- 4442 - NEUTRINOS - The Ghost Particle: What is a “Neutrino” and could it be the key to modern physics? It came from deep space, moving at the speed of light, and crashed into Antarctica. Deep below the ice, it met its end. It wasn't an asteroid or alien spacecraft, but a particle that rarely interacts with matter, known as a “neutrino”.
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- NEUTRINOS - the ghost particle
-
- Though theorized in the 1930s and first
detected in the 1950s, neutrinos maintain a mysterious aura, and are often
dubbed "ghost particles". They
zip through the Earth without us even noticing them.
-
- In recent years, ghost particles have been
making headlines for all sorts of reasons. That Antarctic collision was traced
to a black hole that shredded a star and other neutrinos seem to come via the
sun.
-
- In early 2022, physicists were able to
directly pin down the approximate mass of a neutrino. This is a discovery that could help uncover
new physics or break the rules of the Standard Model.
-
- A neutrino is a fundamental, subatomic
particle. Under the Standard Model of particle physics it's classified as a
"lepton." Other leptons include electrons, the negatively charged
particles that make up atoms, with protons and neutrons.
-
- The neutrino is unique because it has a
vanishingly small mass and no electrical charge and it's found across the
universe. They are made in the sun, in
nuclear reactors, and when high-energy cosmic rays smash into Earth's
atmosphere. They're also made by some of the most extreme and powerful objects
we know of, like supermassive black holes and exploding stars, and they were
produced at the beginning of the universe: the Big Bang.
-
- Like light, they travel in basically a
straight line from where they're created in space. Other charged particles are
at the mercy of magnetic fields, but neutrinos just barrel through the cosmos
without impediment; a ghostly bullet fired from a monstrous cosmic gun.
-
- As you read this, trillions of them are
zipping through the Earth and straight through you. Every second of every day
since the day you were born, neutrinos have been moving through your body. You
just don't know it because they interact with hardly anything.
-
- After studying neutrinos for decades
neutrinos has thrown up a bit of a surprise for scientists. Under the standard
model, neutrinos shouldn't have any mass. But they do. The fact they do points
us to new physics to enhance our understanding of the universe.
-
- The puzzle of the neutrino mass first came
to light in the 1960s. Scientists had suggested the sun should be producing
what's known as “electron neutrinos”, a particular type of the subatomic
particle. But it wasn't. This "solar neutrino problem" led to a
breakthrough discovery: that neutrinos can change “flavor”.
-
- The ghost particle comes in just three
distinct flavors -- electron, muon and tau -- and they can change flavor as
they move through space. For instance, an electron neutrino might be produced
by the sun and then be later detected as a muon neutrino.
-
- And such a change implies the neutrino does
have mass. Physics tells us they couldn't change flavor if they were massless.
Now research efforts are focused on learning what the mass is.
-
- In February 2022, researchers revealed the
mass of a neutrino to be incredibly tiny (but definitely there). Physicists
were able to show directly, using a neutrino detector in Germany, that the
maximum mass for a neutrino is around eight-tenths of an electron volt (eV).
That's an unfathomably tiny mass, more than a million times "lighter"
than an electron.
-
- A neutrino detector? But aren't they...
ghost particles? How do you detect neutrinos?
There are a number of ways to
trap a ghost. One of the key ingredients
you need is space. Physical space, deep underground.
-
- Scientists have built their neutrino
detectors under meters of ice in Antarctica and, soon, at the bottom of the
ocean. This helps keep the data clean from any interference from things like
cosmic rays, which would bombard the sensitive detectors at the surface. The
detector in Antarctica, known as “IceCube', is buried about 8,000 feet straight
down under the ice.
-
- IceCube doesn't hold any neutrinos
prisoner. The particles mostly blast straight through the detector. But on the
way, some very (very!) rarely interact with the Antarctic ice and produce a
shower of secondary particles emitting a type of blue light known as Cherenkov
radiation.
-
- A range of light-sensing spherical modules,
vertically arranged like beads on a string, pick up the light those particles
emit. A similar detector exists in Japan: Super-Kamiokande. This uses a 55,000
ton tank of water instead of ice and is buried under Mount Ikeno.
-
- Both are able to detect which direction the
neutrino came from and its flavor. And so, physicists can see signs the ghost
particle was there, but not the ghost particle itself.
-
- Neutrinos are a fundamental particle in our
universe, which means they underlie, in some way, everything that exists.
Learning more about neutrinos will help unlock some of the mysteries of
physics.
-
- Particle
physicists want to understand if neutrinos violate some of the
fundamental laws of the Standard Model.
This may shed light on why there's more matter than antimatter in the
Universe.
-
- We also know that extreme cosmic objects and
events can produce neutrinos. For instance, exploding stars, or supernovas, are
known to create neutrinos and shoot them across the universe. So are
supermassive black holes chomping on gas, dust and stars.
-
- Detecting neutrinos tells us about what is
going on in these objects. Because they
hardly interact with the surrounding matter, we could use neutrinos to see
these types of objects and understand them in regions of the universe we can't
study with other electromagnetic wavelengths (like optical light, UV and
radio). Scientists could peer into the
heart of the Milky Way, which is hard to observe in other electromagnetic
wavelengths because our view is interfered with by gas and dust.
-
- Reliable detection and tracing could
stimulate an astronomy revolution akin to the one we're currently seeing with
“gravitational waves”. Essentially, neutrinos can give us a whole new eye on
the cosmos, complementing our existing set of telescopes and detectors to
reveal what's going on in the void using light photons.
-
- And then there are "sterile"
neutrinos. Sterile neutrinos are a whole
other class of neutrinos. They're entirely theoretical, but scientists think
they likely exist because of a feature in physics known as “chirality”.
Essentially, the normal neutrinos we've been discussing are what some call
"left-handed." So, some physicists think there may be
"right-handed" neutrinos, called a sterile neutrino.
-
- They give them this name because they don't
interact with other particles via the weak force, like normal neutrinos. They
interact only through gravity. These types of neutrinos are considered a
candidate for dark matter, the stuff that makes up more than a quarter of the
universe but that we've never seen.
-
- That means neutrinos might also help answer
another vexing puzzle in physics: What, exactly, is dark matter? There are lots
of candidates for dark matter theorized by physicists, and there's still plenty
to learn. It may not be related to
neutrinos at all!
-
- 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.
The total area of the array is designed to be about the same size as
Great Britain. The first 10,000 antennas are expected to be placed on the
Tibetan plateau, near the city of Dunhuang, in the next few years.
-
- Though we've been able to detect and trace
only a few neutrinos so far, the next decade should see neutrino astronomy
really take off. Understanding
neutrinos, their flavors and masses, will provide a window into the fundamental
nature of our universe.
-
-
April 24, 2023 NEUTRINOS 4442
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