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

TL;DR

The paper investigates constraints on color-octet fermions, notably gluinos, by performing a global QCD analysis that includes the gluino as an extra parton and allows the strong coupling $\alpha_s$ to float within low- and high-energy constraints. By combining up-to-date DIS and jet data with direct $\alpha_s(Q)$ determinations and using the S-ACOT scheme for mass-dependent SUSY contributions, the study derives mass bounds that depend on $\alpha_s(M_Z)$ and data selection. The results show that gluinos as light as $30$–$50$ GeV can be compatible with current hadronic data, with stronger limits (e.g., $m_{\tilde{g}}\gtrsim25$ GeV) when $\alpha_s(M_Z)$ is fixed; Tevatron jet data offer limited discriminatory power due to systematic uncertainties, while LHC predictions require better control of $\alpha_s$ and PDFs to distinguish SM from SM+SUSY scenarios. Overall, the work provides model-independent gluino constraints from hadronic observables, informs LHC search strategies, and delivers public PDFs that incorporate a light color-octet fermion.

Abstract

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.

Figures (13)

• Figure 2: Running of the strong coupling as a function of the scale $Q$. The red solid line represents the SM evolution, while the dashed lines are plotted for $m_{\tilde{g}}=50,25,10,5$ GeV. The points with the error bands represent the low-$Q$ and high-$Q$ constraints, given in Eqs. (\ref{['alphas:lowQ']}) and (\ref{['alphas:highQ']}), respectively.
• Figure 3: Ratios of $g(x,Q)$ (upper row) and $c(x,Q)$ (lower row) distributions in SUSY fits with floating $\alpha_s(M_Z)$ and the CT10 fit at $Q=2$ GeV (left) and $Q=85$ GeV (right), for the gluino mass $m_{\tilde{g}}$ of 20 and 50 GeV.
• Figure 4: Same as Fig. \ref{['fig:PDFs:floating']}, but for a fixed $\alpha_s(M_Z)=0.118$.
• Figure 5: PDFs for various flavors at $Q$=100 GeV, for $m_{{\tilde{g}}}=50$ GeV.
• Figure 6: Values of $\Delta\chi^2_{h.s.}$ vs. $m_{\tilde{g}}$ are shown for a fixed value of $\alpha_s(M_Z) = 0.118$ for the 2004 study (blue dashed line) and our new one (red solid line).