I know. You know. JUNO what … this rocks!

11 minutes ago

6 min read

Neutrinos are fascinating particles. More than a trillion a second flood through your body, but science has only ever caught a sliver of them. Well now a sphere holding 20,000 tonnes of liquid, buried under southern China, has out-measured every previous experiment combined. It is one of physics’ oldest puzzles. Do these ghosts weigh anything, and how would we ever find out?

(First published on my Substack where you can get #NerdNews, marvellous maths and general geekery.)

(JUNO central detector interior, the acrylic sphere ringed by tens of thousands of photomultipliers. Credit: © JUNO Collaboration)

JUNO what … this rocks!

Meet JUNO. This stunning arrangement of forty-five thousand golden eyes is buried 700 metres under a hill near Kaiping in China’s southern Guangdong province.

Every one of those gold domes is a glass bulb coated with a film sensitive enough to register a single particle of light. Once detected, a special tube sitting behind the face, called a photomultiplier, amplifies that lone speck of light into an electric pulse a million times larger.

The metal stems you see threading up between them are the steel frame that holds all 45,000 sensors pointing them inward, all staring at that same central target.

And the target? Well that’s the jewel in JUNO’s crown. A 35 metre wide acrylic sphere sitting at its centre, containing 20,000 tonnes of a liquid scintillator (yes it is really called that!), a clear oily, hydrocarbon that flashes when the right subatomic streaker slides on through.

JUNO is the largest device of its type ever constructed and took over a decade to piece together. And this whole cosmic cathedral exists to study perhaps the strangest and most elusive particle in the universe, the neutrino.

Meet the particle that barely knows you exist.

Yes it is a building block of nature, but boy a neutrino is a shy little thing. Bearing no electric charge and hardly any mass, it slips through solid matter as though that matter was a mere shadow.

They are so tough to pin down that for decades we have wondered something incredibly basic. Does a neutrino weigh anything at all?

Right now, roughly 100 trillion neutrinos are streaming through your body. They will be replaced by 100 trillion more before you finish this paragraph.

Most emanate from the Sun, pouring out in a constant thin rain. Almost every cosmic raindrop drop passes out the far side of the Earth as if there was nothing there.

Neutrinos are the most abundant particle in the universe that carries any mass. Only the massless photons making up the leftover light of the Big Bang are more numerous.

Everywhere, all at once. But bloody hard to catch.

While neutrinos certainly are not rare, catching one is. That’s because a neutrino almost never bumps into anything. A 5 metre thick wall of steel? A giant chunk of fresh air? Same diff. The neutrino continues on its merry way as though nothing is there.

Physicists first jagged a neutrino 70 years ago, in an underground water tank beside a nuclear reactor in South Carolina.

This was so awesome it won Fred Reines the 1995 Nobel prize.

Quick note to placate any fired-up physicists out there. Strictly speaking, unlike the neutrinos the Sun throws out, near a reactor what we actually catch is the neutrino’s antimatter twin, the antineutrino. Happily, they come in the same three types and carry the same masses as neutrinos, so it’s all one family. Throughout this piece I’ll just call them all neutrinos, if that’s cool with you. And if that genuinely upsets you, chances are you understand the physics here better than I do anyway.

Anyway when I say they are rare to catch; F-dog and his team were only scooping up about 3 per hour, despite the trillions passing through the tank.

So in a single second, very roughly ten million times more neutrinos sail through you than humanity has detected in its entire history. — NerdNews.

Or put it this way, across your entire life, the odds one strikes an atom inside you sit at about one in four.

Time to bring in the big rig.

Catching a ghost.

So how does a machine catch such a slippery thing? Remember we are talking about particles that almost always sail through the machine designed to detect them.

It is all about location, location, location. JUNO sits about 53 kilometres from a gaggle of nuclear reactors. They spew out antineutrinos in staggering numbers.

As we’ve seen practically all of them sail clean through the 20,000 tonnes of liquid scintillator. But every so often one strikes a proton in a molecule of JUNO-juice head-on. This sets off a faint flash of light. JUNO’s 45,000 golden eyes pounce.

Not all the eyes react the same. Complex calculations measuring exactly which eyes lit up, and how brightly, helps the team reconstruct precisely where inside the sphere the flash happened and how much energy it carried.

Almost immediately upon its switch-on last year, JUNO proved remarkably adept at these detections. Safe to say the early numbers stunned everyone.

“JUNO is performing exactly as designed.” — Wang Yifang, JUNO spokesperson.

Geez Wang, feel free to be a bit excited.

(JUNO prompt event (left), delayed event (right). Credit: © JUNO Collaboration)

What have we got here? A thing of pure beauty, that’s what. The prompt event image captures the moment an antineutrino from the nerby reactor strikes a proton in Juon’s liquid. The detector’s 45,000 sensors record a bright flash from this impact. The delayed signal comes about 0.2 milliseconds later and confirms it was a neutrino Juno spotted not just background noise.

And now we have to weigh the bugger!

As if it wasn’t tough enough, we can’t just weigh a neutrino. It depends on what type of neutrino we are talking about.

Like many subatomic particles, neutrinos come in multiple types. In this case three types.

And further as they travel they shape-shift from one to another. Now this quantum oscillation can only happen if neutrinos have mass.

But observing the oscillation reveals only the gaps between the three masses, not the masses themselves. It’s the same as knowing two people’s weights differ by four kilograms, but not knowing what either of them weighs.

JUNO cannot give us the actual weights of the three neutrino types. But it can sort out the order.

And that is what it has done. Using 59 days of incredibly precise measurements, from late 2025, JUNO measured the oscillation values much more accurately than ever before.

“One of the biggest questions in particle physics.” — Davide Castelvecchi, Nature.

We still don’t know the weights involved. That’s a different project altogether.

Seriously, why bother?

So why pour a decade and a fortune into a ball of liquid that hunts particles which barely touch us?

If neutrinos don’t care about us, why should we care for them?

Because, despite their rarity, neutrinos point to a glaring gap in our best theory of nature.

Since we first began to cobble together the Standard Model, the mathematics insisted they had no mass at all. So the discovery in 1998, by the awesomely named Super-Kamiokande observatory in Japan, that they do, was the first solid sign that the model is incomplete.

If we can order the three types of neutrinos, and calculate their mass, perhaps we begin to glimpse the deeper physics. The insights that sit below the impressive amount we already do know about what makes up and governs the amazing universe in which we live.

For all their elusiveness, neutrinos helped shape how galaxies clumped together after the Big Bang. They bring news from the hearts of exploding stars. They may answer one of the deepest puzzles of all: why the universe ended up made of matter rather than nothing.

I think we all agree, that’s pretty important stuff!

Just gettin’ started!

JUNO is built to run for around 30 years, so this spectacular first flash is merely the opening act.

Until next time, take a moment to sit back, and ponder the trillions of neutrinos streaming through you, as if you simply are not there!