Radiative Corrections to a Supersymmetric Relation: A New Approach
  Recently it has been realized that the production and decay processes of charginos, neutralinos, and sleptons receive corrections which grow like log m_squark for large m_squark. In this paper we calculate the chargino pair production cross section at e+e- colliders with quark/squark loop corrections. We introduce a novel formulation, where the one-loop amplitude is reorganized into two parts. One part is expressed in terms of the ``effective'' chargino coupling gbar and mixing matrices U^P, V^P, and includes all O(log m_squark) corrections, while the other decouples for large m_squark. The form of the one-loop cross section then becomes physically transparent. Our formulation can be easily extended to other loops and processes. Numerically, we find significant corrections due to the effective t-channel coupling gbar, for gaugino-like charginos. In the mixed region, where the chargino has large gaugino and Higgsino components, the corrections due to (U^P,V^P) are also significant. Our numerical results disagree with a previous calculation. We revisit previous studies of the determination of gbar through the measurement of the chargino production cross section. We point out that a previous study, which claimed that the measurement suffers large systematic errors, was performed at a ``pessimistic'' point in MSSM parameter space. We provide reasons why the systematic errors are not a limiting factor for generic parameter choices. 
Shingo Kiyoura, Mihoko M. Nojiri, Damien M. Pierce, Youichi Yamada