Same-sign top quarks as signature of light stops at the CERN LHC

TL;DR

This work addresses the challenge of discovering a light stop with $m_{ ilde{t}_1}\lesssim m_t$ in the MSSM by exploiting the Majorana nature of gluinos. The authors propose and test a hallmark signature from gluino pair production: two same-sign top quarks that arise when gluinos decay to $t\tilde{t}_1$ or $\bar{t}\tilde{t}_1$, followed by $\tilde{t}_1\to c\tilde{\chi}^0_1$ and leptonic top decays, yielding $2b$-jets + two same-sign leptons + jets + $E_T^{\rm miss}$. Through a detailed LHC case study (LST1) with Monte Carlo simulation and detector effects, they show that this signal can be extracted from backgrounds for $m_{\tilde{g}}\lesssim 900$ GeV, and they demonstrate mass-determination strategies using invariant-mass distributions $m_{bc}$ and $m_{lc}$ as well as an effective SUSY mass scale from $M_{\text{eff}}$. The analysis also derives analytic shapes for the invariant-mass distributions, enabling fits beyond endpoint methods. Overall, the work provides a viable discovery channel for light stops and a path to extract gluino/stop/LSP mass relations, while highlighting practical considerations such as $c$-jet tagging and tau-induced backgrounds.

Abstract

We present a new method to search for a light scalar top with $m_{\tilde{t}_1}\lsim m_t$, decaying dominantly into a c-jet and the lightest neutralino, at the LHC. The principal idea is to exploit the Majorana nature of the gluino, leading to same-sign top quarks in events of gluino-pair production followed by gluino decays into top and stop. The resulting signature is 2 b-jets plus 2 same-sign leptons plus additional jets and missing energy. We perform a Monte Carlo simulation for a benchmark scenario, which is in agreement with the recent WMAP bound on the relic density of dark matter, and demonstrate that for $m_{\tilde{g}}\lsim 900$ GeV and $m_{\tilde{q}}>m_{\tilde{g}}$ the signal can be extracted from the background. Moreover, we discuss the determination of the stop and gluino masses from the shape of invariant-mass distributions. The derivation of the shape formulae is also given.

Figures (6)

• Figure 1: Neutralino relic density for $M_1=110$ GeV, $M_2=220$ GeV, $\mu=300$ GeV, $\tan\beta=7$: in a) the WMAP allowed band in $m_{\tilde{t}_{1}}$--$m_{\tilde{\tau}_{1}}$ plane, and in b) $\Omega h^2$ as a function of $m_{\tilde{t}_{1}}$ for various $\tilde{\tau}_1$ masses. Computed with micrOMEGAs 1.3. All masses are in [GeV]. The other parameters are as explained in the text.
• Figure 2: WMAP allowed band in the $m_{\tilde{t}_{1}}$--$m_{\tilde{\tau}_{1}}$ plane analogous to Fig. \ref{['fig:mu300']}a, but for $\mu=180$ GeV.
• Figure 3: Effective mass distribution. SM contributions are $t\bar{t}$ (filled circles), $W$+jet (triangles), $Z$+jet (inverted triangles), $WW/WZ/ZZ$ production (stars) and QCD (squares). The sum of all SM events is shown by the hatched histogram. SUSY events are shown as open circles.
• Figure 4: Invariant-mass distributions for LST1. These distributions only take into account the kinematics of the decay, i.e. no spin or width effects are included.
• Figure 5: Invariant-mass distributions without $c$-jet tagging (black with error bars) and best fit. Left panel shows $m_{bc}$, right panel $m_{lc}$. Also shown are the contributions from the SM background (green) and the SUSY background (blue). The SUSY background consists mostly of events with one or more taus (see text).