The very first neutrino experiments discovered that neutrinos exist in three flavors and can oscillate between those flavors as they travel through space. However, many recent experiments have collected anomalous data that contradicts a three neutrino flavor hypothesis, suggesting instead that there may exist a fourth neutrino, called the sterile neutrino, that interacts solely through the gravitational force. While there is no conclusive evidence proving the existence of a fourth neutrino flavor, scientists designed the IceCube laboratory at the South Pole to search for this newly hypothesized particle. Due to its immense size and sensitivity, the IceCube laboratory stands as the most capable neutrino laboratory to corroborate the existence of these particles.
Neutrinos are subatomic, ubiquitous, elementary particles that are produced in a variety of ways. Some are produced from collisions in the atmosphere between different particles, while others result from the decomposition and decay of larger atoms.1,3 Neutrinos are thought to play a role in the interactions between matter and antimatter; furthermore, they are thought to have significantly influenced the formation of the universe.3 Thus, neutrinos are of paramount concern in the world of particle physics, with the potential of expanding our understanding of the universe. When they were first posited, neutrinos were thought to have no mass because they have very little impact on the matter around them. However, decades later, it was determined that they have mass but only interact with other matter in the universe through the weak nuclear force and gravity.2
Early neutrino experiments found that measuring the number of neutrinos produced from the sun resulted in a value almost one third of the predicted value. Coupled with other neutrino experiments, these observations gave rise to the notion of neutrino flavors and neutrino flavor oscillations. There are three flavors of the standard neutrino: electron (ve), muon (vμ), and tauon (v𝜏). Each neutrino is a decay product that is produced with its namesake particle; for example, ve is produced alongside an electron during the decay process.9 Neutrino oscillations were also proposed after these results, stating that if a given type of neutrino is produced during decay, then at a certain distance from that spot, the chance of observing that neutrino with the properties of a different flavor becomes non-zero.2 Essentially, if ve is produced, then at a sufficient distance, the neutrino may become either vμ or v𝜏. This is caused by a discrepancy in the flavor and mass eigenstates of neutrinos.
In addition to these neutrino flavor states, there are also three mass eigenstates, or states in which neutrinos have definite mass. Through experimental evidence, these two different states represent two properties of neutrinos. As a result, neutrinos of the same flavor can be of different masses. For example, two electron neutrinos will have the same definite flavor, but not necessarily the same definite mass state. It is this discrepancy in the masses of these particles that actually leads to their ability to oscillate between flavors with the probability function given by the formula P(ab) = sin2(2q)sin2(1.27Dm2LvEv-1), where a and b are two flavors, q is the mixing angle, Dm is the difference in the mass eigenstate values of the two different neutrino flavors, L is the distance from source to detector, and E is the energy of the neutrino.6 Thus, each flavor is a different linear combination of the three states of definite mass.
The equation introduces the important concept of the mixing angle, which defines the difference between flavor and mass states and accounts for neutrino flavor oscillations. Thus, if the mixing angle were zero, this would imply that the mass states and and flavor states were the same and therefore no oscillations could occur. For example, all muon neutrinos produced at a source would still be muon neutrinos when P(mb) = 0. On the other hand, at a mixing angle of π/4, when P(mb) = 1, all muon neutrinos would oscillate to the other flavors in the probability function.9
Some experimental data has countered the notion of three neutrino flavor oscillations.3 If the experimental interpretation is correct, it would point to the existence of a fourth or even an additional fifth mass state, opening up the possibility of other mass states that can be taken by the hypothesised sterile neutrino. The most conclusive anomalous data arises from the Liquid Scintillator Neutrino Detector (LSND) Collaboration and MiniBooNE. The LSND Collaboration at Los Alamos National Laboratory looked for oscillations between vm neutrinos produced from muon decay and ve neutrinos. The results showed a lower-than-expected probability of oscillation.6 These results highly suggest either an oscillation to another neutrino flavor. A subsequent experiment at Fermilab called the mini Booster Neutrino Experiment (MiniBooNE) again saw a discrepancy between predicted and observed values of ve appearance with an excess of ve events.7 All of these results have a low probability of fit when compared to the standard model of particle physics, which gives more plausibility to the hypothesis of the existence of more than three neutrino flavors.
