HERA events, tevatron jets, and uncertainties on quarks at large x

The recently reported excess of events at HERA compared to QCD calculations impels us to examine all possible Standard Model explanations before invoking ''new physics''. We explore the possibility of adding an unusual, but small, component of additional quarks at large x (beyond x > 0.75) as a way to increase the predicted SM cross-section in the HERA kinematic region by the QCD evolution feed-down effect. We describe various scenarios under which this can be achieved while maintaining good global fits to all established data sets. This implies a much larger SM uncertainty than commonly assumed. In addition, the modified parton distributions provide another possible mechanism to account for the CDF high-p(t) jet excess which occurs at similar x and Q(2) values. (C) 1997 Elsevier Science B.V.