Measuring the top Yukawa coupling at the ILC at √(s)=500 GeV

We report on the feasibility of the direct measurement of the top Yukawa coupling gt at the International Linear Collider during its first phase of operation with a center-of-mass energy of 500 GeV. The signal and background models incorporate the nonrelativistic QCD corrections which enhance the production cross section near the tt threshold. The e+e-→ttH signal is reconstructed in the 6-jet+lepton and the 8-jet modes. The results from the two channels are combined. The background processes considered are e+e-→tbW-/tbW+ (which includes e+e-→tt), e+e-→ttZ, and e+e-→ttg*→ttbb. We use a realistic fast Monte Carlo detector simulation. Signal events are selected using event shape variables, through jet clustering, and by identifying heavy flavor jets. Assuming a Higgs mass of 120 GeV, polarized electron and positron beams with (Pe-,Pe+)=(-0.8,+0.3), and an integrated luminosity of 1 ab-1, we estimate that the e+e-→ttH events can be seen with a statistical significance of 5.2σ, corresponding to the relative top Yukawa coupling measurement accuracy of |Δgt/gt|=10%.