Hunting the ghost particle: What is a neutrino and how could it break the physics?

That came from deep space, moved at the speed of light, and crashed into Antarctica. Deep under the ice, it met its end. It was not an asteroid or an alien, but a particle that rarely interacts with matter, known as a neutrino.

Although theorized in the 1930s and first discovered in the 1950s, neutrinos maintain a mysterious aura and are often called “ghost particles” – they are not haunted or dangerous, but they just glide through Earth without us even noticing them. Oh, “and it’s a cool name,” according to astrophysicist Clancy James of Curtin University in Western Australia.

In recent years, ghost particles have made headlines for all sorts of reasons and not just because they have a fat name. The Antarctic collision was traced to a black hole that e.g. crushed a star, and other neutrinos appear to be coming through the sun. In early 2022, physicists were able to directly determine the approximate mass of a neutrino – a discovery that could help uncover new physics or break the rules of the standard model.

Imagine if we actually caught a ghost and could say that the ghost was from someone who was dead. It would change everything we know about the universe. A ghost particles is pretty much a big deal for the same reason, which is why astrophysicists are trying to capture them. They are thrilled and here is why you should be too.

The IceCube Observatory at the South Pole, surrounded by snow, with a central rectangular unit and two cylindrical towers on each side

The IceCube Observatory in Antarctica.

Erik Beiser, IceCube / NSF

What is a neutrino?

In a nutshell, a neutrino is a basic, subatomic particle. Under the standard model of particle physics, it is classified as a “lepton”. Other leptons include electrons, the negatively charged particles that make up atoms, with protons and neutrons. But look, if we get into all that, we’ll go deep into particle physics, and it’s going to explode our brains.

The neutrino is unique because it has a vanishingly small mass and no electrical charge, and it is found across the universe. “They are made in the sun, in nuclear reactors, and when high-energy cosmic rays smash into the Earth’s atmosphere,” says Eric Thrane, an astrophysicist at Monash University in Australia. They are also made of some of the most extreme and powerful objects we know of, such as supermassive black holes and exploding stars, and they were also produced at the beginning of the universe: the Big Bang.

Like light, they travel in a straight line from the place where they are created in space. Other charged particles are at the mercy of magnetic fields, but neutrinos just run through the cosmos without hindrance; a ghostly bullet fired from a monstrous cosmic gun.

And as you read this, trillions of them are slipping through the Earth and right through you.

Are they running into me right now?

Yes exactly. Every second of every day since you were born, neutrinos have been moving through your body. You just do not know because they hardly interact with anything. They do not smash into the atoms that make you up, and then you do not even know they are there. Just as a shadowy spirit passes through a wall, the neutrino moves straight through. Fortunately, no exorcism is required.

But why should I worry about neutrinos?

Studying them for decades has provided a bit of a surprise to scientists. Under the standard model, neutrinos should have no mass. But they do. “The fact that they do point us to new physics to improve our understanding of the universe,” notes James.

The puzzle of the neutrino mass did not appear until the 1960s. Scientists had suggested that the sun should produce what are known as electron neutrinos, a special type of the subatomic particle. But it was not. This “solar neutrino problem” led to a groundbreaking discovery: that neutrinos can change taste.

Like an almost empty bag of Mentos, the ghost particle comes in only three different flavors – electron, muon and tau – and they can change taste as they move through space (taste is the actual terminology, I do not make it up for this analogy). For example, an electron neutrino may be produced by the sun and then detected as a myone neutrino.

And such a change implies that the neutrino has mass. Physics tells us that they could not change taste if they were massless. Now the research effort is focused on illuminating what the mass is.

In a study published in the prestigious journal Nature in February 2022, researchers revealed that the mass of a neutrino was incredibly small (but definitely there). Physicists were able to show directly using a neutrino detector in Germany that the maximum mass of a neutrino is about eight tenths of an electron volt (eV). It is an unbelievably small mass, more than a million times “lighter” than an electron.

The barrel-shaped detector of KATRIN is run between houses near Leopoldshafen in southern Germany

This is what a ghost hunter looks like: The main spectrometer for the Karlsruhe Tritium Neutrino Experiment (KATRIN) is maneuvered through a road in southern Germany.

Michael Latz / Getty

Wait! A neutrino detector? But aren’t they … ghost particles? How do you detect neutrinos?

As James notes, “the damn things mostly pass right through the detector you build!”

But there are a number of ways to catch a ghost.

