Improving third-generation leptoquark searches with combined signals and boosted top quarks

Search strategies for the third-generation leptoquarks (LQs) are distinct from other LQ searches, especially when they decay to a top quark and a $\tau$ lepton. We investigate the cases of all TeV-scale scalar and vector LQs that decay to either a top-tau pair (charge-$1/3$ and charge- $5/3$ LQs) or a top-neutrino pair (charge-$2/3$ LQs). One can then use the boosted top (which can be tagged efficiently using jet-substructure techniques) and high-$p_{\mathrm{T}}$ $\tau$ leptons to search for these LQs. We consider two search channels with either one or two taus along with at least one hadronically decaying boosted top quark. We estimate the high luminosity LHC (HL-LHC) search prospects of these LQs by considering both symmetric and asymmetric pair and single production processes. Our selection criteria are optimized to retain events from both pair and single production processes. The combined signal has better prospects than the traditional searches. We include new three-body single production processes to enhance the single production contributions to the combined signal. We identify the interference effect that appears in the dominant single production channel of charge-$1/3$ scalar LQ ($S^{1/3}$). This interference is constructive if $S^{1/3}$ is a weak-triplet and destructive for a singlet one. As a result, their LHC prospects differ appreciably.

Figure 1

The parton-level cross sections of different production channels of ${\varphi }_i[(\mathrm{a}),(\mathrm{c}),(\mathrm{e})]/{\chi }_i[(\mathrm{b}),(\mathrm{d}),(\mathrm{f})]$ at the 14 TeV LHC as functions of $M_{{\chi }_n/{\varphi }_n}$. The single production cross sections are computed for a benchmark coupling $\lambda ,\mathrm{\Lambda }=1$ (see Table 1). For the VLQs, we have shown the scenarios with QCD coupling $\kappa =1$. Here, we consider $\tau j$ and $\tau t$ in the single production processes and the $j$ includes all the light jets including $b$-jets. Their cross sections are generated with a cut on the transverse momentum of the jet, $p_{\mathrm{T}}^j>20\mathrm{GeV}$.

Figure 2

Expected significance $\mathcal{Z}$ for observing the ${\varphi }_i/{\chi }_i$ signals over the SM backgrounds. They are plotted as functions of their masses for $3{\mathrm{ab}}^{-1}$ of integrated luminosity at the 14 TeV HL-LHC for different coupling scenarios. The ‘comb’ implies the combined pair and single production processes. We have shown the pair production significance with $\mathrm{BR}=50\%$ and $\mathrm{BR}=100\%$. We have considered $\lambda ,\mathrm{\Lambda }=1$ when computing the signals.

Figure 3

The $5\sigma$ ($2\sigma$) discovery (exclusion) reaches in the mass-coupling plane. These plots describe the lowest values of couplings needed to observe LQ signals with $5\sigma$ and $2\sigma$ significance as functions of masses with $3{\mathrm{ab}}^{-1}$ of integrated luminosity. The pair-production-only regions for 50% and 100% BRs in the $\chi /\varphi \to t\tau$ decay mode are shown in green. The pair production processes are insensitive to couplings.

Figure 4

Same as Fig. 3 for the $t_h{\tau }_h+\mathrm{MET}$ channel. The pair-production-only regions for 50% BRs in the $\chi /\varphi \to t\tau$ and $b\nu$ decay mode are shown in blue.

Figure 5

The bins show the number of signal and background events in the $t_h{\tau }_h{\tau }_h+t_h{\tau }_h{\tau }_{\ell }$ channel [(a) and (b)] and the $t_h{\tau }_h+$ MET channel [(c) and (d)]. The events are obtained applying all the cuts in Table 3, except the ones on the variables used for binning, i.e., $N({\tau }_h)$ and $M_{\mathrm{T}}$ in the $t_h{\tau }_h{\tau }_h+t_h{\tau }_h{\tau }_{\ell }$ channel and $N(b)$ and MET in the $t_h{\tau }_h+$ MET channel.