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Comment by mrlonglong | Hacker News Reader

So if atoms are composed of electrons, protons, and in turn electrons, protons are composed of these particles, can we potentially drill down a bit deeper into these hadrons, might they be composed of further sub particles yet unknown to science yet?

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As far as we know, both leptons (electrons, positrons, muons, taus) and quarks are fundamental, i.e. not composed out of other particles (also, they are pointlike, i.e. they have no extend). That's not true for protons (which are made out of quarks and gluons. It's a little bit complicated because of vacuum polarization, but if you count a certain way, you'll find it's composed out of two up and one down quark). They also have a measurable size.

These new particles are also made from quarks (4 for the tetra quark and 5 for the pentaquark), and they also have a size.

It's not known if quarks are fundamental, nor is it known if they are pointlike.

We've never seen a free quark, so we just don't know.

Doesn't have to be free to be studied. We can do deep inelastic scattering to measure the quark shape, the same way we can do quasi-elastic scattering to study the neutron shape. SM certainly assumes leptons and quarks to be point-like. (of course, in general, we can not prove any theory, only disprove.)

Deep inelastic scattering of the nucleus did not find quarks. It only found nucleons.

The same thing here: it's not known if quarks have an internal structure. And unless your energy is higher than the size of the object being measured you certainly can't tell if it's point like.

No, that's quasi-elastic scattering. DIS was done, for example, by HERA, an elctron-proton collider. SLAC did it in 1968 and essentially discovered the quarks.

Of course there is always a measurement limit. Current limits for the size are "smaller than 1e-19m". That's why I wrote "as far as we know", as we have no evidence that they are not point-like, and we have rather strong limits.

The colorfulness of quarks hints at non trivial internal structure, and given the number of colors, quarks probably consist of three somethings, bound together in a complex dynamically stable motion.

Based on this comment on reddit [1], it looks at some point one needs to use the mass of a solar system.

[1] https://www.reddit.com/r/explainlikeimfive/comments/vri4ih/e...

Excellent article, many thanks.

That's the premise of string theory

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