"Figure 2a" of "Search for low-mass vector and scalar resonances decaying into a quark-antiquark pair in proton-proton collisions at $\sqrt{s}$ = 13 TeV"

Upper limits at 95% CL on the universal coupling $g_q$ between the $Z'$ boson and quarks. The $Z'$ boson is assumed to decay to quark-antiquark pairs with a 100% branching fraction. The solid line represents the observed limits. The dashed line and the inner (yellow) and outer (blue) bands represent the median and the regions containing 68% and 95%, respectively, of the distribution of expected limits under the background-only hypothesis.