Wouldn’t the act of measuring the spin of these quarks effectively change their spin and then change the subatomic particle? Would this go against the law of conservation of charge?
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$\begingroup$ en.wikipedia.org/wiki/R_(cross_section_ratio) $\endgroup$– Jeanbaptiste RouxMar 26, 2021 at 16:19
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$\begingroup$ maybe reading the history of how quarks were deemed necessary to model the protons , even though they could not be seen independently, will help, en.wikipedia.org/wiki/Quark#History $\endgroup$– anna vMar 26, 2021 at 18:24
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They were inferred by your proverbial "we". You are using spin in two senses:
Spin multiplet representation index, i.e. the dimensionality of the spin multiplet reduced by one, and divided by two, $(d-1)/2=s$, so the labelling Casimir of the multiplet s(s+1) is 3/4. This is necessary for working out consistent wave functions of hadrons in accord with the symmetries dictated by the Pauli principle. It took a while, and the introduction of "color" for the quark model to completely gell.
Eigenvalue of $S_z$; for example, the $S_z$ of each of the three quarks of a $\Delta^{+}$ of s=3/2 whose $S_z=3/2$ is measured (notionally) must be 1/2, by the rules of spin addition.
Like most properties of quarks, this is inferred indirectly consistently with basic principles.
I have no clue what the conservation of charge has to do with anything here. Hot tip: quarks do not make up electrons.
answered Mar 26, 2021 at 16:57