Measuring ZγH vertex effects at eγ linear colliders

The one-loop process eγ → eH , for intermediate Higgs masses is considered. Exact cross sections for unpolarized and longitudinally polarized beams are computed and found to be more than two orders of magnitude larger than the rates for the crossed process e+e− → Hγ , in the energy range√s = (0.5÷2) TeV. We show that, apart from being competitive with the γγ → H process for testing the one-loop γγH vertex, by requiring a final electron tagged at large angle the channel eγ → eH provides an excellent way of testing the ZγH vertex. e-mail: [email protected], [email protected], [email protected] To appear in the Report DESY 97-123E Possible ways to test the one-loop couplings ggH , γγH and ZγH have been extensively studied in the literature. Because of the nondecoupling properties of the Higgs boson, these vertices are sensitive to the contribution of new particles circulating in the loops, even in the limit Mnew ≫ mH [1]. A measurement of the γγH and ZγH couplings should be possible by the determination of the BR’s for the decays H → γγ [2, 3] and H → γZ [4, 3], respectively. This is true only for an intermediate-mass Higgs boson (i.e., for 90GeV ∼< mH ∼< 140 GeV), where both BR(H → γγ) and BR(H → γZ) reach their maximum values, which is O(10−3). Another promising way of measuring the γγH coupling for an intermediate-mass Higgs boson will be realized through the Higgs production in γγ collisions [5, 6]. To this end, the capability of tuning the γγ c.m. energy on the Higgs mass, through a good degree of the photons monochromaticity, will be crucial for not diluting too much the γγ → H resonant cross section over the c.m. energy spectrum. In this short note, we sum up the main results recently obtained on the Higgs production in eγ collisions through the one-loop process eγ → eH [7]. This channel turns out to be an excellent means to test both the γγH and ZγH one-loop couplings with relatively high statistics, without requiring a fine tuning of the c.m. energy. While the γ-exchange γγH contribution is dominant in the total cross section, by requiring a large transverse momentum of the final electron (or Higgs boson), one enhances the Z-exchange ZγH contribution, while keeping the corresponding rate still to an observable level. The further contribution given by the box diagrams with W and Z exchange survives at large angles too, but is relatively less important. Furthermore, while the γγH and ZγH channels increase logarithmically with the c.m. collision energy, the contribution from boxes starts decreasing at √ s ∼> 400 GeV. In our study we assumed that the initial photon beam is to a good degree monochromatic, and has an integrated luminosity of O(100) fb. The cross section for the process eγ → eH has previously been studied in the WeizsäckerWilliams (WW) approximation [8], where the only channel contributing is the (almost real) γ-exchange in the t-channel, induced by the γγH vertex [9]. This method provides a rather good estimate of the eγ → eH total cross sections, but it is unable to assess the importance of the ZγH (and box) effects. This we will address particularly in our exact treatment of eγ → eH . Although, the cross sections for the process eγ → eH are quite large also for heavy Higgs masses, we will concentrate on the intermediate Higgs mass case (hence, assuming that the decay H → bb̄ is dominant). The crossed process, ee → Hγ , has been studied in different papers [10, 11, 12]. However, the ee → Hγ channel suffers from small rates, which are further depleted at large energies by the 1/s behavior of the dominant s-channel diagrams. As a consequence, if a eγ option of the linear collider will be realized with similar luminosity of the ee option, the eγ → eH channel will turn out to be much more interesting than the process ee → Hγ for finding possible deviations from the standard-model one-loop Higgs vertices.