Cosmology: standard model and neutrinos (Introduction)

by David Turell , Monday, June 22, 2020, 19:21 (1616 days ago) @ David Turell

Not explained and not understood, but described sort-of:

https://www.forbes.com/sites/startswithabang/2019/09/17/this-is-why-neutrinos-are-the-s...

"Every form of matter that we know of in the Universe is made up of the same few fundamental particles: the quarks, leptons and bosons of the Standard Model...the rules governing these particles explains everything we've ever observed.

"Except, that is, for the neutrino. This one particle behaves so bizarrely and uniquely, distinct from all the others, that it's the only Standard Model particle whose properties cannot be accounted for by the Standard Model alone. Here's why.

"Imagine you have a particle. It's going to have a few specific properties that are intrinsically, unambiguously known. These properties include:
mass,
electric charge,
weak hypercharge,
spin (inherent angular momentum),
color charge,
baryon number,
lepton number,
and lepton family number,

***

"Based on the properties of the particles produced by a neutrino interaction, we can reconstruct various properties of the neutrinos and antineutrinos that we see. One of them, in particular, stands out as incongruent with every other fermion in the Standard Model: spin.

"Remember how there was a 50/50 shot that an electrons would have a spin of either +½ or -½? Well, that's true for every quark and lepton in the Standard Model, except the neutrino.
All six of the quarks and all six of the antiquarks can have spins that are either +½ or -½, with no exceptions.

"The electron, muon, and tau, as well as their antiparticles, are allowed spins of either +½ or -½, with no exceptions.

"But when it comes to the three types of neutrinos and the three types of antineutrinos, their spins are restricted.

***

"All of the neutrinos and antineutrinos we've ever detected are extraordinarily high in energy, meaning that they move at speeds so high that their motion is experimentally indistinguishable from the speed of light. Instead of behaving like electrons and positrons, we find that all neutrinos are left-handed (spin = +½) and all antineutrinos are right-handed (spin = -½). (my bold)

***

"From the neutrino oscillation data, we can determine that at least one of these three neutrinos has a mass that can be no less than a few hundredths of an electron-volt; that's a lower limit.

***

"On the other hand, brand new results from the KATRIN experiment constrain the electron neutrino's mass to be less than 1.0 eV (directly), while astrophysical data from the cosmic microwave background and baryon acoustic oscillations constrain the sum of the masses of all three types of neutrino to be less than about 0.17 eV. Somewhere between these upper limits and the oscillation-informed lower limit lies the actual masses of the neutrinos.
Our Universe, as we understand it today, is full of puzzles that we cannot explain. The neutrino is perhaps the only Standard Model particle whose properties have yet to be thoroughly uncovered, but there's a tremendous hope here. You see, during the earliest stages of the Big Bang, neutrinos and antineutrinos are produced in tremendous numbers. Even today, only photons are more abundant. On average, there are around 300 neutrinos and antineutrinos per cubic centimeter in our Universe.

"But the ones that were made in the Universe's hot, early stages are special: as a result of being around for so long in our expanding Universe, they now move so slowly that they're guaranteed to have fallen into a large halo encompassing every massive galaxy, including our own. These neutrinos and antineutrinos are everywhere, with minute but finite cross-sections, just waiting to be explored. When our experimental sensitivity catches up to the physical reality of relic neutrinos, we'll be one step closer to understanding just how, exactly, our Universe came to be. Until then, neutrinos will likely remain the Standard Model's greatest puzzle."

Comment: All we can do is wait for answers.