GALLEX, an experiment measuring neutrino emissions from the sun and chromium-51 neutrino sources, as well as reactor neutrino experiments gave inconsistent data that did not coincide with the standard model’s predictions for neutrinos. This evidence merely suggests the presence of these new particles, but does not provide conclusive evidence for their existence.4,5 Thus, scientists designed a new project at the South Pole to search specifically for newly hypothesized sterile neutrinos.
IceCube, a particle physics laboratory, was designed specifically for collecting data concerning sterile neutrinos. In order to collect conclusive data about the neutrinos, IceCube’s vast resources and acute precision allow it to detect and register a large number of trials quickly. Neutrinos that come into contact with IceCube’s detectors are upgoing atmospheric neutrinos and thus have already traversed the Earth. This allows a fraction of the neutrinos to pass through the Earth’s core. If sterile neutrinos exist, then the large gravitational force of the Earth’s core should cause some muon neutrinos that traverse it to oscillate into sterile neutrinos, resulting in fewer muon neutrinos detected than expected in a model containing only three standard mass states, and confirming the existence of a fourth flavor.3
For these particles that pass upward through IceCube’s detectors, the Earth filters out the charged subatomic particle background noise, allowing only the detection of muons (the particles of interest) from neutrino interactions. The small fraction of upgoing atmospheric neutrinos that enter the ice surrounding the detector site will undergo reactions with the bedrock and ice to produce muons. These newly created muons then traverse the ice and react again to produce Cherenkov light, a type of electromagnetic radiation, that is finally able to be detected by the Digital Optical Modules (DOMs) of IceCube. This radiation is produced when a particle having mass passes through a substance faster than light can pass through that same substance.8
In 2011-2012, a study using data from the full range of DOMs, rather than just a portion, was conducted.8 This data, along with other previous data, were examined in order to search for conclusive evidence of sterile neutrino oscillations in samples of atmospheric neutrinos. Experimental data were compared to a Monte Carlo simulation. For each hypothesis of the makeup of the sterile neutrino, the Poissonian log likelihood, a probability function that finds the best correlation of experimental data to a hypothetical model, was calculated. Based on the results shown in Figure 2, no evidence points towards sterile neutrinos.8
Other studies have also been conducted at IceCube, and have also found no indication of sterile neutrinos. Although there is strong evidence against the existence of sterile neutrinos, this does not completely rule out their existence. These experiments have focused only on certain mixing angles and may have different results for different mixing angles. Also, if sterile neutrinos are conclusively found to be nonexistent by IceCube, there is still the question of why the anomalous data appeared at LSND and MiniBooNE. Thus, IceCube will continue sterile neutrino experiments at variable mixing angles to search for an explanation to the anomalies observed in the previous neutrino experiments.
- Fukuda, Y. et al. Evidence for Oscillation of Atmospheric Neutrinos. Phys. Rev. Lett. 1998, 81, 1562.
- Beringer, J. et al. Review of Particle Physics. Phys. Rev. D. 2012, 86, 010001.
- Schmitz, D. W. Viewpoint: Hunting the Sterile Neutrino. Physics. [Online] 2016, 9, 94. https://physics.aps.org/articles/pdf/10.1103/Physics.9.94
- Hampel, W. et al. Final Results of the 51Cr Neutrino Source Experiments in GALLEX. Phys. Rev. B. 1998, 420, 114.
- Mention, G. et al. Reactor Antineutrino Anomaly. Phys. Rev. D. 2011, 83, 073006.
- Aguilar-Arevalo, A. A. et al. Evidence for Neutrino Oscillations for the Observation of ve Appearance in a vμ Beam. Phys. Rev. D. 2001, 64, 122007.
- Aguilar-Arevalo, A. A. et al. Phys. Rev. Lett. 2013, 110, 161801.
- Aartsen, M. G. et al. Searches for Sterile Neutrinos with the IceCube Detector. Phys. Rev. Lett. 2016, 117, 071801.