Two-loopO(αsGFMQ2

Abstract

By means of a low-energy theorem, we analyze at O(α s G F M 2 Q ) the shifts in the Standard-Model W + W − H and ZZH couplings induced by virtual high-mass quarks, Q, with M Q ≫ M Z , M H , which includes the top quark. Invoking the improved Born approximation, we then find the corresponding corrections to various four-and fivepoint Higgs-boson production and decay processes which involve the W + W − H and ZZH vertices with one or both of the gauge bosons being connected to light-fermion currents, respectively. This includes e + e − → ffH via Higgs-strahlung, via W + W − fusion (with f = ν e ), and via ZZ fusion (with f = e), as well as H → 2V → 4f (with V = W, Z).1 exploiting the Higgs-strahlung mechanism [3,4], e + e − → ZH → ff H. In next-generation e + e − linear supercolliders (NLC), also e + e − → ν eνe H via W + W − fusion and, to a lesser extent, e + e − → e + e − H via ZZ fusion will provide copious sources of Higgs bosons.The study of quantum corrections to the production and decay processes of the Higgs boson has received much attention in the literature; for a review, see Ref. [5]. Since the top quark, with pole mass M t = (180 ± 12) GeV [6], is so much heavier than the intermediate bosons, the M t -dependent corrections are particularly important. On the other hand, it is attractive to consider the extension of the SM by a fourth fermion generation, where such corrections may be even more significant. Some time ago, Hill and Paschos [7] proposed an interesting fourth-generation scenario with Majorana neutrinos, which exploits the see-saw mechanism to evade the LEP1/SLC constraint on the number of light neutrinos. The novel charged fermions of this model are assumed to be of Dirac type and to have standard couplings. Subsequently, this model was further elaborated, and the precise triviality bounds, renormalization-group fixed points, and related dynamical mechanisms were discussed [8].In particular, it was demonstrated how this model is reconciled with the fermion-mass constraints established in Ref. [9]. In Ref.[10], it was shown that this model is compatible with precision data from low energies and LEP1/SLC. Very recently, it was noticed [11] that arguments favouring the presence of a fourth fermion generation may be adduced on the basis of the democratic mass-matrix approach [12]. The possible existence of a fourth fermion generation is also considered in the latest Particle Data Group Report [13], where mass bounds are listed. For a recent model-independet analysis, see Ref. [14].It is advantageous to trace such novel fermions via their loop effects in the Higgs sector, since these effects are also sensitive to mass-degenerate isodoublets via fermion-mass power corrections. This has originally been observed in Ref. [15] in connection with the ff H, W + W − H, and ZZH couplings. Moreover, the ggH coupling may serve as a device to detect mass-degenerate isodoublets of ultraheavy quarks [16], although power corrections do not occur here. By contrast, in the gauge sector, power corrections only appe...

Radiative corrections to single Higgs boson production in e+e− annihilation

“…In this region, √ s > 500 GeV, one can compare this constant correction with the effective approximation of Ref. [10], where one only corrects the W W h 0 coupling. Although the sign of this approximation is correct, it does not fully account for the whole effect.…”

Higgs production and decay with a fourth Standard-Model-like fermion generation

“…The results for the two-loop corrections δ tot(2) V can be found in Ref. [33] for the QCD corrections of O(α s G µ m 2 f ′ ) and in Ref. [25] for the EW corrections of O(G 2 µ m 4 f ′ ).…”

Low-mass Higgs decays to four leptons at one loop and beyond

“…Including the corrections of O(x t ) [23,41,43,58], O(x 2 t ) [19], O(x t α s ) [45], and O(x t α 2 s ) [36], we have…”

“…In the case of Higgs-boson decays to two light-fermion pairs, this has the advantage that the branching fractions B(Z → f f ) and B(W ± → f f ′ ) may be taken to be the experimentally measured values, which naturally contain all radiative corrections. As for the partial decay widths Γ(H → W + W − ) and Γ(H → ZZ), which presuppose that m H > 2m V with V = W, Z, the full one-loop O(α) corrections [23,41,42,43], also including the subsequent decays into massless-fermion pairs for off-shell V bosons [44], the dominant two-loop terms of O(x t α s ) [45,46], O(x 2 t ) [19], and O(G 2 F m 4 H ) [47], and the three-loop term of O(x t α 2 s ) [36] are available. In Ref.…”