Effective fermion–Higgs interactions at an e+e− collider with polarized beams

We consider the possibility of new physics giving rise to effective interactions of the form e+e−Hff¯, where f represents a charged lepton ℓ or a (light) quark q, and H   the recently discovered Higgs boson. Such vertices would give contributions beyond the standard model to the Higgs production processes e+e−→Hℓ+ℓ−e+e−→Hℓ+ℓ− and e+e−→Hqq¯ at a future e+e−e+e− collider. We write the most general form for these vertices allowed by Lorentz symmetry. Assuming that such interactions contribute in addition to the standard model production processes, where the final-state fermion pair comes from the decay of the Z   boson, we obtain the differential cross section for the processes e+e−→Hℓ+ℓ−e+e−→Hℓ+ℓ− and e+e−→Hqq¯ to linear order in the effective interactions. We propose several observables with differing CP and T properties which, if measured, can be used to constrain the couplings occurring in interaction vertices. We derive possible limits on these couplings that may be obtained at a collider with centre-of-mass energy of 500 GeV and an integrated luminosity of 500 fb−1fb−1. We also carry out the analysis assuming that both the electron and positron beams can be longitudinally polarized, and find that the sensitivity to the couplings can be improved by a factor of 2–4 by a specific choice of the signs of the polarizations of both the electron and positron beams for the same integrated luminosity.