When will we 'weigh' the neutrino? A new Project 8 study etches closer

Fundamental particles fall into two broad categories— . Fermions (quarks and leptons) make up matter, and bosons (photons and Higgs) govern the forces that hold matter together. The neutrino is a subatomic particle belonging to the lepton family. They are neutral, with minuscule masses and no electric charge.

Neutrinos are famously elusive. Unlike their charged counterparts, neutrinos interact very weakly with matter, making them . They can traverse vast distances through dense materials without leaving a trace. Neutrinos are commonly observed in various astrophysical phenomena, nuclear reactions, and cosmic processes.

They play a crucial role in the dynamics of supernovae, the sun's , and cosmic ray interactions. The neutrino's mass has been the subject of much physics research. One such project is a collaborative neutrino mass experiment that uses the cyclotron radiation emission spectroscopy (CRES) technique to enable the precise measurement of the neutrino's mass.

Recently, the team made progress in the CRES technique, and this advancement was detailed in a study published in . However, the precise measurement of the neutrino's mass is still pending. spoke to the researchers involved about their work and the motivation behind measuring the neutrino's mass.

Walter C. Pettus, Assistant Professor at Indiana University and co author of the study, told , "Studying neutrinos, particularly their mass, is driven by curiosity. These elusive defy the norms of fundamental particles. They're harder to measure, which means they are still hiding some of their secrets, which makes it fun." "They're also important because their impact straddles many subfields of physics, such as cosmology, astrophysics, and ," added Elise Novitski, Assistant Professor at the University of Washington and co author of the study.

The missing piece of the puzzle In the of particle physics, the neutrino stands out as the sole fundamental particle with an unmeasured mass. "Neutrinos were supposed to be massless in the original formulation of the theory; that makes them interesting because this is already an example of physics .

We've found one chink in the armor. If we keep investigating in this direction, what other interesting discoveries might be made?" posed Prof. Novitski. Additionally, neutrinos played a crucial role in matter formation in the , influencing how matter clumped together. This influence was dependent on the mass of neutrinos.

When cosmologists observe phenomena like the cosmic microwave background, traces of neutrinos are scattered throughout. However, various processes in the universe can produce similar effects to changes in . By measuring neutrino mass in a controlled environment like a laboratory, cosmologists gain a reference point.

This allows them to quantify the impact of neutrinos on their observations, providing a clearer understanding of the broader workings of the universe. As mentioned, one of the main challenges of measuring the neutrino's mass is its size and weakly interacting nature. The is more than 500,000 times lighter than an electron.

So, when neutrinos and electrons are created at the same time, the neutrino mass has only a tiny effect on the electron's motion. Seeing that small effect requires an extremely precise method for measuring how fast the electrons move. Detecting this subtle impact is akin to finding a needle in a trillion haystacks.

We asked Prof. Pettus why the neutrino's mass is so small. "That's a great open question," answered Prof. Pettus. "Perhaps the neutrino is a Majorana particle, in which case it would be its own anti particle and would get its mass in a completely new and different way from all the other fundamental fermions of the Standard Model." "Maybe neutrinos interact with the , but they do so much more weakly than the other fermions," he continued.

Measuring the neutrino's mass may help us disentangle these questions to understand the fundamental rules governing the behavior of these elusive particles. Project 8 and the CRES technique Unlike traditional methods, CRES doesn't directly observe the neutrino. Instead, it focuses on the electron produced in beta decay, a process integral to neutrino studies.

This process involves the transformation of a neutron into a proton or vice versa, accompanied by the emission of a beta particle—either an electron or its , a positron. "We look at the energy that's missing at the end and work backward from there to deduce the role and properties of the neutrino," explained Prof.

Pettus. "The innovation in CRES comes from observing the electron by cleverly linking its energy, which we want to measure, to a frequency, which we can measure extremely precisely (think )," added Prof. Novitski. This precision is essential in overcoming the challenges posed by the small mass and weak interaction of neutrinos.

The heart of CRES lies in linking the energy of the emitted electron to a measurable frequency. This is achieved by subjecting the electron to a strong and ensuring that the electron follows a curved path and generates radio waves, which can be detected with an antenna. The CRES technique exploits a special relativity effect in emitted during beta decay.

Amplifiers and cooling the apparatus to 200 degrees Celsius (°C) — or 328 degrees Fahrenheit (°F) improve precision in measuring the energy of the emitted electron despite its weak signals. CRES's unique ability to observe multiple decay events simultaneously increases sensitivity to neutrino mass, providing valuable insights into neutrino research.

Project 8's advancements in CRES enable the setting of tighter limits on neutrino masses. "A measurement or tighter limit would affect our understanding of the universe. It could also potentially determine which of the three types of neutrinos is the heaviest, which would help us better understand the results from other neutrino ," explained Prof.

Novitski. Advantages, challenges, and scaling up The three known neutrinos are electron neutrino, muon neutrino, and tau neutrino. Associated with the corresponding leptons, the distinctions of the neutrinos arise from the interactions each has with specific particles and processes. The electron neutrino is associated with processes like in the sun and beta decay in nuclear reactors on Earth.

The muon neutrino is often observed in cosmic ray interactions and muon decay, while the tau neutrino is associated with processes involving the tau particle, such as tau decay. Primarily designed for measuring the masses of known neutrinos, the experiment also considers a potential fourth type called a .

A sterile neutrino is a hypothetical particle that does not interact via the weak nuclear force, unlike the three known active neutrinos, and instead would only interact gravitationally. It may potentially be a candidate for . Prof Novitski shed light on how scaling up the experiment could help with the search for the sterile neutrino.

"With some adaptation, the CRES method could extend this search for sterile neutrinos to be sensitive to ones with an even broader range of masses." "CRES could also be used in a variety of other experiments beyond neutrinos; it is already starting to be used to investigate the very nature of the weak interaction, which is the force that causes this type of process (beta decay).

It's also being adapted for ultra high resolution X ray spectroscopy," she explained. When asked what sets Project 8 apart from traditional research methods, the researchers emphasized the novel approach of detecting from electrons instead of measuring the electrons directly. This distinction brings a significant advantage, as radio waves can travel through gas without interference, allowing the experiment to be scaled up.

Unlike traditional methods that rely on solid state detectors and can be disrupted by interference, Project 8's unique methodology minimizes background noise, enhancing precision and making it less susceptible to typical sources of interference. Project 8's unique perspective is highlighted by its departure from the conventional approach of building large .

Instead, it follows a tradition of smaller scale experiments, focusing on extreme precision at lower energies. "We are chasing a handful of low energy photons emitted by a solitary electron. It's surprising and beautiful that this may give us the clearest view of this curious facet of the workings of the universe," said Prof.

Pettus. A bright future As Project 8 celebrates a critical technical triumph, the path forward involves building on this success to create more advanced apparatus. "The sensitivity is compelling, and if nature is kind, we will directly measure the neutrino mass. If the neutrino mass is even smaller and remains out of reach, having learned that it is so extremely small will still have provided vital input to understanding the overall landscape in ," said Prof.

Novitski hopefully. Perhaps the most intriguing prospect lies in the scenario where Project 8 measures the neutrino mass, and this value deviates from other measurements, such as those derived from cosmological observations. "That would create a real puzzle for experimentalists and theorists alike to untangle, charting a way forward to investigate an even deeper mystery of the universe," said Prof.

Pettus..