Boosted Semileptonic Tops in Stop Decays

TL;DR

This work revisits the viability of semileptonic stop decays at the LHC by leveraging boosted leptonic tops. It introduces a two-step strategy that first exploits the top-3-momentum direction and then reconstructs the full top 4-momentum, using MET correlations to suppress the ttbar background. Through hadron-level studies at 14 TeV and ~20 fb^-1, it shows that stops in the 340–540 GeV range can achieve S/√B ~ 5 with S/B ≈ 1–2, and it demonstrates how an M_T2 endpoint could enable stop-mass determination when combining orthogonal and decay-plane approximations. The approach links boosted-top tagging techniques with leptonic-top reconstruction to expand the observable toolkit for stop searches and mass measurements.

Abstract

Top partner searches are one of the key aspects of new physics analyses at the LHC. We correct an earlier statement that supersymmetric top searches based on decays to semileptonic tops are not promising. Reconstructing the direction of the boosted leptonic top quark and correlating it with the measured missing transverse energy vector allows us to reduce the top pair background to an easily manageable level. In addition, reconstructing the full momentum of the leptonic top quark determines the stop mass based on an M_{T2} endpoint.

Figures (4)

• Figure 1: Top: one-dimensional $\Delta\phi$ distributions with an additional cut $\slashchar{p}_T > 200$ GeV for the stop signal (red) and the top pair background (black). Below: two-dimensional $\slashchar{p}_T$ vs. $\Delta\phi$ distributions for the $t\bar{t}$ background (2nd row) and for the $\tilde{t}\tilde{t}^\ast$ signal (3rd row). From left to right we show $\Delta \phi(\slashchar{\vec{p}}_T,\hat{p}^{\perp}_{t,\ell})$, $\Delta \phi(\slashchar{\vec{p}}_T,\hat{p}^{\perp}_\nu)$ and $\Delta \phi(\slashchar{\vec{p}}_T,\hat{p}_{b\ell})$. All plots are at the hadron level and based on the orthogonal approximation. We apply the universal selection cuts 1-5 and the border of the cut 8 is shown in the two-demensional plots.
• Figure 2: $M_{T2}$ distributions for the stop signal (red) and the $t\bar{t}$ background (black). From the left we show the orthogonal and the decay plane approximations without the $\Delta \phi$ vs $\slashchar{p}_T$ cut but with $\slashchar{p}_T > 200$ GeV and then both of them with the $\Delta \phi$ vs $\slashchar{p}_T$ cut. The expected endpoint is $M_{T2} < m_{\tilde{t}}(1 -{m_{\tilde{\chi}^0_1}^2}/{m_{\tilde{t}}^2})= 418~{\rm GeV}$.
• Figure 3: $(\hat{x}_\parallel, \hat{x}_\perp)$ distributions as defined in Eq.(\ref{['eq:def_xhat']}) for top pair production. The four panels correspond to slices in the top momentum $200-300$ GeV, $400-500$ GeV, $600-700$ GeV, and $800-900$ GeV. All events fulfill $\slashchar{p}_T>150$ GeV.
• Figure 4: Absolute and relative $\Delta^\text{est} |p|$, $\Delta^\text{est} |p|/|p^\text{true}|$, $\Delta^\text{est} \phi$ and $\Delta^\text{est} \theta$ distributions in the decay plane approximation (black) and the orthogonal approximation (red). The upper two rows show the neutrino momentum estimate, the lower two rows the top momentum estimate. The first of the two rows is at parton level, the second row at hadron level. Only semileptonic top pair events are included. At hadron level we only require $\slashchar{p}_T>150$ GeV while at parton level all events also fulfill $p_{T,\ell} > 20$ GeV and $p_{T,b} > 25$ GeV.