They hold the keys to new physics. If only we could understand them.

Somehow, neutrinos went from just another random particle to becoming tiny monsters that require multi-billion-dollar facilities to understand. And there’s just enough mystery surrounding them that we feel compelled to build those facilities since neutrinos might just tear apart the entire particle physics community at the seams.

It started out innocently enough. Nobody asked for or predicted the existence of neutrinos, but there they were in our early particle experiments. Occasionally, heavy atomic nuclei spontaneously—and for no good reason—transform themselves, with either a neutron converting into a proton or vice-versa. As a result of this process, known as beta decay, the nucleus also emits an electron or its antimatter partner, the positron.

There was just one small problem: Nothing added up. The electrons never came out of the nucleus with the same energy; it was a little different every time. Some physicists argued that our conceptions of the conservation of energy only held on average, but that didn’t feel so good to say out loud, so others argued that perhaps there was another, hidden particle participating in the transformations. Something, they argued, had to sap energy away from the electron in a random way to explain this.

Eventually, that little particle got a name, the neutrino, an Italian-ish word meaning “little neutral one.” //

All this is… fine. Aside from the burning mystery of the existence of particle generations in the first place, it would be a bit greedy for one neutrino to participate in all possible reactions. So it has to share the job with two other generations. It seemed odd, but it all worked.

And then we discovered that neutrinos had mass, and the whole thing blew up. //

Nazgutek Ars Scholae Palatinae
23y
866
That was a fun read. I feel like I've climbed a single Dunning-Kruger step and now I feel like I know that I know less about the universe than I did before reading this article! //

NameRedacted Ars Praetorian
7y
445
Subscriptor
karadoc said:
such that relative to you the neutrino's direction of motion would then be reversed (compared to before you overtook it)... so then I'd expect that to be a right-handed neutrino from the point of view of that speedy observer.

I may be very wrong here, but I think that the entire point of chirality is that you can’t just reverse it by changing your perspective.NameRedacted Ars Praetorian
7y
445
Subscriptor

karadoc said:
such that relative to you the neutrino's direction of motion would then be reversed (compared to before you overtook it)... so then I'd expect that to be a right-handed neutrino from the point of view of that speedy observer.

I may be very wrong here, but I think that the entire point of chirality is that you can’t just reverse it by changing your perspective. //

NameRedacted Ars Praetorian
7y
445
Subscriptor
Back when I first graduated with my engineering degree, I really wanted to go back and get a PHD in physics because I loved QM so much.

Every time I read one of these articles, I’m glad I didn’t. Don’t get me wrong, this stuff is exciting: but I don’t think I could handle how much the universe “wants” to perplex us.

I have little doubt that the physics world will need to completely change everything to figure out all four of the big “mysteries”: Neutrinos, Dark Matter, Dark Energy, and the Hubble Constant. I also have little doubt that the solution will be complex, expensive, and be an advancement on the level of QM (I.e. atomic energy and semiconductors).

I hope I’m alive for when it happens, but *$&@ am I ever glad I haven’t spent my career trying to sort it out. //

Simk Smack-Fu Master, in training
4y
56
Subscriptor++
I really enjoyed that article! I'm none the wiser for having read it, but that seems fitting for the subject matter. //

neil_w Ars Praetorian
13y
464

Well, the properties of neutrinos don’t line up like this. They’re weird. When we see an electron-neutrino in an experiment, we’re not seeing a single particle with a single set of properties. Instead we’re seeing a composite particle—a trio of particles that exist in a quantum superposition with each other that all work together to give the appearance of an electron-neutrino.

For a moment I considered just closing the browser tab after reading this paragraph.

This was a very good article, trying to explain the nearly unexplainable. Hat tip to the physicists who are able to grasp it all. //

dmsilev Ars Praefectus
14y
5,375
Subscriptor
The sum of all three neutrino masses cannot be more than around 0.1 eV/c2
The absolute value of the square of the difference between m2 and m1 is 0.000074 eV/c2
The absolute value of the square of the difference between m2 and m3 is 0.00251 eV/c2

One thing which the article didn't mention is that there's an additional question hiding in these constraints. Usually, mass scales with family; the electron is lighter than the muon is lighter than the tau, and similarly for the quarks. We assume that that's the case for neutrinos as well, that m1 (the major constituent of electron neutrinos) is less than m2 is less than m3. That's called the "normal hierarchy" solution. However, the data doesn't prove that. There's also an "inverted hierarchy" fully consistent with the data which swaps the ordering. And we can't tell which one is correct. The only reason for the somewhat prejudicial names "normal" and "inverted" is the sense of elegance that the laws of physics should be somewhat consistent.