Mixed stops and the ATLAS on-$Z$ excess

The ATLAS experiment has recently observed a $3\sigma$ excess in a channel with a leptonically decaying $Z$, jets, and $E_T^{\text{miss}}$. It is tantalizing to interpret the signal as the first sign of a natural supersymmetric spectrum. We study such a possibility in a minimal model containing light stops and a neutralino lightest supersymmetric particle. The signal is characterized by a novel topology (compared to previous attempts) where the $Z$ is emitted from a colored particle in the first step of a decay chain, namely ${\tilde{t}}_2\to {\tilde{t}}_{1}Z$, which is characteristic of mixed stops. We show that the excess is compatible with a compressed stop spectrum and is not excluded by any other relevant search, finding some regions of parameter space with signal strength within $1\sigma$ of that measured by the ATLAS Collaboration. In addition, we notice that the corresponding CMS search could be prone to background contamination in unexpected topologies of this kind.

Figure 1

Flavor-conserving (left panel) and flavor violating (right panel) decays contributing to the on-$Z$ excess. In the flavor violating case, $q$ can be either an up or a charm quark.

Figure 2

Heavy stop branching ratios in the split scenario, for $m_{{\tilde{t}}_2}=450\mathrm{GeV}$, $m_{{\tilde{t}}_1}=250\mathrm{GeV}$, $m_{{\tilde{\chi }}^0}=210\mathrm{GeV}$, $\tan \beta =20$.

Figure 3

Scans of the three scenarios. The regions favored by the ATLAS on-$Z$ excess are shaded green, with contours indicating 95% and 68% confidence intervals. Additional solid lines indicate the 95% limits described in the text. The dashed lines indicate limits under specific model assumptions that do not necessarily apply, as described in the text. The band on the $\text{jets}+\mathrm{MET}$ limit illustrates the considerable uncertainty on the strength of this limit. Black stars indicate the benchmark points chosen from each scenario, and they also indicate the region of parameter space that is not excluded by the other searches.

Figure 4

The kinematic distributions compared to those of ATLAS in the on-$Z$ search. The simulation predictions are summed with the ATLAS-calculated SM background to produce the model predictions.

Figure 5

JZB probability distributions for the benchmark points when compared with a typical gluino production scenario ($\tilde{g}\to qq{\chi }_2^0\to qq{\chi }_1^{0}Z$). The gluino scenario has the most positively skewed JZB distribution, while the $\cancel{\mathrm{F}}$-S has almost symmetrical JZB.