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Diphoton resonance at the ILC

In this paper we study the direct production of the diphoton resonance $X$ which has been suggested by 2015 data at the LHC, in $e^+e^-\to Xγ/XZ$ processes at the ILC. We derive an analytic expression for the scattering amplitudes of these processes, and present a comprehensive analysis for determining the properties of $X$ at the ILC. A realistic simulation study for $e^+e^-\to Xγ$ is performed based on the full detector simulation to demonstrate the capabilities of the ILC experiment. Complementary to the searches at the LHC, prospects of the measurement of the absolute values of production cross-section are obtained for the ILC using recoil technique without assuming decay modes of $X$. In addition, we have studied the searches for $X\to\rm{invisible}$ and $X\to b\bar{b}$ modes, which are challenging at the LHC, and found that these decay modes can be discovered with high significance if their branching ratios are large enough.