By Dev Kapadia, ’23, Physical Science, 7/26/20
Figure 1: The above table depicts the elementary particles dictated by the standard model of particle physics. As shown, the standard model contains three different flavors of neutrinos and their corresponding leptons: electron neutrino, muon neutrino, and tau neutrino. However, recent research has suggested that there is a fourth neutrino, named the “sterile” neutrino, that is larger in mass than the other flavors and could provide explanations for strange findings resulting from experiments in particle physics.
Source: Wikimedia Commons
One of the biggest ideas fascinating virtually every scientific discipline is the question of how our universe was created, and further, why it is the way it is today. A multitude of intricate processes went into creating our world, including the presence of more matter as opposed to antimatter.
Antimatter was first theorized by Paul Dirac, as seen in his papers published in 1928.1 Antimatter is in essence exactly what it sounds like: the opposite of matter. The sub-atomic particles in antimatter have opposite properties of the sub-atomic particles in matter, thereby constituting antimatter.2 For instance, the positron, which is a common product of beta decay, behaves like electrons, but positrons have a positive charge instead of the negative charge of electrons. Similarly, protons have a positive charge and behave like the negatively charged antiprotons, their corresponding anti-matter sub-atomic particle.1
It was theorized that at the beginning of the universe, there was an equal amount of matter and anti-matter. However, it is a mystery as to why we see much more matter than anti-matter in our world today. This question is thought to be answered by the presence of neutrinos.3 Neutrinos are particles that are believed to have an extremely small, almost negligible, amount of mass. Because of their virtual weightlessness, they travel near the speed of light and rarely interact with matter; in fact, you have numerous neutrinos passing through you every second. It is theorized that their involvement in the universe’s founding caused the abundance of matter relative to antimatter that we see today. However, because of their weightlessness and subsequent weak interactions, it is extremely hard for them to be studied, and so their exact involvement is still unknown.4
Originally, scientists believed that neutrinos were so elusive that it was likely they might not be constituted of matter at all. The main piece of evidence that tipped scientists off to the fact that neutrinos might have a small amount of mass instead of no mass is their ability to change between what are called “flavors.” Each neutrino comes in one of three types of flavors: electron neutrino, muon neutrino, and tau neutrino. Each of these flavors is considered indivisible and is therefore within the category of “fundamental particles.” The only means scientists have of differentiating between these neutrinos are from the corresponding non-neutrino fundamental particles they attract: electron, muon, and tau. The interesting property that scientists saw was that neutrinos attracted different sub-atomic particles as they moved, indicating that neutrinos actually change flavors when they travel! Scientists theorized that this particle change was a result of neutrinos changing their mass states as they move through space.5 Scientists predict that the small masses of the different neutrino flavors are composed of three different mass states. However, scientists are not sure about the value of these mass states or even the mass state composition of the three flavors; for example, electron neutrinos could be 60% mass state 1, 30% mass state 2, and 10% mass state 3. Their reasoning for suggesting the existence of mass states is supported by Einstein’s theory of relativity because if neutrinos were massless, they would be able to move at the speed of light, where time would stand still. But since neutrinos change flavors and mass states over time, they must experience changing time and thus must have a small amount of mass to facilitate these changes in mass state.3
However, there is now new evidence that suggests there might not just be three flavors at all! Through data collected during the Mini Booster Neutrino Experiment (MiniBooNE) at Fermi National Accelerator Laboratory, scientists William Louis, Richard Van de Water, and their team now believe there is “more than a 99.999999 percent chance” that there is a new neutrino flavor. Named the “sterile” neutrino, this flavor is suggested to be the most elusive of all. While the small size and the neutral charge inhibit neutrinos from interacting with matter through the strong force and electromagnetic force, they are still able to interact with matter through the two weaker forces: weak force and gravitational force. Surprisingly, these “sterile” neutrinos are theorized to be massive enough that they escape even the weak force.
Though they do not have photographical evidence of these “sterile” neutrinos, the research team saw that muon neutrinos were disappearing faster than expected while electron neutrinos were being formed more frequently than expected. In order to explain the high number of muon neutrinos that are changing to electron neutrinos quicker than expected, they predict that they must be changing into these sterile undetectable neutrinos while they are traveling through space. These sterile neutrinos are thought to be more massive than the other three flavors, thereby speeding up the oscillation process between the mass states and reducing the distance needed to be travelled for a muon neutrino to transform into an electron neutrino.3
The team also cites various other studies with unexpected results that may be explained by the presence of sterile neutrinos. For instance, the sun, like a nuclear reactor, produces neutrinos due to the numerous chemical reactions that are carried out. However, when studies attempted to estimate the number of neutrinos emitted, it was much lower than expected. This might be because many of the neutrinos are in the sterile form and are not detected.3
Though there is no observable evidence of sterile neutrinos, there are numerous laboratories currently investigating their existence. Their detection would have important implications for physics, especially the Standard Model of particle physics that only allows for three neutrinos. Louis, Water, and the rest of their team are hopeful that someone will be able to find evidence. The existence of the sterile neutrino might play an important role in explaining the lower abundance of antimatter than expected, especially considering the fact that the sterile neutrino might have been the crucial tipping point that allowed for the world that we see today.
- Antimatter. (n.d.). Retrieved July 12, 2020, from https://www.iop.org/resources/topic/archive/antimatter/index.html#gref
- Live Science Staff. (2014, June 20). What is Antimatter?Com. https://www.livescience.com/32387-what-is-antimatter.html
- Louis, W., & Van de Water, R. (2020, July 1). Hidden Neutrino Particles May Be a Link to the Dark Sector. Scientific American. https://doi.org/10.1038/scientificamerican0720-46
- Marder, J. (2011, January 25). What is a Neutrino…And Why Do They Matter? | PBS NewsHour. https://www.pbs.org/newshour/science/what-is-a-neutrino-and why-should-anyone-but-a-particle-physicist-care
- Neutrino flavors | All Things Neutrino. (n.d.). Retrieved July 13, 2020, from https://neutrinos.fnal.gov/types/flavor/