Increasing Rb and decreasing Rc with new heavy quarks.

If the b and c quarks mix with new heavy quarks of weak isospin I3 = −1 and 0 respectively, then the Z → bb̄ (cc̄) rate is necessarily greater (smaller) than that of the standard model. This may be the reason for the Rb excess and Rc deficit observed at LEP. A possible consequence of this scenario is the prospective discovery of a new quark x with the dominant decay x → ch, then h → bb̄, where h is a neutral Higgs boson. It has been known for some time[1] that the experimentally measured Z → bb̄ (cc̄) rate is greater (smaller) than that of the standard model. With the recent observation of the top quark[2] at the Tevatron and more precision data[3] from the four LEP experiments, the two discrepancies have become even sharper, as summarized below. Measurement SM Pull Rb 0.2219± 0.0017 0.2156 3.7 Rc 0.1543± 0.0074 0.1724 −2.5 Here Rb ≡ Γ(Z → bb̄)/Γ(Z → hadrons), Rc ≡ Γ(Z → cc̄)/Γ(Z → hadrons), SM stands for the standard-model fit with mt = 178 GeV and mH = 300 GeV, and “pull” is defined as the difference between measurement and fit in units of the measurement error. If these results are taken at face value, physics beyond the standard model is indicated. Previous attempts in this direction have dealt mostly with Rb. Its excess has been interpreted as due to one-loop corrections of the Zbb̄ vextex coming from extensions of the standard model, such as the two-Higgs-doublet model,[4] or the minimal supersymmetric standard model,[5] or the SU(3) × SU(2)L × U(1)Y model.[6] However, the first two scenarios are in potential conflict with top quark decay[7] and all three fail to account for the large Rc deficit. The purpose of this note is to point out that the Rb excess and the Rc deficit are naturally explained by the mixing of the b and c quarks with new heavy quarks of weak isospin I3 = −1 and 0 respectively. The idea is very simple. Consider first the mixing of the c quark with a new heavy isosinglet quark x of charge 2/3.[8] Since both cR and xR are singlets, we can define xR to be that which appears in the gauge-invariant mass term x̄LxR. We then have both c̄LcRφ̄ 0 and c̄LxRφ̄ 0 Yukawa terms, where (φ, φ) is the usual Higgs doublet of the standard model. As a result, the mass matrix linking (c̄L, x̄L) to (cR, xR) is given by M = 