Particles are available in a variety of flavors
There are three types of charged leptons, for example, the lightest of which is the well-known electron that keeps your life running. Quarks are made up of six different types, two of which are found in the protons and neutrons that make up the screen you’re reading this on. With neutrinos, though, things are always more difficult. So, how many different types of neutrinos are there?
So far, scientists have identified three neutrino flavors (electron neutrino, muon neutrino, and tau neutrino) that are related to the three charged lepton tastes (the electron, muon and tau). However, there could be other types of neutrinos, such as the suggested “sterile” neutrinos, which would interact with matter only through gravity and could arrive in a variety of flavors. Researchers are unsure if there are none, one, or a large number of these sterile neutrinos.
Sterile neutrinos would not interact with the weak force in the same manner that regular neutrinos do, making them much more difficult to detect than regular (so-called “active”) neutrinos. After all, a particle that interacts solely through gravity and is as light as a neutrino will be virtually invisible.
Scientists may still catch a glimpse of the stealthy particles because the standard flavors of neutrinos may oscillate, or shift, into sterile neutrinos. It’s possible that if a muon or electron neutrino has to “oscillate” through a sterile phase, the rate at which electron neutrinos appear will change. This is the type of anomaly that scientists have found in several studies, when there were more electron neutrinos than expected based on what they knew about neutrinos.
These anomalies hint to a distance that is optimal for looking for sterile neutrinos. For accelerator experiments, this distance is typically a few hundred meters, allowing detectors to be erected relatively close to the neutrinos’ source—though longer-distance investigations hundreds of kilometers away might also catch sterile neutrinos. Many experiments are now seeking for them, notably Fermilab’s Short-Baseline Neutrino Program, which consists of three investigations.
You might be wondering why scientists don’t know how many different forms of neutrinos there are, especially since they know how many different types of quarks and leptons there are. It originates from some experimental findings that contradict the three-neutrino model. Scientists added an extra neutrino to try to explain these unusual observations.
But first, let’s go through why scientists believe there are three different mass neutrinos to begin with.
If three neutrinos have different masses, there can only be two independent mass differences: a minor mass difference between two neutrinos of similar mass (v1 and 2), and a huge mass difference between the outlier mass neutrino (3) and its nearest neighbor. The total of the other two mass differences equals the third mass difference. It can be compared to a number line. The difference between one and four is three, four and ten is six, and one and ten is three plus six.
Scientists have been able to pinpoint the minuscule mass difference by studying how neutrinos oscillate. The easiest approach to quantify the mass difference is to look at the energy of neutrinos that emerge from reactors that are about 200 kilometers away from their source and have energies of a few million electronvolts.
Scientists have also been able to pinpoint the substantial mass differential. The best way to determine the mass difference is to examine the energy of neutrinos produced by accelerators. The neutrinos have an electronvolt mass of a billion or a few billion, and the detectors are typically 300 to 800 kilometers away from the source.
Scientists have calculated the chances of one of those neutrinos remaining the same flavor. It appears like this in a simplified figure, where there is only one mass difference (or two neutrinos):
P = 1 – sin2 (1.27m2 L/E) P = 1 – sin2 (1.27m2 L/E) P = 1 – sin2 (1.
P is the probability that the neutrino will maintain its original flavor, L is the distance (in kilometers) between where the neutrino was produced and where it was detected, E is the neutrino energy (in billions of electronvolts), and m2 is the difference between the two different neutrino masses squared in this equation (in squared electronvolts).
Scientists investigated whether the prediction would hold true for both reactor and accelerator observations if each experiment measured only one mass change. The energies—E—from the two measurements, however, are substantially different, as we saw earlier. Millions of electronvolts against billions of electronvolts, the accelerator neutrinos had a thousand times greater energy than the reactor neutrinos. Those two vastly different E values result in two dramatically different P values. And if P—the probability that a neutrino retains its original flavor—is different, then the original flavor can transform into two separate tastes. And the only way for it to work is for there to be three different neutrino masses: the original, as well as the two it would oscillate into. This is one of the evidences that scientists use to come to the conclusion that there are three different mass neutrinos.
The “disappearance” of a neutrino flavor has been noticed by scientists. They may transmit 300 electron neutrinos to a detector and only see 100 arrive. When one neutrino flavor “disappears” in a detector, another neutrino flavor should appear to take its place. Otherwise, where are all those nagging neutrinos going? This is exactly what scientists discovered. Experiments using neutrino beams from accelerators revealed that electron-flavored neutrinos appeared in the beam at the same energy as muon-flavored neutrinos were vanishing.
However, this is where the fourth (or fifth, or other) neutrinos enter the picture.
Imagine seeing electron neutrinos arrive in a muon beam at a considerably smaller distance/energy combination — a smaller L/E — than anything scientists have set up before, knowing what you now know. When you perform the arithmetic, you’ll notice that a smaller L/E suggests a significantly higher mass difference than the huge or small mass differences we’ve discussed. One way to make sense of this is to create a new neutrino mass, making the mass difference between that neutrino and the others we know about much bigger, and determined by the new distance/energy combination.
This scenario has now occurred twice, in the LSND (Liquid Scintillator Neutrino Detector) experiment at Los Alamos National Laboratory and MiniBooNE at Fermilab. However, in the two circumstances, the electron neutrino excess did not occur at exactly the same distance/energy combinations. There could be more than three mass neutrinos flying through our cosmos if each of these experimental data is interpreted as oscillations.
Why do neutrino physicists continue to depict a three-neutrino image despite the fact that these abnormalities have been observed in experiments? The Z boson, an extremely hefty force particle, is one of the reasons. Scientists can forecast how often this particle will decay into neutrinos by looking at all of the possible ways it can decay. Scientists know there are only three types of neutrinos into which the Z boson can decay based on the theory of Z boson decays and how often the particle has been measured to decay into neutrinos.
That is, if scientists add a fourth (or fifth, or twentieth) neutrino mass, they will also add a fourth (or fifth, or twentieth) flavor. Then they have to figure out why that flavor doesn’t care as much about the Z boson as the other neutrinos. The Z boson would have to be invisible for the other flavors to come along for the voyage, making the extra flavors a form of “sterile” or noninteracting neutrino.
The jury is still out on whether the universe contains more than three neutrinos, and a number of studies are underway to confirm or disprove any of these extra neutrinos.