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e $^{+}$e − depends on N$_{c}$ ; again, experimentally N$_{c}$ = 3. Similarly, the inclusive semi-hadronic decay rate of the τ lepton to its semi-leptonic one is a direct measure of N$_{c}$ with N$_{c}$ = 3. And the decay of neutral pions to two photons is a direct measure of the square of the number of colors; the theoretical prediction 

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where f$_{π}$ = 92 . 4 MeV is the pion decay constant controlling the π − → µν , is in remarkable agreement with data (7 . 7 ± 0 . 6 eV). Four jet measurements identified directly the triple gluon vertex and found that the gauge group is SU(3) instead of SO(3) or perhaps U(1)$_{3}$ [43, 44]; see Fig. 1.4. Similarly the running of the coupling agrees well for N$_{c}$ = 3 [66]. 

