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

We report on the feasibility of the direct measurement of the top Yukawa coupling g_t 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 tt̄ threshold. The e ⁺e^-→tt̄H 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 ^-→tb̄W^-/t̄bW⁺ (which includes e⁺e^-→tt̄), e⁺e ^-→tt̄Z, and e⁺e^-→tt̄g ^*→tt̄bb̄. 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 1ab^-1, we estimate that the e⁺e^-→tt̄H events can be seen with a statistical significance of 5.2σ, corresponding to the relative top Yukawa coupling measurement accuracy of |Δg _t/g_t|=10%.