What is "the famous 3R experiment" for quark colours? This page says: "The famous $3R$ experiment also suggests that whatever force binds the quarks together has 3 types of charge (called the 3 colors)."
Google seems to think that the $3R$ experiment isn't at all famous! Does it have a different name nowadays?  Can anyone tell me what it is?
 A: The experiment is the measurement of the total cross section for electron-positron annihilation into hadronic final states, $e^{-}+e^{+}\rightarrow\, {\rm hadrons}$.  Because the hadrons interact strongly, the details of the sub-processes that make up the inclusive $e^{-}+e^{+}\rightarrow\, {\rm hadrons}$ are very difficult to work out quantitatively.  However, the total for all external hadron states tells us quite a bit of useful information, and precise measurements of the total rate of hadron production have been goals at successive high-energy electron-positron colliders for decades.
The rate for the process with hadrons in the final state is compared to the total rate for annihilation into muons, $e^{-}+e^{+}\rightarrow\mu^{-}+\mu^{+}$ (which is easy to calculate, because it involves only leptons at tree level and is totally dominated by a single QED diagram).  The ratio of rates is the quantity $R$:
$$R=\frac{\sigma(e^{-}+e^{+}\rightarrow\, {\rm hadrons})}{\sigma(e^{-}+e^{+}\rightarrow\mu^{-}+\mu^{+})}.$$
From the quark model predictions of hadron charges and masses, it was inferred the the up and down(/strange) quarks had charges $+\frac{2}{3}|e|$ and $-\frac{1}{3}|e|$, respectively.  This was further confirmed (although with a lot noise) by deep elastic scattering experiments, which saw pointlike charged quarks when high-energy electrons were scattered off nucleons.  With those charges known, it was possible to calculate the expected rate for $e^{-}+e^{+}\rightarrow u+\bar{u}$, $e^{-}+e^{+}\rightarrow d+\bar{d}$, and $e^{-}+e^{+}\rightarrow s+\bar{s}$ (in the ultrarelativistic limit of QED).  The process of $e^{-}+e^{+}\rightarrow\, {\rm hadrons}$ begins with a very hard (large momentum exchange) $e^{-}+e^{+}\rightarrow q+\bar{q}$, reaction, which controls the overall cross section; after this, there are soft final state interactions that cause hadrons or hadron jets to coalesce out of the initial quark and antiquark products.
So measuring $e^{-}+e^{+}\rightarrow\,{\rm hadrons}$ is a good experimental proxy for measuring the total rate of $e^{-}+e^{+}\rightarrow q+\bar{q}$.  The total rate is approximately proportional to the number of quark types that can be created, and when the $R$ was measured, it was about three times larger than what naively have been expected.  They reason is that there are three kinds of up quark, three kinds of down quark, etc.  These three kinds are the colors.  That factor of three is what gives the $3R$ measurement its name.
The state of the art of electron-positron annihilation experiments at the time when these questions became important and interesting is explained in detail in R. F. Schwitters, K. Strauch, Annual Reviews of Nuclear Science 26, 89–149 (1976).
