  In a recent publication by ATLAS collaboration a new observable, the
so-called phi-star angle, was used for precise measurement of transverse Z
momentum. One of the dominant systematic errors for this measurement originates
from the theoretical control of QED final-state bremsstrahlung. At present, it
is estimated at the 0.3% level for the shape of the phi-star distribution. In
this paper we discuss the possible effects of the second-order QED matrix
element for that quantity. For that purpose, results from simulations based on
the Yennie--Frautchi--Suura (YFS) exponentiation and featuring the second-order
matrix elements are used and compared with the case when the matrix element is
restricted to the first order. From this study we conclude that in order to
reach the precision below 0.3% for the phi-star distribution at the LHC,
inclusion of the second-order QED matrix element in a respective Monte Carlo
event generator is necessary.
