I know that looking at the invisible decay width of the $Z$-boson at the LEP collider at CERN leads to the evidence of the existence of three (light) lepton generations but I can't find any information on the experimental evidence for 3 quark generations. Thus: what experimental evidence suggests the existence of three quark generations?

  • $\begingroup$ Check Griffiths, Introduction to fundamental particles. In the historical part I think you'll find what you want. And a quick answer: u and d are obvious, c, s comes from J/Psi particle experiment I think? Can't recall how b and t were found. $\endgroup$
    – Turgon
    Apr 3, 2017 at 7:07
  • $\begingroup$ Wikipedia's page on the Quark gives a history section that should lead you to what you want. I don't think a short answer can be provided given that it required some forty years of experiment and theory to build the model. $\endgroup$ Apr 3, 2017 at 8:05
  • $\begingroup$ @StephenG From what I can tell, the history page only contains info on how each quark was found. This is not what I am after. I am looking for experimental evidence that there are only three quark generations (i.e. no more) $\endgroup$ Apr 3, 2017 at 12:01
  • $\begingroup$ I'd suggest you modify the question and title to reflect that. This page might be what you're looking for. $\endgroup$ Apr 3, 2017 at 12:09

2 Answers 2


The data that provides direct measurement of the charges of high-mass quarks also exhibits the existence of those heavier states. By comparing the cross sections for $$ e^+ + e^- \longrightarrow \mu^+ + \mu^-$$ with that for $$ e^+ + e^- \longrightarrow q + \bar{q} \longrightarrow \text{hadrons} $$ we get a fairly direct measurement of $$ \sum_\text{accessible quark masses} q^2_\text{quark} \;.$$

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All by itself that's pretty strong evidence, but when combined with the group structure of the hadronic n-tuplets it's about as close to iron clad as evidence comes in the particle physics world.


This link explains the experimental discovery of each quark.

Here is the rational for three generations

One of the definitive experiments which supports the quark model is the high energy annihilation of electrons and positrons. The annihilation can produce muon-antimuon pairs or quark-antiquark pairs which in turn produce hadrons. The hadron events are evidence of quark production. The ratio of the number of hadron events to the number of muon events gives a measure of the number of "colors" of the quarks, and the evidence points to five quarks with three colors. With the more recent evidence for the top quark, these experiments provide support for the standard model of six quarks with three colors.

Experimenters were not giving up that maybe a fourth generation of quarks and leptons exists, though by now it can be excluded at five sigma:

According to the results of the statistical analysis by researchers from CERN, and Humboldt University of Berlin, the existence of further fermions can be excluded with a probability of 99.99999% (5.3 sigma). The researchers combined latest data collected by the particle accelerators LHC and Tevatron with many known measurements results relating to particles, such as the Z-boson or the top-quark. The most important data used for this analysis come from the discovery of the Higgs particle. In the Standard Model, the Higgs particle gives all other particles their mass. As additional fermions were not detected directly in accelerator experiments, they have to be heavier than the fermions known so far. Hence, these fermions would also interact with the Higgs particle more strongly. This interaction would have modified the properties of the Higgs particle such that this particle would not have been detected


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