The Juno Experiment: Unveiling Neutrino Mysteries

Located 700 meters beneath the surface near Jiangmen in southern China, a colossal sphere, 35 meters in diameter and filled with over 20,000 tons of liquid, has embarked on a multi - decade mission. This is Juno, the Jiangmen Underground Neutrino Observatory, a cutting - edge, large - scale scientific endeavor dedicated to exploring some of the most enigmatic and evasive particles in the scientific realm.

Neutrinos: The Elusive Particles of the Universe

Neutrinos are the most prevalent massive particles in the universe. As fundamental particles, they do not decompose into smaller components, rendering them minuscule and lightweight. Additionally, they possess zero electric charge, being electrically neutral, which is the origin of their name. These properties result in their infrequent interaction with other matter; they can traverse through it unimpeded, making their observation a formidable challenge. Consequently, they are sometimes aptly referred to as "ghost particles."

Neutrinos also exhibit the remarkable ability to transition (or "oscillate") among three distinct forms, known as "flavors": electron, mu, and tau. It is crucial to note that electron - flavored neutrinos differ from electrons; the latter are a distinct type of fundamental particle with a negative charge.

The phenomenon of neutrino oscillation was experimentally verified by physicists Takaaki Kajita and Arthur Bruce McDonald. Through separate experiments, they observed the oscillation of electron - flavored neutrinos into mu - and tau - flavored neutrinos. This discovery not only demonstrated that these particles have mass but also that the mass of each flavor varies. For this groundbreaking work, they were awarded the Nobel Prize in Physics in 2015.

However, a pivotal yet unresolved question remains: how are these masses ordered? Which of the three flavors has the greatest mass, and which the least? A more profound understanding of neutrino mass could significantly enhance our description of the universe's behavior and evolution. This is where the Juno experiment comes into play.

A Distinctive Experimental Setup

Neutrinos elude detection by conventional particle detectors. Instead, scientists must search for the rare instances of their interaction with other matter, and this is precisely the purpose of Juno's massive sphere. Known as a scintillator, it is filled with a sensitive internal liquid composed of a solvent and two fluorescent compounds. When a passing neutrino interacts with this matter, it generates a flash of light. Surrounding the liquid is a substantial stainless - steel lattice that supports an extensive array of highly sensitive light sensors, called photomultiplier tubes. These tubes can detect even a single photon generated by a neutrino - liquid interaction and convert it into a measurable electrical signal.

Gioacchino Ranucci, the deputy head of the Juno experiment and former head of Borexino (another neutrino - hunting experiment), states, "The Juno experiment builds upon the legacy of its predecessors, with the key distinction being its significantly larger scale." Ranucci further elaborates that one of Juno's primary features is its ability to "detect" both neutrinos and their antimatter counterparts, antineutrinos. Neutrinos are typically produced in Earth's atmosphere, through the decay of radioactive materials in the Earth's crust, or arrive from outer space, originating from stars, black holes, supernovae, or even the Big Bang. In contrast, antineutrinos are artificially generated, in this case, by two nuclear power plants in close proximity to the detector.

Ranucci explains, "As they propagate, neutrinos and antineutrinos continuously oscillate, transforming into each other." Juno will be capable of capturing all these signals, revealing their oscillation patterns "with unprecedented precision."

Juno's Objectives

Juno's primary objective is to resolve the neutrino mass - ordering problem. It is established that the electron - flavored neutrino is lighter than the mu - flavored neutrino. However, it remains unclear whether the third flavor, tau, is heavier than the other two (indicating a direct hierarchy) or not (suggesting an inverse hierarchy). By measuring the energy spectrum of reactor - produced antineutrinos with high resolution, Juno aims to determine whether the hierarchy is normal or inverse. The research collaboration anticipates that after approximately six years of data collection, they should obtain a statistically significant result to answer this question.

Yet, Juno's potential extends beyond this. At a later stage, it could contribute to solving an even more profound mystery related to the so - called Majorana neutrino, a hypothetical particle that has yet to be observed. (Majorana particles are unique in that they are their own antiparticles; thus, a Majorana neutrino is both a neutrino and its antineutrino.) Determining whether neutrinos are Majorana particles could potentially unlock the answer to one of the most complex questions in modern physics: why there is more matter than antimatter in the universe, a phenomenon for which there is currently no complete, consistent, and definitive explanation.

This story originally appeared on WIRED Italia and has been translated from Italian.