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In a recent publication by ATLAS collaboration a new observable, theso-called phi-star angle, was used for precise measurement of transverse Zmomentum. One of the dominant systematic errors for this measurement originatesfrom the theoretical control of QED final-state bremsstrahlung. At present, itis estimated at the 0.3% level for the shape of the phi-star distribution. Inthis paper we discuss the possible effects of the second-order QED matrixelement for that quantity. For that purpose, results from simulations based onthe Yennie--Frautchi--Suura (YFS) exponentiation and featuring the second-ordermatrix elements are used and compared with the case when the matrix element isrestricted to the first order. From this study we conclude that in order toreach 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 Carloevent generator is necessary.

Observable phi-eta-star at LHC and second-order QED matrix element in Z/gamma to l+ l- decays