Constraints on color-octet fermions from a global parton distribution analysis

We report a parton distribution function (PDF) analysis of a complete set of hadron scattering data, in which a color-octet fermion (such as a gluino of supersymmetry) is incorporated as an extra parton constituent along with the usual standard model constituents. The data set includes the most up-to-date results from deep-inelastic scattering and from jet production in hadron collisions. Another feature is the inclusion in the fit of data from determinations of the strong coupling alpha_s(Q) at large and small values of the hard scale Q. Our motivation is to determine the extent to which the global PDF analysis may provide constraints on the new fermion, as a function of its mass and alpha_s(M_Z), independent of assumptions such as the mechanism of gluino decays. Based on this analysis, we find that gluino masses as low as 30 to 50 GeV may be compatible with the current hadronic data. Gluino masses below 15 GeV (25 GeV) are excluded if alpha_s(M_Z) varies freely (is equal to 0.118). At the outset, stronger constraints had been anticipated from jet production cross sections, but experimental systematic uncertainties, particularly in normalization, reduce the discriminating power of these data.