Boosting searches for natural supersymmetry with $R$-parity violation via gluino cascades

In the presence of even minuscule baryonic $R$-parity violation (RPV), the stop can be the lightest superpartner and evade LHC searches because it decays into two jets. In order to cover this interesting possibility, we here consider new searches for RPV stops produced in gluino cascades. While typical searches for gluinos decaying to stops rely on same-sign dileptons, the RPV cascades usually have fewer hard leptons, less excess missing energy, and more jets than $R$-parity conserving cascades. If the gluino is a Dirac fermion, same-sign dilepton signals are also often highly depleted. We therefore explore search strategies that use single-lepton channels, and combat backgrounds using $H_T$, jet counting, and more detailed multijet kinematics or jet substructure. We demonstrate that the stop mass peaks can be fully reconstructed over a broad range of spectra, even given the very high jet multiplicities. This would not only serve as a “double-discovery” opportunity, but would also be a spectacular confirmation that the elusive top partner has been hiding in multijets.

Figure 1

Left panel: Constraints on a Majorana gluino. The blue line is from the ATLAS same-sign dilepton search (LHC8, $6{\mathrm{fb}}^{-1}$). The purple line is from the ATLAS $b^{\prime }$ search (LHC7, $1{\mathrm{fb}}^{-1}$). The red lines are our estimates for our $l+\mathrm{\text{jets}}$ ($N_j$, $H_T$) style search (thin: assuming LHC7, $1{\mathrm{fb}}^{-1}$; thick: assuming LHC8, $5{\mathrm{fb}}^{-1}$), with the boundary defined by $S/\sqrt{S+B}=2$. Right panel: Constraints on a Dirac gluino. The green line is from the CMS opposite-sign dilepton SUSY search (LHC7, $5{\mathrm{fb}}^{-1}$). The black line is from the ATLAS black hole search (LHC7, $1{\mathrm{fb}}^{-1}$). The purple line is again ATLAS $b^{\prime }$, and the red lines are again our ($N_j$, $H_T$) counting estimates. We do not consider regions with a $\tilde{g}$ LSP, indicated with dark gray. The light gray line indicates $m_{\tilde{g}}=m_{\tilde{t}}+m_t$.

Figure 2

Left panel: $N_j$ distributions of signals for $m_{\tilde{g}}=600\mathrm{GeV}$ (black dotted line), 800 GeV (black dashed line), and 1000 GeV (black solid line) with $m_{\tilde{t}}=1/2\times m_{\tilde{g}}$, and backgrounds $t\overline{t}+\mathrm{\text{jets}}$ (solid red line) and $W+\mathrm{\text{jets}}$ (solid blue line). Right panel: $H_T$ distributions of the corresponding signals and backgrounds. The gluinos used for these plots are Dirac, but we obtain equivalent distributions with Majorana.

Figure 3

Reconstructed stop mass distributions using our traditional jet analysis for 800 GeV Dirac gluinos with $20{\mathrm{fb}}^{-1}$ at LHC8. Black histograms are signal, red histograms are $t\overline{t}+\mathrm{\text{jets}}$ background, and blue histograms are $W+\mathrm{\text{jets}}$ background. We consider three different stop masses and three different choices for the number of jets used in the reconstruction. Rows correspond to 600 GeV stops (top row), 400 GeV stops (middle row), and 200 GeV stops (bottom row). Columns correspond to reconstructions using the $n$ leading jets with $n=4$ (left column), 5 (middle column), and 6 (right column). We restrict to events with $N_j\ge 7$ and $H_T>1200\mathrm{GeV}$. Vertical dashed lines represent the truth stop masses. (All plots are unstacked.)

Figure 4

Reconstructed stop mass distributions using jet substructure for 800 GeV Dirac gluinos with $20{\mathrm{fb}}^{-1}$ at LHC8. Black histograms are signals with 250 GeV (solid lines) and 140 GeV (dashed lines) stops, the red histogram is the $t\overline{t}+\mathrm{\text{jets}}$ background, and the blue histogram is the $W+\mathrm{\text{jets}}$ background. We restrict to events with $N_j\ge 7$ and $H_T>1200\mathrm{GeV}$. (The plot is unstacked.)

Figure 5

The significance ($S/\sqrt{B}$, left) for $20{\mathrm{fb}}^{-1}$ integrated luminosity, and signal/background ratios ($S/B$, right), for an 800 GeV gluino and varying stop masses at LHC8. The red curves are the simple ($N_j$, $H_T$) analysis, the blue curves are with stop peaks reconstructed using traditional jets, and the pink curves use jet substructure. We have used the cuts given in Table .

Figure 6

Discovery potential ($S/\sqrt{B}\ge 5$) for a Majorana (left panel) and Dirac (right panel) gluino with $20{\mathrm{fb}}^{-1}$ at LHC8. The red line is our simple ($N_j$, $H_T$) cut-and-count style search. The black line is the better of our two searches using mass windows around the reconstructed stop peaks (traditional and substructure), which improve discrimination against backgrounds. The blue line is traditional only, without substructure. We do not consider regions with a $\tilde{g}$ LSP, indicated with dark gray. The light gray line indicates $m_{\tilde{g}}=m_{\tilde{t}}+m_t$.