This is a demo article about neutrinos and the JUNO Experiment
Did you know that, right now, you are traversed by many many particles ? Indeed, there is natural radioactivity. More than radioactivity, there are the less-observable Neutrinos. To give you on how secretive they are, out of ten trillions of Neutrinos passing through Earth, only one will interact. The neutrinos were theorized by Pauli in 1930 to solve an energy disappearance problem. Indeed, some energy vanished from the decay of some radioactive elements. There has to be a missing particle! However, the neutrino is so evasive that it hadn’t be detected for 26 years and without a better technology. It was indeed detected in 1956 by Reines & Cowans, using a nuclear reactor.
The neutrinos are a kind of leptons, the particle kind of the electrons, muons and tau particles. There are three kinds of neutrinos and the associated antiparticles – electron neutrinos, muon neutrinos, and tau neutrinos. Those kinds are called « flavours ».
But what can this particle tell ?
This question gets along with another one – How are the neutrinos generated ?
Well, neutrinos are produced by the decay of more unstable particles from the weak interaction. So where there is unstable particles, there’s neutrinos. This includes radioactivity as well !
Neutrinos comes from environments where unstable particles can be generated and decay. Those environments are usually the theaters of high-energetic events, like supernovae – the collapse of heavy stars – or more humbly nuclear reactions.
Since neutrinos do barely react, taping them could lead to probe and detect those environments. A supernova happens ? A neutrino signal can make you aware of it. Since neutrinos are even less reactive than light itself, they arrive before light, noticing you of the event before you can see it ! Likewise, neutrinos can be used to probe opaque environments like the inner sun or the inner earth.
Neutrino oscillation – Challenging the standard model of Particle Physics
On a more fundamental level, this particule has an interesting property, called « neutrino oscillation ». This property is not allowed by the standard model of particle physics, the most complete set of theories explaining how matter interacts on a fundamental level. This model predicts that the neutrino is massless. However, neutrino oscillation, is possible only if the neutrino has masses. Like flavours, neutrinos have three states of mass. More specifically, neutrino oscillation is possible if each flavour is composed by a superposed state of different masses.
Traveling through matter, the neutrino shifts from one flavour to another. It’s kinda like having a blue pencil which turns red to turn green.
Okay, but if they barely interact, how do we detect them ?
Short answer – We use a lot of matter !
The neutrinos interact through the weak interaction. This interaction is mediated by a neutral or electrically-charged boson. A boson is a particle that transmits physical properties, like energy or momentum, but also electrical charge, from a sender to a receiver.
This boson can knock off a nucleus, setting it into motion. Otherwise, it can break it apart. The children particles will be able to interact with the environment. Moreover, with the help of this boson, the neutrinos (or more accurately the electron anti-neutrinos) can use a process called inverse beta decay, where they turn into a positron transmute the proton in a neutron.
An example of an ambitious neutrino observatory program – JUNO
JUNO, from Jiangmeng Underground Neutrino Observatory, is a brand new research complex built in the Guandong Province, in the south of China. It should be active this decade, during at least six years. The core of JUNO is located under 693 meters of rock to shield it from atmospheric muons.
The core of JUNO is a liquid scintillator ball of about 20 kilotons. On this ball, the inverse beta decay induced positron will be annihilated with an electron and produce a pair of photons. The induced neutron will be captured by another atom, and liberates a photon in the process as well. The emitted photons will be collected by a surrounding array of photomultipliers.
The main aim of JUNO is to give a precision measurement of the relative masses of the mass states of the neutrinos through a measurement of neutrino oscillation. During its activity, the observatory will keep an eye on other neutrino sources, like the Sun, Supernovae, the atmosphere, inner Earth or more miscellaneous sources.
To optimise those researches, JUNO is located exactly at 53 kilometers from two nuclear power plants. This is the adequqte distance to get most of the oscillation. Indeed, at this distance, most of the electron anti-neutrinos produced by the reactors have switched to another flavour.
A tiny particle which has a lot to tell
To sum it up, this « ghost particle », as Pauli named it himself, will tell us of the hidden mysteries of Earth and Sun, and keep us aware of terrible events of the Sky, if we are heavy enough to listen. Its inner properties are even challenging one of the most robust theories of physics !
This decade, watch out to JUNO but other detectors as Hyper-Kamiokande, KM3NeT or DUNE to learn about this reliable messenger !
Tell us what do you think on this charming particle, or if you wanna explore other topics in the comments !
This article was inspired by academical knowledge and the article « JUNO Physics and Detector » by the JUNO Collaboration, published the 14th of May, 2021. You can find it here : https://arxiv.org/abs/2104.02565 (DOI : https://doi.org/10.48550/arXiv.2104.02565)
To go further, I recommend this cute manga about Neutrinos published in 2019 : https://www-he.scphys.kyoto-u.ac.jp/nucosmos/en/index.html
If you want to learn more on the discovery of the Neutrino, you can check this article (The Reines-Cowans Experiment ~ Detecting the poltergeist, published in Los Alamos Science in November 25, 1997) :
This is a demo articleabout neutrinos and the JUNO Experiment
Jean-Baptiste Cognée