If a neutrino do not interact with anything and can pass through a light year of lead, how humans manage to detect them?
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Neutrinos interact via two of the four fundamental forces: gravity and the weak force, which is… well, weak.
When a neutrino passes through a detector, there is a *chance* of it being detected, but that chance is low. Thankfully, there are so many neutrinos passing through earth that some will be detected. What we do is wait for an interaction to happen via the weak force (eg transfering some of its momentum to a nearby electron via the weak force (specifically a W boson)), and then we detect the effects on the things that were interacted with (eg in our example, the electron can be accelerated to relativistic speeds, and so if we put it in heavy water then there will be a flash of light from Cherenkov radiation).
Neutrinos interact with matter via the [weak interaction](https://en.wikipedia.org/wiki/Weak_interaction). So while indeed most of them will pass through a detector without interaction, a small fraction will interact. When a neutrino scatters off an atom, it can create a charged particle and/or break apart the nucleus. It’s these things that neutrino detectors detect. Neutrino detectors are large volumes of some kind of material that gives a detectable signal (e.g. a flash of light) in response to ionising radiation. They also need to be very well shielded from other sources of radiation, so that the background noise doesn’t overwhelm the neutrino signal.
With a lot of difficulty.
Neutrino detectors [tend to be really, really big](https://www.businessinsider.com/super-kamiokande-neutrino-detector-is-unbelievably-beautiful-2018-6).
Usually giant water tanks (some using heavy water), buried deep underground, surrounded by detectors that look for interactions between neutrinos and stuff in the water.
Neutrinos do interact with things, but only via the weak interaction (and gravity, but only in negligible amounts). That detector linked above is the [Super-Kamiokande detector](https://en.wikipedia.org/wiki/Super-Kamiokande) in Japan; a tank containing 55,000 tons of water, 1,000m underground, with 13,000 light detectors around it. Specifically it detects Cherenkov radiation which is a weird thing you can get when neutrinos interact (weakly) with electrons or the nuclei of atoms.
About 10^11 neutrinos pass through each square centimetre of the Earth’s surface each second. The Super-Kamiokande detector (with a surface area of over a thousand square metres) detects about 8 per day.
We detect neutrinos with a lot of difficulty, by having really big detectors, and knowing that huge numbers of neutrinos pass through every second.
Well, the chance of a neutrino interacting with any matter is really really small. Luckily, though:
* The nuclear reactions in the Sun put out a HUGE amount of neutrinos; 100 billion neutrinos pass through your fingernail every second. If we could see in “neutrino” the Sun would shine extremely brightly. We’re basically positioned very close to a huge and constant source of neutrinos.
* The detector facilities are [huge](https://i0.wp.com/eusci.org.uk/wp-content/uploads/2020/05/Kamioka.jpg?fit=786%2C427&ssl=1) and those cameras are sensitive enough to detect the flash of energy from a single interaction.
So basically, the size of the detector is huge, and the amount of neutrinos passing through the Earth is huge, and scientists have built a number of detector facilities. So overall we detect about 30 per day, about one per hour, across all the detector facilities that have been built.
The premise of the question is incorrect, neutrinos do interact with other things.
The probability of the interaction is just very low so you need a lot of them to get a few interactions.
Interaction is not a binary always interact or never interact. If you shine visible light on a wooden wall it will not pass through. But instead, use radio waves and X-rays and a large part of them will get through. These are just photons with different wavelengths and they interact with matter in quite different ways.
So radio waves can pass through matter a lot easier than visible light, The same way neutrinos can pass through matter many times easier than radio waves. If the chance of interacting is low enough it can likely pass through a light year of lead. But that does not mean that some neutrons interact with the first meter of the lead, it will only be a minuscule fraction that does that.
It isn’t that neutrinos don’t interact at all, but that the probability of an interaction is extremely, low. We basically detect them by setting up a huge vat of liquid (water, liquid argon, mineral oil, etc) with a lot of photomultiplier tubes (these detect light). After, we use a particle accelerator to send a beam of neutrinos through the earth towards the aforementioned detector, and because of the low probability of interaction, they just make it there. Also because of the low interaction probability, you have to send a lot of neutrinos over a relatively sustained period of time for you to get any real data. In addition, neutrinos coming from space also pass through and may be detected.
The photons resulting from particle interactions within the detector gives us more information about the particles passing through, like mass, speed, etc. Using this information, we can find out which of these interactions are neutrinos, as neutrinos are not the only particles that pass through the detector. There’s more involved, but this is one facet.
If you want to read up on neutrino experiments:
[https://en.wikipedia.org/wiki/Super-Kamiokande](https://en.wikipedia.org/wiki/Super-Kamiokande)
[https://en.wikipedia.org/wiki/MiniBooNE](https://en.wikipedia.org/wiki/MiniBooNE)
[https://www.dunescience.org/](https://www.dunescience.org/)
[https://www.symmetrymagazine.org/article/protodune-in-pictures](https://www.symmetrymagazine.org/article/protodune-in-pictures) –this one may be of particular interest to you, as it concerns the construction of a liquid argon detector
I’m no expert, but this is my understanding of the topic! Hope it helped. If anyone wants to add to/correct anything, please do.
A lot of comments here on modern neutrino experiments which are great, but it sounds like you are interested in the very first experiment to do so. It won its creators a Noble Prize and was called “Project Poltergeist”. The name was somewhat a joke as seeing a neutrino would be like seeing a ghost.
The experiment was completed in 1956 by physicists Cowan and Reines. Reins had worked on the Manhanttan project and knew that an atom bomb would create absolutely huge amounts of neutrinos. As other commentors have said, Neutrinos do interact, it’s just incredibly rare. So if you are going to detect one you need a butload of neutrinos.
So Reines designed a method for detection. If you have a massive tank of water near a source of neutrinos there is a small chance the neutrino will interact with the hydrogen atoms creating a neutron and positrons. The positron would then interact with electrons in the water and annihilate each other and release 2 gamma rays. There was already a method to detect Gamma rays.
There was only one problem with this experiment. How do you build a water tank and gamma ray detector that can survive being 100m away from a nuclear explosion?
Turns out you can’t. Reines sat on this idea for many years (Cowan helped improve the design by adding Cadium Cholride to the water but thats beyond ELI5). Eventually another colleague let him know that these new devices called “Nuclear reactors” may create the butload of neutrinos needed to dectect.
So Reines and Cowan set up their dector at the Savannah River Site where there were 5 reactors running. Their detector worked, they won a Noble Prize and the rest is history…
What would be the chance hitting the bulls eye if you put a darts board on its back next to the Empire State Building and threw a single dart of the observation deck?
It would be pretty much zero. Same as finding a single neutrino.
But if you would throw billions and billions of darts of the building, eventually one would land by chance in the bulls eye.
Neutrino detectors work the same. Literally gazillions of neutrinos pass through, but only a few hit the core of a heavy water atom, generating a detection event.
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