The discovery of the electron by Sir Joseph John Thomson in 1897 is a huge landmark in our seemly endless quest for the fundamental components that make up the universe. Since then, more than 200 other subatomic particles have been detected, although most of them are highly unstable, existing for less than a millionth of a second. Historically, each step we made towards answering the question of what the universe is made of may feel like digging a bottomless pit. Yet, physicists all around the world keep on digging, and with that, interesting discoveries are bound to happen from time to time.
Over 30 years after the discovery of the electron, Wolfgang Ernst Pauli postulated the existence of one of the most elusive particles known to this day, the neutrino. Neutrinos are virtually devoid of mass and have no electric charge. These particles were proposed to explain the energy that seems to vanish during radioactive decay. Today, we know that there are a billion times more neutrinos than atoms in the universe, and scientists estimate that 100 trillion of them pass through our bodies every second without pause. Yet, it took twenty-six years for scientists to finally be able to experimentally detect the existence of neutrinos.
The peculiar nature of neutrinos has always attracted the attention of particle physicists.
Throughout the years, conflicting experimental results challenged the original idea around these fundamental particles, calling for new theories. Scientists explored models all the way from the simplest concept of a single unknown particle to more complex ideas proposing a class of particles encompassing multiple species. Recently, the MicroBooNE experiment at the Fermi National Accelerator Laboratory and the IceCube detector at the South Pole both published independent analyses that suggest we may be close to the solution of this puzzle. Now, some physicists suspect that neutrinos might be the key to understand several mysteries of the universe.
The number of things we don’t know is vastly greater than what we know.
A simple piece of evidence for the inarguable truth of this is that most of the universe is beyond the reach of our observations. As the frontiers of the universe expand, many portions of the universe are so distant from us that their light simply did not have enough time to reach us yet. Furthermore, visible matter – the stuff that makes up the Earth and everything on it, including us and everything we have ever observed with all of our instruments, only makes up about 5% of the universe. The rest is composed of 27% dark matter and 68% dark energy, about which little is known because they are invisible to our measuring instruments.
Particle physicists always suspected that neutrinos could be a window to at least some of these otherwise inaccessible portions of the universe.
These particles have been so relevant that most of the noteworthy developments in the history of neutrino research have resulted in Nobel Prizes in Physics to the scientists behind them. The first example was in 1988 when the winners were the discoverers of the muon neutrino in 1962. In 1995, the award honored the first team to detect the neutrino back in 1956. In 2002, the detection of cosmic neutrinos was the highlight. More recently, in 2015, Dr. Takaaki Kajita and Dr. Arthur B. McDonald received the award for demonstrating that neutrinos have mass.
At first, Pauli thought neutrinos had no mass and no electric charge, making him doubt if an experiment could ever detect them. The first time neutrinos were detected in 1956 was a triumph in particle physics. But, as scientists gathered data from different sources and experimental setups, conflicting results kept arising. When physicists detected neutrinos coming from the sun, they found fewer than half the number predicted by theory. In other cases, like the experiment at the Liquid Scintillator Neutrino Detector (LSND), where scientists looked for bursts of radiation created by neutrinos, they found far more than the theory predicted.
Looking for models that could explain the odd behavior of neutrinos, Bruno Pontecorvo proposed that neutrinos are like shape-shifters, and they would transform between three types as they travel – electron, muon, and tau neutrino. This theory would explain why neutrinos seem to vanish in some experimental setups – instead, they would be transforming, which creates the impression of disappearing. The mathematics of this idea required that each of the three neutrino species existed as a mix of three different masses in quantum mechanics. Thus, it also helped physicists to conclude that neutrinos are not massless after all.
The results of the LSND experiment corroborated the theory, but the inferred rate of transformation among species overshot the rate implied by solar and atmospheric neutrinos, which affected the credibility of the results. The authors of the experiments proposed that rather than being errors, the unexpected results might be caused by the existence of an unknown type of neutrino. They named this new type “sterile neutrino.” It would be indifferent to the force that ropes the other species of neutrinos into interactions with atoms, making it undetectable.
To investigate this inconsistency, Dr. Janet Conrad, professor at the Massachusetts Institute of Technology, led a team to set up a neutrino detector at Fermilab – the MiniBooNE – and collected data on neutrino oscillation from 2002 to 2019. Their results were similar to those from the LSND. But at the same time, many other independent experiments supported the three-neutrino model. Overall, there was more evidence supporting the existence of three neutrinos than four, including analyses of data extracted from a picture of the early universe produced by the Planck space telescope in 2013.
But the three-neutrino models still couldn’t account for the conflicting evidence found on different experimental setups. Errors are rarely as consistent as the results found by LSND and MiniBooNE, so scientists started exploring new possibilities, including the existence of several types of neutrinos. This assembly of particles could interact through unknown forces, forming an invisible “dark sector.”
This large family of neutrinos and their peculiar nature inspired new possibilities and even resurrected old ideas. Scientists have long wondered if neutrinos could account for the dark matter in the universe. That would not be possible if only the three known types of neutrinos existed, but it might be if that is not the case. From the rate of expansion of the universe to the reason why galaxies do not cluster as much as they should if dark matter was a single particle, a dark sector could be the key to many unanswered questions.
Following that line of thought, Dr. Carlos Argüelles-Delgado, assistant professor at Harvard University, and colleagues recently proposed a new dark sector model with a combination of heavy neutrinos decaying and light-weight neutrinos oscillating, which accounts for the data from the LSND and MiniBooNE experiments. If correct, this model can lead us to the answers like the origin of neutrino mass, why there is so much more matter than anti-matter in the universe, and even what dark matter is.
Preliminary results of Fermilab’s MicroBooNE and the data collected by the IceCube experiment in the South Pole seem to corroborate the dark sector model. MicroBooNE is an amped-up version of the MiniBooNE, which, unlike its predecessor, can distinguish between signals created by electron neutrinos and those from particle decays. With that, researchers expect to determine if the unexpectedly high abundance of signals in previous experiments could derive from heavy invisible neutrinos decaying inside the detector. In addition, analyses of the results of the IceCube experiment as well as considering all neutrino experiments together also seem to fit the dark sector model.
As we deepen our knowledge of neutrinos, scientists contemplate the immensity of unanswered questions moving us forward.
Now, almost a hundred years after the first conception of the neutrino, we face the challenge of accessing this hypothetical dark sector with the same type of doubt that Pauli faced, wondering if we could ever detect these elusive particles.
The big difference is that today we have a better-coordinated effort of many research groups worldwide simultaneously exploring multiple approaches. For example, Fermilab is investing in exploring the complete pattern of neutrino oscillation and in the capacity to detect heavier dark sector particles. In turn, IceCube can detect very energetic neutrinos from the collisions of cosmic rays with the atmosphere, which could provide experimental evidence of the existence of the dark sector particles. And, even though heavy neutrinos “may be completely inaccessible to any reasonable experiment,” according to Dr. Joshua Spitz, professor at the University of Michigan, there is no doubt that scientists will keep on digging.