Finite Lifetime Effects in Top Quark Pair Production at Threshold

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In this work electroweak effects to the e+e− top pair production cross section at threshold are determined that are associated to the finite top lifetime. A new effective theory approach is developed that involves complex Wilson coefficients and anomalous dimensions and that allows to treat these effects systematically at NNLL order in a non-relativistic expansion. The imaginary parts are associated to experimental cuts that are imposed to define the cross section in analogy to the optical theory in an absorptive medium. In addition electroweak effects at NNLL order not related to instability are computed. The results are important for an extraction of Standard Model parameters at a future Linear Collider.