Hunting the Mass of a Neutrino

Roughly 65 billion neutrinos from the sun pass through every square centimeter of the Earth every second without making the slightest impact. Besides being mind-boggling small and cruising around at nearly the speed of light, these particles only interact with the rest of the universe through the weak interaction. This force is so weak, in fact, neutrinos simply pass right through the Earth, right along with any detectors physicists build to design to catch them.

Well, most of them do, but not quite all of them.

In 1956, scientists devised a way to directly detect the nearly invisible particle, a full 26 years after their existence was first proposed. But it wasn’t until the late 1990s that scientists discovered the tiny particles actually have a bit of mass. Since then, scientists have learned a lot about neutrinos.

For example, there are at least three “flavors” of neutrinos that can oscillate from one into another. However, there are still many open fundamental questions about the nature of neutrinos and even though scientists now know that neutrinos have mass, exactly how massive they are is yet to be determined.

But all of that may change soon.

Scientists all over the world are working on new experiments aimed at revealing more information about neutrinos. Nuclear scientists like those at NSCL play an important role in these experiments, as many of these studies involve a special type of radioactive nuclear decay. Known as neutrinoless double beta decay, the process is a close cousin of a well-known decay that emits neutrinos.

Recently, scientists from NSCL contributed to this worldwide effort by investigating the nuclear structure of the double beta decay of neodymium-150, one of the main candidate nuclei for catching this rare process in action.

Neutrinoless Double Beta Decay

Beta Decay

Beta (β) decay is a type of radioactive decay in which a beta particle – an electron or a positron – is emitted in conjunction with a neutrino or antineutrino due to a proton turning into a neutron or vice versa. In double beta decay, two neutrons turn into protons and two beta particles (electrons) are emitted. In the (regular) double beta decay process, depicted on the left, two neutrinos are emitted in the process as well. But in neutrinoless double beta decay, shown on the right, the neutrinos are internally absorbed and only two electrons are emitted. The regular double beta decay processes is well-known; observation of neutrinoless double beta decay would reveal that neutrinos are their own anti particles and allow for the determination of the neutrino mass.

A standard beta decay involves protons or neutrons spontaneously turning into one another, which creates byproducts like electrons, positrons and neutrinos. In double beta decay, two neutrons in a nucleus decay simultaneously, emitting two neutrinos and positrons. Depending on the still unknown fundamental nature of neutrinos, this standard process of double beta decay could take place without the neutrinos being emitted from the nucleus. In this extremely rare case, neutrons in the nucleus absorb the neutrinos emitted before they can escape the nucleus.

However, there is a catch.

Neutrinoless double beta decay is still a theoretical process. Only one claim has ever been made for its observaton, and it is heavily debated. If its existence were to be confirmed unambiguously, it would stun the physics world. Not only would the detection allow scientists to measure the mass of the neutrino, it would prove the neutrino as the first Majorana particle ever discovered – a particle that is its own anti-particle. Revisions would have to be made to the Standard Model, a theory that has withstood 35 years of rigorous testing.

There are many large experiments being constructed around the world to try to detect neutrinoless double beta decays. Each involves large quantities of isotopes that potentially could decay by this rare process. One such experiment – called SNO+ - currently is being built two kilometers underground in VALE's Creighton mine near Sudbury, Ontario, Canada.

In SNO+, the source of the potential decay is neodymium-150, which transforms into samarium-150 during double beta decay. Compared to other nuclei that could be used, neodymium-150 is predicted to have the shortest half-life, which makes it easier to catch the decay in action. The downside is that neodymium-150 is relatively heavy, which makes the description of the nuclear structure theory very difficult. And since this is required to determine the neutrino mass once neutrinoless double beta decay is observed, this difficulty is a problem.

NSCL Sheds Light on Decay

Nuclear Matrix

In the picture above, the different horizontal lines stacked on each other represent some of the different energy states of promethium-150. The red and green arrows represent the measurements taken of the transformation of either neodymium-150 or samarium-150 to the different energy states of promethium-150. By combining the data from each leg, information of relevance for the description of the nuclear matrix element for double beta decay can be obtained, which in turn is critical for calculating the neutrino mass in case neutrinoless double beta decays are observed by SNO+.

This is where the study conducted by researchers at the NSCL and their collaborators from Europe and Japan comes in handy.

The researchers performed two experiments involving promethium-150, which sits between neodymium-150—the double-beta decay mother—and samarium-150—the double beta-decay daughter.
The first experiment was performed at NSCL and was aimed at investigating transitions from samarium-150 to promethium-150. For that purpose, the researchers impinged a beam of hydrogen-3 particles on a thin foil of samarium-150 and detected helium-3 particles created in the reaction. From the measurement of the velocity and angle of these helium-3 particles, the team could infer information about the resulting state of promethium-150.

The second experiment was performed at the Research Center for Nuclear Physics at Osaka University in Japan and focused on transitions from neodymium-150 to promethium-150, the other “leg” of the double beta decay. In this experiment, a beam of helium-3 particles was impinged on a foil of samarium-150 and hydrogen-3 particles were observed.

The goal of the two experiments was to constrain the theoretical estimates for one of the pieces of the equation that describes the half-life of the neutrinoless double beta decay of neodymium-150. This piece—known as the nuclear matrix element—represents the sum of all of the possible contributions to the total transition. Unfortunately, there is no direct way to measure the nuclear matrix element and the best scientists can do is to constrain the theoretical calculations by testing them against the experimental data.

Prior to these experiments, very little was known about these two transitions. Should SNO+ successfully detect the neutrinoless double beta decay of neodymium-150, the data taken here will be very valuable in the subsequent calculations that will unlock many of the neutrino’s lingering secrets, including its true nature and its exact mass. In addition, the measurements are important for estimating what the expected number of neutrinoless double beta decay events will be, assuming the process takes place at all.

The two experiments and the analysis of the data were the subject of the Ph.D. thesis work of Carol Guess and the results were published in Physical Review C 83, 064318 (2011). This work was supported by the US National Science Foundation.