  The $\phi_{\eta}^{*}$ distribution of the $Z/\gamma^{*} \rightarrow
\ell^{+}\ell^{-}$ production in hadron collisions is simulated using a
leading-order event generator, GR@PPA. The initial-state parton shower, which
simulates the multiple QCD-radiation effects in the initial state, plays the
dominant role in this simulation. The simulation in the default setting agrees
with the high-statistics measurement by ATLAS at LHC with the precision at the
level of 5%. The observed systematic deviation, which can be attributed to the
effects of ignored higher-order contributions, can be reduced by adjusting the
arbitrary energy scales in the simulation. The agreement at the level of 1% can
be achieved over a very wide range without introducing any modification in the
implemented naive leading-logarithmic parton shower.
