Negative S and Light New Physics *

The several sigma difference between SLD's recent precise measurement of A LR and the corresponding LEP results tends to reinforce the earlier trends in the data towards negative values for the Peskin-Takeuchi parameter S and T. Motivated by this not yet statistically significant, but suggestive, trend, we explore which kinds of new particles can (1) contribute dominantly to new physics through oblique corrections, (2) produce negative values for S and T , and (3) not be in conflict with any other experiments, on or off the Z resonance. We are typically led to models which involve new particles with masses that are not much heavier than M Z /2, and so which would also have implications for other experiments in the near future. We show how the analysis of such 'light-new-physics' models in terms of oblique parameters requires the interpretation of the data in terms of modified parameters, S ′ and T ′ , whose difference from S and T improves the available parameter space of the models.