I never thought I’ll get my overalls on, I’ll get my hard hat on.
I just drive like into a mountain, stand on top of this big tank of water and see what’s what.
For more than 50 years, Japan has been at the forefront of big physics,
creating experiments of such size and ambition that they need to be seen to be believed.
My name is Davide Castelvecchi, and I’m one of Nature’s physics reporters.
In this series, I will be travelling to Japan to visit three of its flagship experiments,
delving inside Japan’s big physics.
This is Kamioka, a tiny village nestled in the shadow of Mount Ikeno in Gifu Prefecture, Japan.
It is an unassuming village, but there is more than meets the eye.
It is home to the control centre of the Kamioka Observatory and the world’s largest neutrino detector,
Super-Kamiokande, located under the neighbouring mountain.
The neutrino is one of the elementary particles. Sometimes we call neutrinos ghost particles.
That means neutrinos travel in light speed everywhere.
The neutrino passes through everything, so it’s a really difficult particle to detect.
That’s why the neutrino has many mysteries.
And that is where Super-Kamiokande comes in,
the machine under the mountain designed to detect the ghost particle.
We’re standing here at the entrance of the tunnel that leads to the Super-Kamiokande detector.
It’s about 2,000 metres down this tunnel and to the left.
But neutrinos aren’t the only particles racing through the atmosphere.
Super-K is built under a mountain to make sure that neutrinos aren’t drowned out by other particles.
Only neutrinos can get through that much rock.
But even after filtering out the noise, scientists still can’t detect neutrinos until they interact with something.
The neutrino basically passes through everything.
However, with very, very small probability, it interacts with the matter.
So, to increase the tiny odds, they built a water tank 10 storeys high, filled with 50,000 tonnes of pure water.
And the water in the tank is what the neutrinos will occasionally smash into.
If the neutrino interacts with this water then it causes a flash of light,
and this flash of light can be observed by the photo sensors, so that’s how we observe the neutrinos.
The photo sensors are called photomultiplier tubes or PMTs, and they look somewhat like giant lightbulbs.
They are so sensitive they could spot a candle on the surface of the Moon.
So, this is one of the photomultiplier tubes.
It’s a high vacuum inside and whenever there is a neutrino event happening in the water,
the light that is produced by that event hits maybe a few dozen or a few hundred of these photomultiplier tubes.
When I visited, Super-Kamiokande’s water tank had been emptied for the first time in 12 years for maintenance.
I’ll just open. I’ll just show you. This is the Super-K tank. So, you can see clearly.
I did not expect to see it right away.
I was lucky enough to have Masayuki Nakahata invite me to take a look inside. But first, we had to suit up.
So, I’m putting on this vest to reduce the amount of contamination
that we can leave inside the Super-Kamiokande tank.
The plan was to lower us into the tank on a tiny gondola.
The first glimpse down the hatch feels like looking through a portal into another universe
or through the door of Doctor Who’s Tardis.
This is just so unreal.
As the gondola is lowered down, one feels the stare of 11,000 giant orange eyes.
The PMTs aren’t really orange.
They are reflecting the colour of the sodium lamps inside used to prevent damage to the sensors.
You don’t really understand the scale until you get to the maybe the middle,
and then you’ve got half of the PMTs above you.
It’s awesome and really weird to be in there at the same time.
The PMTs form a regular geometric pattern that confuses one’s sense of proportions and distances,
as if seen from the inside, the tank could be even bigger than it already is.
From the bottom of the tank, I could see the space above us as well as the work being carried out beneath.
It is hard to overestimate the engineering feat that creating this experiment has been, and it is paying off.
Super-K detector, hosted by Japan, discovered neutrino oscillations.
Neutrino oscillations showed that neutrinos must have some tiny but non-zero mass,
and that was a huge surprise.
So much so that the discovery won a Nobel Prize,
the second Nobel to come out of this formidable observatory.
This neutrino oscillation is clearly a breakthrough beyond the Standard Model,
and we believe there is more higher theory beyond this Standard Model,
and then this discovery of a neutrino oscillation provided some window to see the new physics.
Next, the researchers at Super-K are setting their sights on trying to spot the decay of protons,
which has never been seen before.
They will also be adding a metal called gadolinium to the water
to make Super-K more sensitive to stray neutrinos from ancient supernovas.
And they are about to start building an even bigger experiment, Hyper-Kamiokande,
a detector that will be five times larger than Super-Kamiokande.
We aim to discover proton decay. That’s not yet discovered in Super-K.
Another goal is we aim to discover the difference between the neutrino and the antineutrino,
so Hyper-K has many physics goals.
For decades, scientists here at Kamioka have been pushing the boundaries of neutrino research,
and the discoveries that they’re making are changing our understanding of the Universe.
In the next episode, we’re heading back under Mount Ikeno but to see a very different experiment, KAGRA,
Japan’s answer to LIGO – it’s very own gravitational wave detector.