Hamish Robertson was a Michigan State professor with tenure in 1980. Since his postdoctoral year in 1971, he had been there, and he was happy. I want to emphasize how appreciated and content I felt there, he says. It was and still is a fantastic location. However, he had started to formulate a concept with his buddy and coworker Tom Bowles that would take him far from MSU. They were coming up with a fresh experiment to determine the mass of the mysteriously light and elusive neutrino.
The only fundamental particles whose mass is yet unknown are neutrinos. Neutrinos are extremely tiny particles, as their name suggests. But they are 10 billion times more numerous than the other elementary particles. Knowing their mass is essential to filling in the gaps in our knowledge of the cosmos because of their collective abundance, which makes it likely that they had an impact on the emergence of structures in the early universe. But how can you measure something whose mass is almost zero? This issue has occupied the careers of hundreds of physicists, including Robertson, and they are making progress. There is hope that the mission can be completed thanks to research initiatives being carried out in Europe and the United States.
To measure directly or indirectly, that is the question
How would you weigh anything, such as your dog, if you were asked? You might place the dog in your automobile, observe the compression, measure how many inches the car is displaced, and then use this information to calculate the dog’s mass. You would need to know the car’s weight, the shocks’ technical parameters, the tire pressure, and something about spring constants. This is a difficult and indirect route. You may also simply place the dog on a bathroom scale.
The choice between direct and indirect approaches depends on the resources at your disposal. In the absence of a bathroom scale, an indirect measurement using your vehicle may be your best alternative. Regarding neutrinos, scientists have encountered a similar issue. Astrophysicists interested in the mass of neutrinos received assistance in 1987 from a rare local supernova. Using the spectrum data acquired from the star explosion, scientists were able to produce an indirect measurement that established a maximum value for the neutrino mass.
As in the car example, they performed a calculation using mathematical models, known quantities, and interactions between various system components. According to Diana Parno, associate professor at Carnegie Mellon University and US spokesperson for the Karlsruhe Tritium Neutrino direct mass experiment, or KATRIN, cosmologists have also made indirect measurements by looking for the imprint of neutrino mass on the cosmic microwave background, a faint radiation in space.
“Ideally, we’d prefer an Earth-based measurement of neutrino mass, so that we can compare it to the cosmological measurement,” explains Parno. Consequently, while cosmologists and astrophysicists turn to the heavens, experimentalists such as Robertson, Bowles, and Parno take a direct approach in their search for the neutrino mass. The phantom particles have little interaction with electromagnetic fields or the nuclear strong force, and they are so light that gravity exerts minimal impact on them. The experimentalist must be inventive.
Building a bathroom scale for neutrinos
Robertson and Bowles conceived a novel method for directly measuring the neutrino mass in 1980. However, only a few locations possessed the necessary technology, finances, and other resources for the proposed experiment. The Los Alamos National Laboratory of the US Department of Energy was one of them. Bowles was already employed in Los Alamos, and he wished for Robertson to join him there. Robertson faced adversity. “Ever since I became conscious of science as a child, I was familiar with Los Alamos. He says, “I’ve always believed that would be the most fantastic location to be.”
Bowles summoned Robertson to the laboratory to convince him to participate in the dive. One evening, he took Robertson to the Southwestern restaurant Rancho de Chimayo for dinner. Robertson says, “On the way back, we stopped the car and turned off the lights.” “We stepped out of the car so I could admire the amazing starry carpet in the crisp mountain air.” Robertson was acquired. He was prepared for the journey to determine neutrino mass. So how does one go about constructing a neutrino bathroom scale? If you have a dog that refuses to sit on the scale, you can first weigh yourself alone, then add your pet.
The difference between them is the dog’s weight. Utilizing the same concept, neutrino researchers take use of processes that produce neutrinos. To reestablish equilibrium, a neutron in an unstable nucleus undergoes beta decay and becomes a proton. As a neutron transforms into a proton, it emits an electron with a negative charge and a neutrino. Electron capture is another process that generates neutrinos: A proton in an unstable nucleus catches an electron from the inner shell, transforms into a neutron, and emits a neutrino. In both circumstances, the occurrences generate an exact amount of energy, which can be found in a table.
The exact quantity of energy is equal to the difference between the masses of the father and daughter atoms. And this energy is shared by the products: neutrino and electron in beta decay, or neutrino and excited daughter atom in electron capture. Since experimentalists measure energies, they can determine the neutrino’s energy. Then, utilizing the time-tested equation E = mc2, they convert the neutrino’s energy to mass.
