  We investigate the global existence and optimal time decay rate of solution
to the one dimensional (1D) two-phase flow described by compressible Euler
equations coupled with compressible Navier-Stokes equations through the
relaxation drag force on the momentum equations. First, we prove the global
existence of strong solution to 1D Euler-Navier-Stokes system by using the
standard continuity argument for small $H^{1}$ data while the second order
derivative can be large. Then we derive the optimal time decay rate to the
equilibrium state $(\rho_*, 0, n_*, 0)$. Compared with multi-dimensional case,
it is much hard to get time decay rate by direct spectrum method due to a
slower convergence rate of the fundamental solution in 1D case. To overcome
this main difficulty, we need to first carry out time-weighted energy estimates
for higher order derivatives, and based on these time-weighted estimates, we
can close a priori assumptions and get the optimal time decay rate by spectrum
analysis method. Moreover, due to non-conserved form and insufficient decay
rate of the coupled drag force terms between the two-phase flows, we
essentially need to use momentum variables $(m= \rho u, M=n\omega)$, not
velocity variables $(u, \omega)$ in the spectrum analysis, to fully cancel out
those non-conserved and insufficiently decay drag force terms.
