Observable ϕη⁎ at LHC and second-order QED matrix element in Z/γ⁎→l+l− decays

In a recent publication by ATLAS Collaboration a new observable, the so-called ϕη⁎ 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 ϕη⁎ distribution. In this Letter 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 ϕη⁎ distribution at the LHC, inclusion of the second-order QED matrix element in a respective Monte Carlo event generator is necessary.