Demystifying the compressed top squark region with kinematic variables

The ongoing perplexing scenario with no hints of new physics at the Large Hadron Collider can be elucidated amicably if the exotic particle spectrum in many of the well-motivated theoretical models possesses degenerate mass. We investigate the usefulness of different kinematic variables sensitive to the compressed mass region, and propose a search strategy considering a phenomenological supersymmetric scenario where the top squark undergoes a four-body decay due to its extremely narrow mass difference with the lightest supersymmetric particle. Considering a challenging but relatively clean dileptonic decay channel, we demonstrate that one can effectively restrain the significant background from the top quark, which provides a complementary approach to the present CMS analysis. With the new strategic approach the current limit can be extended to a phase-space region that was not explored before.

Figure 1

The right panel shows the top squark four-body decay producing top and $W$ boson off shell. The left panel shows the top decay via the leptonic channel.

Figure 2

The distribution of four variables $M_{b\ell }$, $R_{bE}$, $R_{\ell E}$, and $M_{T2}(b\ell \ell )$ is shown here (clockwise). The blue (dash-dotted), red (solid), and green (dashed) distributions are for the signal, $t\overline{t}$, and $tW$ events, respectively. The distribution was simulated for $\mathrm{\Delta }M=50\mathrm{GeV}$ and $m_{\tilde{t}}=400\mathrm{GeV}$.

Figure 3

An exclusion limit with 13 and $300{\mathrm{fb}}^{-1}$ data and a discovery plot for $300{\mathrm{fb}}^{-1}$ data are shown. The inset shows an exclusion limit in the $m_{\tilde{t}}-\mathrm{\Delta }M$ plane with $13.3{\mathrm{fb}}^{-1}$ luminosity.