One of the most important ingredients you need is space. Physical space, deep underground. To achieve good results, scientists have built their neutrino detectors under meters of ice in Antarctica and soon on the bottom of the ocean. This helps keep the data clean from any interference from things like cosmic rays that would bombard the sensitive detectors on the surface. The detector in Antarctica, known as the IceCube, is buried about 8,000 feet straight down.

“Capturing” a ghost particle may not actually be the best terminology for what these detectors do. IceCube, for example, keeps no neutrinos trapped. The particles blow mostly straight through the detector. But along the way, some very (very!) Rarely interact with the Antarctic ice and produce a shower of secondary particles that emit a type of blue light known as Cherenkov radiation. A series of light-sensing spherical modules, arranged vertically like beads on a string, pick up the light emitted by these particles. A similar detector is found in Japan: Super-Kamio jug. This uses a 55,000 ton tank of water instead of ice and is buried under Ikeno Mountain.

Both are able to detect which direction the neutrino came from and its taste. And then physicists can see signs that the ghost particle was there, but not the ghost particle itself. It’s a bit like a poltergeist – you can see how it interacts with chairs (throws them after you) and lights (threatening to turn them on and off), but you can not see the phantom itself. Spooky!

The sun is known to produce a specific type of neutrino

NASA / SDO / Spaceweather.com

Store. So what can we learn from neutrinos?

Neutrinos are a fundamental particle of our universe, which means that they somehow underlie everything that exists. Learning more about neutrinos will help unlock some of the mysteries of physics.

“Particle physicists study neutrinos to look for traces of physics beyond the standard model,” says Thrane. He notes that physicists want to understand whether neutrinos violate some of the fundamental laws of the Standard Model. “This may shed light on why there is more matter than antibody in the universe,” says Thrane, noting that the problem has been referred to as one of the great mysteries of physics.

We also know that extreme cosmic objects and events can produce them. For example, exploding stars or supernovae are known to create neutrinos and shoot them across the universe. So do supermassive black holes that corrode gas, dust and stars.

“Discovering neutrinos tells us about what’s going on in these objects,” James says.

Because they hardly interact with the surrounding matter, we could use neutrinos to see these types of objects and understand them in areas of the universe that we cannot study with other electromagnetic wavelengths (like optical light, UV, and radio). For example, scientists could look into the heart of the Milky Way, which is difficult to observe in other electromagnetic wavelengths because our view is disturbed by gas and dust.

Reliable detection and tracking can stimulate an astronomy revolution similar to the one we are currently seeing with gravitational waves. Basically, neutrinos can give us a whole new eye on the cosmos that complements our existing set of telescopes and detectors to reveal what’s going on in the void.

And then there are “sterile” neutrinos, which …

Oh God. What are sterile neutrinos?

I probably should have kept them hidden, but when you’re here, sterile neutrinos are a whole other class of neutrinos. They are completely theoretical, but scientists believe they probably exist because of a function in physics known as chirality. Basically, the normal neutrinos we have discussed are what some call “left-handed”. So some physicists believe that there may be “right-handed” neutrinos – sterile neutrinos.

They give them this name because they do not interact with other particles via the weak force of normal neutrinos. They interact only through gravity. These types of neutrinos are considered a candidate for dark matter, the things that make up more than a quarter of the universe but that we have never seen.

This means that neutrinos can also help answer another annoying puzzle in physics: What is dark matter really? There are plenty of candidates for dark matter theorized by physicists, and there is still plenty to learn – it may not be related to neutrinos at all!

A graphic image of the four types of neutrino: electron neutrino, myon neutrino, tau neutrino, sterile neutrino

The three variants of neutrino and the theoretical “sterile” neutrino.

IceCube collaboration

Cool. Is there anything else I need to know about neutrinos?

As Deborah Conway once sang, “It’s only the beginning, but I’m already gone and lost my mind.”

We have not gotten into some of the more mind-numbing theories about neutrinos, such as neutrino-double beta decay, and the idea of ​​the neutrino as a Majorana particle.

Several new neutrino experiments have been proposed, including the Giant Radio Array for Neutrino Detection, or GRAND, which would see up to 200,000 receivers placed. The total area of ​​the array is designed to be approximately the same size as the UK. The first 10,000 antennas are expected to be located on the Tibetan plateau, near the city of Dunhuang, over the next few years.

Although so far we have only been able to detect and track a few neutrinos, neutrino astronomy should really take off in the next decade. The bottom line is that understanding neutrinos, their tastes and masses, will provide a window into the fundamental nature of our universe.

And it’s always cool to chase ghosts.

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