Motivation for polarised $e^-$ and $e^+$ Beams
  A future Linear Collider is well suited for discovering physics beyond the Standard Model, for revealing the structure of the underlying physics as well as for performing high precision tests of the Standard Model. The use of polarised beams will be one of the powerful tools for reaching these goals. This paper highlights some recent studies of having both beams simultaneously polarised which allows to use longitudinally as well as transversely polarised beams. 
Gudrid Moortgat-Pick