Awkward space in the Standard Model
Why is the study of neutrino mass so intriguing to scientists such as Robertson and Bowles? Because the unresolved issue has disturbed the scientific community’s conception of the universe. Neutrinos should be massless, according to the Standard Model of particle physics, our current best explanation of the fundamental forces and particles that make up everything. However, investigations conducted in the 1990s demonstrated that they must have mass.
“Neutrino mass is the first experimental evidence of physics outside the Standard Model, which is really cool,” says Parno. According to Robertson, neutrinos are the only matter particles for whom the Standard Model produced a mass prediction. And that forecast was incorrect. “There are instances of items not included in the Standard Model. Not included in the Standard Model is gravity. The quark mass cannot be predicted. Only in this one instance has the Standard Model actually made a prediction, and it was incorrect.”
Indirectly determined by cosmology, the neutrino mass upper limit is around one millionth the mass of the next-lightest particle, the electron. That is comparable to the difference between a mouse, which weighs approximately 25 grams, and five elephants, which collectively weigh over 25,000 kilograms. Parno states, “That’s a tremendous disparity.” This vacant region within the Standard Model is odd. The difference suggests that neutrino masses may be exceptional. Or they could be false. However, unless we have a metric to work with, we cannot say anything definitively. Consequently, this measurement is an essential initial step.
Even if scientists determined the neutrino’s mass, the study would not be complete. Neutrino theorist and head of the Center for Neutrino Physics at Virginia Tech, Patrick Huber, thinks it would help rule out some theories and models, but there would still be issues. “It’s not as if, once you have the measurement, you automatically choose the appropriate model,” he explains. “It’s not as though every theorist has predicted a specific neutrino mass, and someone will have the correct theory once it’s measured. However, it would raise a multitude of bigger problems.”
Because of this, Huber has dedicated his career to neutrinos. “If they discover the neutrino mass, this will be a really interesting development. But what would be even more interesting is if the neutrino mass is not where it should be, because that would indicate that something new is occurring,” he explains. Then we are compelled to begin thinking anew.
A ghost worth chasing
Rewind to the 1980s: Bowles’ argument was successful. Robertson claims that “the siren melody of Los Alamos was too much to refuse.” Robertson’s spouse was also considering moving to the West. She had to fight an uphill struggle to advance her career in the 1970s as a female nuclear physicist at a lab where men predominated. Together, the couple obtained positions at the lab and flew to the area to purchase a home. Robertson relates, “I still recall the sensation of euphoria on the glide path into Albuquerque. “It was a fresh start.”
They moved formally a few months later, bringing their son, who was then 6 months old, along with them. Karl-Erik Bergkvist, a Swedish physicist, announced a new neutrino mass limit of 55 electronvolts in 1972. In the opinion of Robertson, Bowles, and their coworkers, they could do better. Their plan was to use tritium, a radioactive hydrogen isotope, to examine beta decay. Tritium had also been employed by Bergkvist, although it had been implanted in metal. It was intended to be used in a purer, gaseous form by Robertson and Bowles.
The lab’s management was supportive of their proposal. How much money do you need, they enquired? Well, I didn’t know,” Robertson admits. He then announced a number. They responded, “Okay, fine. That was it, then. We departed. They were able to lower the upper limit to roughly 10 electronvolts with the help of that financing, which ultimately wasn’t quite enough but at least got them started. Importantly, they had demonstrated the viability of tritium decay tests. Robertson changed his focus and enlisted in the Sudbury Neutrino Observatory in 1988. The team first showed neutrino oscillation in 2001, proving neutrinos having mass and ultimately winning the 2015 Nobel Prize in Physics.
After establishing that neutrinos have mass, scientists are now focused on creating a neutrino bathroom scale. Everyone who had previously participated in a tritium experiment was invited to join the KATRIN experiment as it was taking shape. By that time, Robertson had relocated to the University of Washington, but he was still happy to join the collaboration and even offered to build and supply the detection system. The KATRIN experiment, located in Karlsruhe, Germany, now depends on contributions from 150 researchers from seven other nations. One of the researchers is Parno. She, like others, was drawn to the project by the excitement of pursuing an uncharted territory. She remarks, “Neutrinos are extremely strange. They never cease to astound and perplex us. I believe there is still more that neutrinos can teach us.
