interact through any of the fundamental means of interaction of the standard model but instead they interact by gravity. They are termed sterile so as to differentiate them from the already known active neutrinos which have been proven to exist. Sterile neutrinos are different from active neutrinos as they are said to possess no charge whereas in the case of the normal active neutrinos they possess a weak nuclear charge.
For over half a century, the understanding of the universe and subatomic particles by scientists have been dominated by the standard model which tends to dictate the behavioral characteristics of every subatomic particle. And after carrying out many experiments scientists have confirmed the standard model to be the accurate predictor of reality. Although it is generally accepted the standard model does not really account totally or in other words I'll say does not give the clear picture of how the universe operates.
Particles like electrons and quarks for instance are the simple easily imagined particles that are the basic building blocks of atoms that makes up matters. The understanding of these simple particles along with some other more complex and abstract ones like the neutrino particles mentioned earlier helps in explaining how matter and energy interact with the cosmos.
All of the matter in the universe are governed by four fundamental forces which are the weak force, the strong force, gravity and electromagnetism. Neutrinos only interact with matter via the weak force and gravity as have been mentioned before, this then tells us that with the aid of specialized detectors we can find neutrinos. The discovery in the LSND experiment brought the possibility that what could be detected might not be the full picture, bringing the possibility of the existence of other types of neutrino dubbed sterile neutrino.
Waves of neutrinos periodically oscillate, randomly moving from one neutrino flavor to another as they stream through space. The LSND and MiniBooNE experiment involve firing beams of neutrinos at a detector hidden behind an insulator to block out all other radiation. In the LSND experiment the insulator used was water while in the MiniBooNE experiment a vat of oil was used as detector. They observed carefully and counted how many of each type of neutrino strike the detector.
Same as the LSND experiment, the MiniBooNE experiment reported more neutrino detection than can be accounted for by the standard model's description of neutrino oscillation.
According to the authors of the report, that suggests, they wrote, that the neutrinos are oscillating into hidden, heavier, "sterile" neutrinos that the detector can't directly detect before oscillating back into the detectable realm. The MiniBooNE result had a standard deviation measured at 4.8 sigma, just shy of the 5.0 threshold physicists look for. (A 5-sigma result has 1-in-3.5-million odds of being the result of random fluctuations in the data.) The researchers wrote that MiniBooNE and LSND combined represent a 6.1-sigma result (meaning more than one-in-500 million odds of being a fluke), though some researchers expressed a degree of skepticism about that claim.
Perhaps this result would have been accepted had it been that the MiniBooNE and LSND experiments were the only neutrino experiment on earth and probably the standard model would have been adjusted to include some sort of sterile neutrinos but other major experiments around the world haven't found any result that will concur with that of LSND and MiniBooNE with the results being doubted by many scientists.
According to the author of the report they haven't found reasons to believe there were errors in the experiment itself, however it could be that there is a way the neutrinos are interacting with the experimental set up in the MiniBooNE and LSND experiments that scientists do not understand at this point. Whatever the case is with time we will see how it plays out.
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