Extending the STU formalism for electroweak oblique corrections to the case of light new physics

The STU formalism, which parametrizes oblique electroweak corrections due to heavy new physics, has recently been extended to the case of light new physics. In principle, this extended formalism, which involves the six parameters STUVWX, must be used if loop contributions to the oblique corrections entail light new particles with masses in the range ∼~M_Z or less. Applying classical equations of motion to manipulate the effective lagrangian, we present here a thorough introductory discussion of the STUVWX formalism, elucidating such subtleties as the connection between S - X and the star formalism of Kennedy and Lynn, the connection between S - X, and the ε parameters of Altarelli and Barbieri, and the loss of definite symmetry properties which S and U undergo as the STU parameter set is extended to the case of light new physics. This paper provides a significantly more detailed, expanded, and pedagogic presentation than previous expositions of the STUVWX parameters.