The most sophisticated direct mass measurement experiment is KATRIN. In February 2022, the KATRIN cooperation announced an intriguing finding: Neutrinos must weigh less than 0.8 electronvolts. The KATRIN finding offers a ceiling, and oscillation trials gave a floor. Robertson declares, “We are drawing closer. The range from the indirect astrophysical supernova measurement is substantially wider than this one. According to Parno, “the supernova limit is about 5.7 electronvolts, which is approximately seven times looser than the existing KATRIN limit.” In addition, according to Robertson, it is near to the limit from cosmological indirect measurements, which is between 0.12 and 0.5 electronvolts depending on the model’s chosen parameters. The current standard for all direct mass investigations is the 0.8 electronvolt figure from KATRIN, although other creative researchers are hot on his heels.
Another tritium-based concept was developed in 2009 by Case Western Reserve University associate professor Benjamin Monreal and Massachusetts Institute of Technology professor Joe Formaggio. This concept served as the basis for Project 8, a neutrino mass experiment. They presented a really lovely proposal, according to Robertson. It’s one of those concepts that makes you think, “Oh, I wish I had thought of that.”
Since the beginning of Project 8, Robertson has collaborated with the team. Although Project 8 utilizes a different method to estimate the energy of the released electron, the experiment similarly involves tritium decay. The microwave radiation that is emitted by charged particles in a magnetic field in a circular orbit is known as cyclotron radiation, and it is the frequency of this radiation that is being measured.
Although it is really very little, Robertson claims that you can quantify the power that is radiated. The project is continuously being developed and tested in many ways. The Project 8 team anticipates measuring the neutrino mass with a sensitivity of roughly 0.04 electronvolts if the new method performs as expected. Molecules of the holmium isotope 163Ho are used in the third method that is now being researched as a direct method to determine neutrino mass. According to researcher Loredana Gastaldo, a junior professor at the Kirchhoff Institute for Physics at the University of Heidelberg and the ECHo experiment’s spokeswoman, it is also difficult.
Find an innovative and intelligent way that enables you to actually learn something about neutrinos, she advises, if you want to understand more about them. You have to put in a lot of effort to learn even the slightest bit more about neutrinos because they don’t offer anything as a gift. Electron capture events in 163Ho are essential to the ECHo experiment. Scientists employ microcalorimeters, a completely different kind of detector than those used in KATRIN and Project 8, to implant 163Ho ions. The concept is that if energy is added to the detector, the temperature will rise. and we can use exceedingly accurate thermometers to monitor this really modest temperature increase, said Gastaldo. Direct mass measuring investigations are progressing thanks to human ingenuity (and perseverance), yet each study is unique.
Robertson, who is currently a professor emeritus at the University of Washington’s Center for Experimental Nuclear Physics and Astrophysics in Seattle, formally resigned in 2017. However, he is still engaged with Project 8. The cooperation is newer than KATRIN and has only carried out proof-of-concept studies thus far. The creation of a complete, large-scale detector is the next phase. When you wake up on some days, you find out something is simply not going to work, adds Robertson. We’re doomed, you say. Oh my gosh. “And after working for a few more days—or weeks—or months or years, you chat to your friends and someone comes up with a solution. Suddenly, the sun comes out again, the issue is resolved, and you can move on.
I continue to work on this endeavor round-the-clock because I genuinely enjoy it. He is not alone either. Among the hundreds of other neutrino specialists who have devoted their professional lives to determining the neutrino’s mass are Parno, Gastaldo, and Huber. They all depend on one another. Gastaldo claims, “We are all learning from all the other groups. And because of this, the partnerships are quite lively and full of talks that are extremely insightful and in which we are all learning a lot. In order to maximize the capacity of neutrino physicists to learn from one another, Gastaldo organized the NuMass conference in 2016. A total of 40 scientists from the US and Europe, each with a distinct area of expertise, attended the inaugural meeting. Gastaldo remarks that the discussions were unbelievably in-depth.
In 2018, 2020, and 2022, the conference was repeated by the group. According to Parno, “I think it’s extremely fascinating to have so many different domains of expertise that turn out to be required in order to unravel the secrets of this incredibly light, incredibly infrequently interacting particle that somehow changed the universe.” Researchers studying neutrino masses have contagious optimism. They keep working to push the boundaries of human creativity with a real passion. Newcomers are always welcome, too. Young people are necessary because they are the ones who can truly accomplish things, according to Robertson. There is a sizable crowd of individuals. I’m just walking around, leaping from one gigantic to another’s shoulder. That’s one of the fun things about science.