A primer on neutrinoless double-beta decay

At Sanford Underground Research Facility, we often talk about the Majorana Demonstrator’s search for “neutrinoless double-beta decay.” We say that this process could be incredibly important to understanding the imbalance of matter and anti-matter in the early universe. We explain how it is difficult to detect, demanding a minuscule background. We show photos of germanium detectors and ultra-pure copper shields, then describe immaculate cleanrooms and show off stylish Tyvek garb. 

But what exactly is neutrinoless double-beta decay?

Dr. Vincente Guiseppe is the co-spokesperson for the Majorana Demonstrator collaboration and an assistant professor of physics and astronomy at the University of South Carolina. 

The best way to explain this mysterious process, Guiseppe said, is to work backward, defining one word at a time. So, let’s start at the end.

Decay

“There are two types of isotopes,” Guiseppe explains, “stable and radioactive.”

The nuclei of a stable isotope are relaxed, meaning, they have a very low energy state. The nuclei of a radioactive isotope, on the other hand, are in a high energy state—they are very excited. But objects in nature prefer to be relaxed, Guiseppe said.

So how do nuclei achieve a lower energy state? Through radioactive decay.

“In nuclear physics, decay means a relaxation or a change of an atomic nucleus,” Guiseppe explained. “Nature allows protons and neutrons to change their makeup to achieve a desirable equilibrium. Once a nucleus is at the lowest energy state, we call it a stable isotope.”

A lot of times, the words “radioactive decay” sound threatening. That’s because they often are used in the context of radiation you don’t want—radiation that is dangerous or destructive. In reality, though, radioactive decays are taking place all the time.

“Potassium 40 is an isotope in our bodies,” said Guiseppe. “These isotopes decay 200,000 times per minute.”

Radioactive decay is simply a nucleus re-configuring itself through an interplay of matter and energy. Researchers with Majorana are looking for a natural process in which nuclei undergo such a change.

Double-beta

Every time an isotope decays, it loses a bit of energy in the form of a particle. Scientists classify types of decays by defining what type of particle comes out of the decay. In the case of beta decay, the particle emitted is an electron, or a beta particle.

While there are multiple types of decays that could occur within the detector, Majorana researchers are looking specifically for a decay in which a beta particle is emitted. 

“And by ‘double-beta,’ we just mean we are looking for two of these decays simultaneously,” Guiseppe said.

Neutrino(less)

All reactions in nature, including beta decays, require symmetry, or a balance. Because of this symmetry, scientists originally assumed that every time an isotope underwent beta decay, it would emit an electron with a uniform energy. The problem was, it didn’t. 

“Electrons emitted from beta decays have a range of energies,” Guiseppe said. “Sometimes it is low, sometimes it is high, but it has this average value that was more or less half of what the scientists thought it should be.”

This inconsistency lead researchers to realize that there must be another particle emitted—one that could not easily be detected, having no charge and very little mass. That missing particle was a neutrino. 

“When neutrinos were discovered in 1956, their addition to the beta-decay equation was confirmed,” said Guiseppe. “The neutrino balances this fundamental symmetry. With beta decay, there has to be both an electron and a neutrino produced.”

Hold on a second. By definition, a beta decay must have an electron. By the laws of physics, it must have a neutrino. So why is Majorana looking for neutrinoless double-beta decay?

“I just spent all this time explaining why you need a neutrino for a beta decay,” Guiseppe said with a smile. “And now, I’m going to say, no, you might not need a neutrino every time.”

Scientists, Guiseppe said, have good reason to believe that neutrinos have the ability to do something very interesting—the ability to act like anti-neutrinos. 

Neutrinos — the maverick of the early universe

To better understand the theory, we must first examine what is called the matter and antimatter asymmetry problem. 

According to the Big Bang theory, when the universe first formed, it had equal parts of matter and antimatter. This is a conundrum because, when matter and antimatter meet, they annihilate, leaving a universe filled with pure energy—no planets, stars or comets. And, most certainly, no life. 

So, what happened? Why did matter win out in the cosmic battle? Scientists are seeking an answer to how matter became the dominant form of matter in the universe. 

Many scientists believe there must have been a particle—very much like a neutrino—that acted very inconsistently with our current understanding of the laws of physics. This inconsistency, if detected, could answer the matter and anti-matter asymmetry puzzle. If just one particle acted differently, it could have upset the balance and allowed a remnant of matter to survive.

For most particles, there exists matter and anti-matter. These types of matter are mirror images of each other—100 percent different. In the early 1930s, however, physicist Ettore Majorana theorized that neutrinos could be their own anti-particle—or what we call today, a Majorana particle. 

“The claim is that maybe there’s no difference between neutrinos and what we call anti-neutrinos. They may be indistinguishable from each other,” said Guiseppe. “If they have that quality, it could help explain matter and antimatter asymmetry."

Neutrinoless double-beta decay — putting it all together 

If neutrinos have this property, it could answer a lot of questions for scientists; for example, how matter became the dominant form of matter in the universe, allowing for the creation of everything we see. But how might Majorana help discover it?

Researchers are waiting for a double-beta decay to occur inside the Majorana. If it does, and if neutrinos can indeed act like their own antiparticle, then the two neutrinos necessary may interact, possibly being absorbed, making the double-beta decay seem neutrinoless.

“If two beta decays occur in the Majorana Demonstrator, in close proximity to each other, and neutrinos do have this property, then we will detect the absence of neutrinos,” Guiseppe said. 

Should this rare event be detected, it will require rewriting the Standard Model of Particles and Interactions, our basic understanding of the physical world. 

“What isn’t up for debate,” Guiseppe concluded, “is that if neutrinos are indistinguishable from their anti-particle, then they will allow this neutrinoless double-beta decay process to take place. If they have this property, we will see the decay in Majorana. This is the best type of experiment we have to learn that.”