Initially there were mathematical reasons why people suspected that all boson needed to have 0 mass as is the case for photons a gluons, see Goldstone's theorem.

However, experiments showed that the W boson and the Z boson both has large non-zero masses.

So people started theorizing some hack that would fix up the equations, and they came up with the higgs mechanism.

Can be contrasted with baryons as mentioned at baryon vs meson vs lepton.

Behavior fully described by quantum electrodynamics.

Experiments to measure it:

The 2019 redefinition of the SI base units defines it precisely and uses it as a measure of charge: en.wikipedia.org/wiki/2019_redefinition_of_the_SI_base_units#Ampere

Given the view of the Standard Model where the electron and quarks are just completely separate matter fields, there is at first sight no clear theoretical requirement for that.

As mentioned e.g. at QED and the men who made it: Dyson, Feynman, Schwinger, and Tomonaga by Silvan Schweber (1994) chapter 1.6 "Hole theory", Dirac initially wanted to think of the holes in his hole theory as the protons, as a way to not have to postulate a new particle, the positron, and as a way to "explain" the proton in similar terms. Others however soon proposed arguments why the positron would need to have the same mass, and this idea had to be discarded.

Clear experiment diagram which explains that the droplet mass determined with Stoke's law:

American Scientific, LLC sells a ready made educational kit for this: www.youtube.com/watch?v=EV3BtoMGA9c

Here's some actual footage of a droplet on a well described more one-off setup:

This American video likely from the 60's shows it with amazing contrast: www.youtube.com/watch?v=_UDT2FcyeA4

Hypothesized as the explanation for continuous electron energy spectrum in beta decay in 1930 by .

First observed directly by the Cowan-Reines neutrino experiment.

composite particle made up of an odd number of elementary particles.

The most important examples by far are the proton and the neutron.

"Barys" means "heavy" in Greek, because protons and neutrons was what made most of the mass of known ordinary matter, as opposed notably to electrons.

Baryons can be contrasted with:

I copied this grading scale mechanism from a failed Google interview ;-)

One problem with it is that I am always very hesitant to put a 5 on anything, who can not look at the documentation?

It is also hard to scope things right. Who can claim to be a C++ or Linux kernel expert, even if you wrote a book about it, since those are such humongous topics?

As a result, I haven't updated this in a while, and things may be out of date.

If your project does something that interests me, I can what it takes to contribute. Tell me what I must know, how long I have to learn it, and I'll call you back when I've mastered it.

composite particle made up of an even number of elementary particles, most commonly one particle and one anti-particle.

This can be contrasted with mesons, which have an odd number of elementary particles, as mentioned at baryon vs meson vs lepton.