Exploring compressed supersymmetry with same-sign top quarks at the Large Hadron Collider

TL;DR

This work investigates LHC phenomenology of compressed supersymmetry in which a light top squark mediates neutralino annihilation to set the dark matter relic density, focusing on same-sign leptonic top decays from gluino and squark production with $\tilde{g}\to t\tilde{t}_1$ and $\tilde{t}_1\to c\tilde{N}_1$. A one-parameter model line is defined at the GUT scale with $M_1,M_2,M_3$ relations and relic-density constraint, yielding a spectrum where the stop is light, the LSP is bino-like, and gluinos are within reach; sleptons decouple. The paper develops methods to extract information about the superpartner masses from (i) same-sign dilepton events with heavy-flavor tags, (ii) scalar-summed transverse momenta observables $M_A$–$M_D$, and (iii) invariant-mass endpoints of visible gluino-decay products, while discussing the limitations due to jet misassignment and MET reconstruction. It finds that heavy-flavor tagging and mass-estimator correlations provide rough gluino mass estimates and can constrain the gluino–stop–LSP mass relations, though endpoints are ambiguous without additional information; combined with DM relic-density considerations, these observables offer a practical strategy to test this compressed-SUSY scenario at the LHC.

Abstract

In compressed supersymmetry, a light top squark naturally mediates efficient neutralino pair annihilation to govern the thermal relic abundance of dark matter. I study the LHC signal of same-sign leptonic top-quark decays from gluino and squark production, which follows from gluino decays to top plus stop followed by the stop decaying to a charm quark and the LSP in these models. Measurements of the numbers of jets with heavy-flavor tags in the same-sign lepton events can be used to confirm the origin of the signal. Summed transverse momentum observables provide an estimate of an effective superpartner mass, which is correlated with the gluino mass. Measurements of invariant mass endpoints from the visible products of gluino decays do not allow direct determination of superpartner masses, but can place constraints on them, including lower bounds on the gluino mass as a function of the top-squark mass.

Figures (13)

• Figure 1: The mass spectrum for a sample point on the model line described in the text, with $M_1 = 500$ GeV at the GUT scale, $C_{24} = 0.21$, $A_0/M_1 = -1$, $m_0 = 314$ GeV, $\mu = 361$ GeV, and $\tan\beta = 10$. The columns contain, from left to right, Higgs scalar bosons, neutralinos, charginos, the gluino, first and second family squarks and sleptons, and third family squarks and sleptons.
• Figure 2: The mass difference $m_{\tilde{t}_1} - m_{\tilde{N}_1}$ as a function of $m_{\tilde{g}}$, for the model line described in the text. The solid line is the model line with $\Omega_{\rm DM} h^2 = 0.11$, and the shaded region denotes the approximate region favored by the thermal relic abundance constraints. The model line is cut off on the left by the LEP2 Higgs mass constraint.
• Figure 3: The NLO production cross-section for superpartner pairs in $pp$ collisions at $\sqrt{s} = 14$ TeV for selected points along the model line described in the text, as a function of the gluino mass. Prospino2 prospino was used. The most important contributions, from stop pair production ($\tilde{t}_1 \tilde{t}_1^*$), gluino-squark production ($\tilde{g} \tilde{q}$ and $\tilde{g} \tilde{q}^*$), gluino pair production ($\tilde{g} \tilde{g}$), and squark pair production ($\tilde{q} \tilde{q}$ and $\tilde{q} \tilde{q}^*$ and $\tilde{q}^* \tilde{q}^*$) are also shown separately.
• Figure 4: The number of LHC signal events with two same-sign leptons, two $b$ tags, and two additional jets, per fb$^{-1}$, after the cuts described in the text, for the model line described in section \ref{['sec:modelline']}.
• Figure 5: Representative transverse momentum distributions for $\ell^\pm \ell^{\prime \pm} bbjj + E_T^{\rm miss}$ events from 100 fb$^{-1}$ of superpartner production, after the cuts described in the text. The upper left panel shows the $E_T^{\rm miss}$ distribution (here without the $E_T^{\rm miss}$ cut) for two points on the model line with $m_{\tilde{g}} = 569$ and $702$ GeV. The upper right panel shows the leading and subleading lepton $p_T$ distributions for the model with $m_{\tilde{g}} = 569$, and the lower panels show the leading and subleading $b$-jet distributions (left) and the three leading jet distributions (right) for the same model point.