Introduction : $R_b$, $R_c$ and QCD

Precision measurements using the b quark, i.e. the isopartner of the heaviest fermion top, probe new physics that could have escaped the more precise tests using light fermions and leptons in particular. At present, the measured decay rate of the Z boson in b quarks is 3 standard deviations away from the Standard Model (SM) value. Is this the long-sought new physics? Or could this be an experimental artefact? After a brief introduction of the important observables in this game, I recall the accuracy (10-3) with which the Standard Model (SM) works for the light fermions (section 2) In comparison, the 2% (resp. 7%) discrepancy in R b (resp. R c) seems enormous (section 3). Of course, given the difficulty of the measurement, one should first ask how it could be wrong, (section 4), before trying to find how new physics could explain it (section 5). Although this topic is at first sight the concern of electroweak afficionados, I will explain in section 6 how it is linked, via the total hadronic width, to the determination of the strong coupling constant s, and therefore constitutes an adequate topic of discussion for a forum of hadronic interaction experts.