Third generation in cascade decays

In supersymmetric models with gluinos $m(\tilde{g})\sim 1000-2000\mathrm{GeV}$, new physics searches based on cascade decay products of the gluino are viable at the next run of the LHC. We investigate a scenario where the light stop is lighter than the gluino and both are lighter than all other squarks, and show that its signal can be established using multi $b$-jet, multi $W$ and/or multi-lepton final state topologies. We then utilize both boosted and conventional jet topologies in the final state in conjunction with di-tau production as a probe of the stau-neutralino coannihilation region responsible for the model’s dark matter content. This study is performed in the specific context of one such phenomenologically viable model named no-scale $\mathcal{F}\text{−}SU(5)$.

Figure 1

Events with (0,1,2) leptons and no like-sign dilepton (category I) are studied at a luminosity of 30 events per femtobarn. Two heavy-flavor tagged jets with $P_{\mathrm{T}}>80\mathrm{GeV}$ are required, and the leading eight jets (with or without a b-tag) must carry $P_{\mathrm{T}}>(400,200,80,80,40)\mathrm{GeV}$. Left: Absolute signal and background component event counts are binned as a function of the missing transverse energy ${\cancel{E}}_{\mathrm{T}}$. Right: Signal significance relative to the leading $t\overline{t}$ background is evaluated for four signal regions as a function of the ${\cancel{E}}_{\mathrm{T}}$ cut.

Figure 2

Events with a like-sign dilepton topology (category II) are studied at a luminosity of 30 events per femtobarn. Two heavy-flavor tagged jets with $P_{\mathrm{T}}>80\mathrm{GeV}$ are required, and the leading four jets (with or without a b-tag) must carry $P_{\mathrm{T}}>(400,200,80,80)\mathrm{GeV}$. Two additional jets (for a total of 6) must carry $P_{\mathrm{T}}>40\mathrm{GeV}$. Left: Absolute signal and background component event counts are binned as a function of the missing transverse energy ${\cancel{E}}_{\mathrm{T}}$. Right: Signal significance relative to the leading $t\overline{t}$ background is evaluated for four signal regions as a function of the ${\cancel{E}}_{\mathrm{T}}$ cut.

Figure 3

Events with a trilepton topology (category III) are studied at a luminosity of 30 events per femtobarn. Two heavy-flavor tagged jets with $P_{\mathrm{T}}>80\mathrm{GeV}$ are required, and the leading four jets (with or without a b-tag) must carry $P_{\mathrm{T}}>(400,200,80,80)\mathrm{GeV}$. Left: Absolute signal and background component event counts are binned as a function of the missing transverse energy ${\cancel{E}}_{\mathrm{T}}$. Right: Signal significance relative to the leading $t\overline{t}$ background is evaluated for four signal regions as a function of the ${\cancel{E}}_{\mathrm{T}}$ cut.

Figure 4

Left: Normalized distribution of $b$-jets retained after soft kinematic grooming in the background, the unified signal, and the isolated gluino pair production channel. The background remains larger than the unified signal by a factor of about 80 at this level, with cross sections of $8.8\times {10}^3$ and $1.1\times {10}^2\mathrm{fb}$, respectively. Restricting to four or more b-jets produces order parity in the event expectations, reducing the cross sections to 5.5 and 2.0 fb, respectively. The gluino pair production sub-channel yields a somewhat larger density of jets in isolation than the full signal in aggregate, to which it contributes approximately one third of net events at this level. Right: The signal significance is evaluated as a function of the missing transverse energy ${\cancel{E}}_{\mathrm{T}}$ for the category I selections, adding a 4 $b$-jet requirement relative to the selection depicted in Fig. 1.

Figure 5

Normalized distribution of reconstructed hadronic $W$’s in the category $\mathrm{I}\ge 8$ jets with $\ge 2b$-jets plus large ${\cancel{E}}_{\mathrm{T}}$ final state for both signal and background. The net signal counts are larger than the background after these hard cuts by a factor of about 65, with cross sections of 2.9 and $4.5\times {10}^{-2}\mathrm{fb}$, respectively. Restricting to two or more $W$ reconstructions effectively suppresses the background and further reduces the signal cross section by a factor of about 20 to $1.5\times {10}^{-1}\mathrm{fb}$.

Figure 6

Feynman diagrams (adapted from MadGraph5 [20] output) contributing to strong interaction analogs of the electroweak “vector boson fusion” boosted event topology with dijet plus $\tilde{g}\tilde{g}$, $\tilde{g}\tilde{q}$, and $\tilde{q}{\tilde{q}}^{*}$ final states.

Figure 7

Normalized signal and background distribution shapes are compared as a function of the missing transverse energy ${\cancel{E}}_{\mathrm{T}}$ for the boosted and conventional event topologies.

Figure 8

Di-tau events are studied at a luminosity of 30 events per femtobarn under application of the conventional event topology. All additional cuts documented in Sec. 1 are imposed as a prerequisite. Left: Absolute signal and background component event counts are binned as a function of the missing transverse energy ${\cancel{E}}_{\mathrm{T}}$. Right: Signal significance relative to the leading $t\overline{t}$ background is evaluated for four signal regions as a function of the ${\cancel{E}}_{\mathrm{T}}$ cut.

Figure 9

Di-tau events are studied at a luminosity of 30 events per femtobarn under application of the boosted event topology. All additional cuts documented in Sec. 1 are imposed as a prerequisite. Left: Absolute signal and background component event counts are binned as a function of the missing transverse energy ${\cancel{E}}_{\mathrm{T}}$. Right: Signal significance relative to the leading $t\overline{t}$ background is evaluated for four signal regions as a function of the ${\cancel{E}}_{\mathrm{T}}$ cut.

Figure 10

The ratio of boosted to conventional signal to background ratios is displayed as a function of the gaugino scale parameter $M_{1/2}$ for various ${\cancel{E}}_{\mathrm{T}}$ cut thresholds.

Figure 11

Expected $\mathcal{F}\text{−}SU(5)$ signal and $t\overline{t}$ background counts at a luminosity of 300 events per femtobarn are depicted for the boosted and conventional event topologies as a function of the gaugino scale parameter $M_{1/2}$ for various ${\cancel{E}}_{\mathrm{T}}$ cut thresholds.

Figure 12

The conventional event topology opposite-sign minus like-sign di-tau invariant mass $M_{\tau \tau }$ distribution shape is depicted for various gaugino mass $M_{1/2}$ scales and mass gaps $\mathrm{\Delta }M\equiv M_{{\tilde{\tau }}_1}-M_{{\tilde{\chi }}_1^0}$.

Figure 13

The distribution of transverse momenta $P_{\mathrm{T}}$ associated with individual hadronic tau elements is depicted in terms of the luminosity-independent shape, and in terms of the absolute event yield at fixed luminosity for various values of the mass gap $\mathrm{\Delta }M\equiv M_{{\tilde{\tau }}_1}-M_{{\tilde{\chi }}_1^0}$.