Searches for direct pair production of supersymmetric top and supersymmetric bottom quarks in $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$

We search for direct pair production of supersymmetric top quarks and supersymmetric bottom quarks in proton-antiproton collisions at $\sqrt{s}=1.96\mathrm{TeV}$, using $295{\mathrm{pb}}^{-1}$ of data recorded by the Collider Detector at Fermilab (CDF II) experiment. The supersymmetric top (supersymmetric bottom) quarks are selected by reconstructing their decay into a charm (bottom) quark and a neutralino, which is assumed to be the lightest supersymmetric particle. The signature of such processes is two energetic heavy-flavor jets and missing transverse energy. The number of events that pass our selection for each search process is consistent with the expected standard model background. By comparing our results to the theoretical production cross sections of the supersymmetric top and supersymmetric bottom quarks in the minimal supersymmetric standard model, we exclude, at a 95% confidence level in the frame of that model, a supersymmetric top quark mass up to $132\mathrm{GeV}/c^2$ for a neutralino mass of $48\mathrm{GeV}/c^2$, and a supersymmetric bottom quark mass up to $193\mathrm{GeV}/c^2$ for a neutralino mass of $40\mathrm{GeV}/c^2$.

Figure 1

Leading-order Feynman diagrams for pair production of stop and sbottom at the Tevatron.

Figure 2

The decay channels of stop and sbottom considered in this paper.

Figure 3

The ${\mathrm{E̸}}_T$ distribution in the stop (top) and sbottom (bottom) signal regions for data (solid points) compared to the SM background (filled histograms). Also shown (open histograms) are the expected distributions arising from stop pair production and decay at $m_{{\tilde{t}}_1}=120\mathrm{GeV}/c^2$ and $m_{{\tilde{\chi }}_1^0}=50\mathrm{GeV}/c^2$, and sbottom pair production and decay at $m_{{\tilde{b}}_1}=160\mathrm{GeV}/c^2$ and $m_{{\tilde{\chi }}_1^0}=80\mathrm{GeV}/c^2$.

Figure 4

The 95% CL exclusion region in the mass plane of $m_{{\tilde{\chi }}_1^0}$ vs $m_{{\tilde{t}}_1}$, assuming that stop decays exclusively into $c{\tilde{\chi }}_1^0$. For the region excluded by the present search (area inside the solid curve), the one sigma lower bound of the predicted NLO cross section is used to extract the mass limits. The expected exclusion contour is shown as a dashed curve. The result from the DØ Collaboration, using data from the Tevatron Run II, is presented in the same manner [16]. Results from other previous searches are also indicated [9, 10, 11]. The LEP results are presented for the case of no mixing in the stop sector ($\theta =0°$).

Figure 5

The 95% CL exclusion region in the mass plane of $m_{{\tilde{\chi }}_1^0}$ vs $m_{{\tilde{b}}_1}$, assuming that sbottom decays exclusively into $b{\tilde{\chi }}_1^0$. For the region excluded by the present search (area inside the solid curve), the one sigma lower bound of the predicted NLO cross section is used to extract the mass limits. The dashed line indicates the expected exclusion region. The result from the D0 Collaboration, using data from the Tevatron Run II, is presented in the same manner [17]. Results from other previous searches are also indicated [9, 10]. The LEP results are presented for the case of no mixing in the sbottom sector ($\theta =0°$).