HIGGS RADIATION OFF QUARKS IN SUPERSYMMETRIC THEORIES AT ee COLLIDERS

Yukawa couplings between Higgs bosons and quarks in supersymmetric theories can be measured in the processes ee → QQ̄ + Higgs. We have determined the cross sections of these processes in the minimal supersymmetric model including the complete set of next-to-leading order QCD corrections for all channels. Supported by the EU FF Programme under contract FMRX-CT98-0194 (DG 12 MIHT). Heisenberg-Fellow 1. In the Standard Model (SM) and its supersymmetric extensions, the masses of electroweak gauge bosons, leptons, and quarks are generated by interactions with Higgs fields [1]. The Yukawa couplings between Higgs particles and fermions therefore grow with the masses Mf of the fermions. The couplings obey a universal scaling law gffH = Mf/v in the SM, with v ≈ 246GeV being the ground-state value of the Higgs field. In supersymmetric models, which involve at least two Higgs doublets, the size of the Yukawa couplings is also set by the fermion masses, yet the relationship is more complex owing to the mixing among the Higgs fields. The Yukawa couplings [2] of the two CP-even light/heavy Higgs bosons h/H and of the CP-odd Higgs boson A in the minimal supersymmetric extension of the Standard Model (MSSM) [3,4], expressed in units of the SM couplings, are collected in Table 1. The parameter tanβ = v2/v1 is the ratio of the vacuum expectation values φ uūφ dd̄φ ZZφ ZAφ h cosα/ sin β − sinα/ cosβ sin(β − α) cos(β − α) H sinα/ sin β cosα/ cosβ cos(β − α) − sin(β − α) A 1/ tanβ tanβ — — Table 1: Higgs Yukawa couplings in the MSSM in units of the SM couplings Mf/v, and gauge couplings in units of the ZZH coupling MZ/v in the SM. of the two Higgs fields generating the masses of upand down-type particles, and α is the mixing angle in the CP-even sector. In the decoupling limit, in which the light Higgs mass reaches the maximum value for a given parameter tanβ, the h Yukawa couplings approach the SM values. In general, they are suppressed for up-type fermions and enhanced for down-type fermions; the enhancement increases with tan β and can therefore be very strong. Higgs radiation off top or off bottom quarks in ee collisions, ee → QQ̄h/H/A [Q = t, b], (1) lends itself as a suitable process for measuring the Yukawa couplings in supersymmetric theories [6], particularly for the light Higgs boson h and for moderately heavy Higgs bosons H and A. In this letter we present the cross sections for these processes including the nextto-leading order (NLO) QCD corrections. This problem has been solved before in the SM, where the QCD corrections significantly increase the cross section at moderate collider energies [7,8]. Moreover, it has been shown in detailed simulations that the SM top-quark Yukawa coupling can be measured with an accuracy close to 5% [9] (see also Ref. [10]) in high-luminosity operations of ee linear colliders. For the MSSM, first steps towards an NLO analysis have been made in Ref. [11], adopting either a global K factor from For large tanβ, SUSY loop corrections to the bb̄–Higgs vertices become very important [5]. In the present QCD analysis, these corrections can be absorbed into effective Higgs Yukawa couplings, since emission and reabsorption of virtual heavy SUSY particles is confined to small space-time regions compared with QCD subprocesses involving massless gluons.