Extending the reach of compressed gluinos at the LHC

Conventional supersymmetry searches rely on large missing momentum and, on that account, are unsuitable for discovering superpartners nearly degenerate with the LSP. Such “compressed regions” are best probed by dedicated strategies that exploit their unique kinematic features. We consider a case study of a compressed gluino-bino simplified spectrum, motivated by its ability to set the dark matter relic abundance via coannihilation. A kinematic variable suited to this spectrum is introduced, by which, for a gluino-bino mass splitting of 100 GeV, the discovery reach is extendable to $m_{\tilde{g}}=850\mathrm{GeV}$ (1370 GeV) at LHC center-of-mass energy 8 TeV (13 TeV) with luminosity $20{\mathrm{fb}}^{-1}$ ($3000{\mathrm{fb}}^{-1}$). The nontrivial role played by soft triggers is also discussed.

Figure 1

Area-normalized event distributions of the quantities on the lhs of Eq. (8). We take $\sqrt{s}=8\mathrm{TeV}$, $\mathcal{L}=20{\mathrm{fb}}^{-1}$ and $m_{\tilde{g}}=1\mathrm{TeV}$ for illustration. From these distributions we can infer that the combination of quantities that defines $\rho$ peaks at smaller values for the signal. See also Fig. 2 that validates this.

Figure 2

Area-normalized event distributions in $\rho$ [defined in Eq. (8)] after imposing cuts I and II. We take $\sqrt{s}=8\mathrm{TeV}$, $\mathcal{L}=20{\mathrm{fb}}^{-1}$ and $m_{\tilde{g}}=1\mathrm{TeV}$ for illustration. The signal distinctly peaks near low $\rho$ because of kinematic features (ii)–(iv) described in the text.

Figure 3

Significances [as defined in Eq. (9)] of the compressed gluino search after all our cuts are imposed. The plot on the left (right) corresponds to $\sqrt{s}=8(13)\mathrm{TeV}$. The green (purple) curves denote an integrated luminosity of $20{\mathrm{fb}}^{-1}(3{\mathrm{ab}}^{-1})$. The bands depict background systematic uncertainties in the range [$-50\%$,100%] due to matching procedures and factorization scale choice.

Figure 4

95% exclusion contours in the $m_{\tilde{g}}\text{−}{m}_{{\tilde{\chi }}^0}$ plane at $\sqrt{s}=13\mathrm{TeV}$ and $\mathcal{L}=12.9{\mathrm{fb}}^{-1}$. The dashed black (blue) line is where $m_{\tilde{g}}=m_{{\tilde{\chi }}^0}$ ($m_{\tilde{g}}=m_{{\tilde{\chi }}^0}+100\mathrm{GeV}$). The magenta curves depict our strategy, and the brown curves are CMS exclusion limits. This plot illustrates that a dedicated search strategy for the compressed region ($\mathrm{\Delta }M\lesssim 200\mathrm{GeV}$) can provide better limits than existing $\mathrm{jets}+\mathrm{MET}$-based cut-and-count strategies.