Hunting scalar leptoquarks with boosted tops and light leptons

The LHC search strategies for leptoquarks that couple dominantly to a top quark are different than for the ones that couple mostly to the light quarks. We consider charge $1/3$ ($\phi_1$) and $5/3$ ($\phi_5$) scalar leptoquarks that can decay to a top quark and a charged lepton ($t\ell$) giving rise to a resonance system of a boosted top and a high-$p_{\rm T}$ lepton. We introduce simple phenomenological models suitable for bottom-up studies and explicitly map them to all possible scalar leptoquark models within the Buchm\"{u}ller-R\"{u}ckl-Wyler classifications that can have the desired decays. We study pair and single productions of these leptoquarks. Contrary to the common perception, we find that the single production of top-philic leptoquarks $\phi = \{\phi_1,\phi_5\}$ in association with a lepton and jets could be significant for order one $\phi t\ell$ coupling in certain scenarios. We propose a strategy of selecting events with at least one hadronic-top and two high-$p_{\rm T}$ same flavour opposite sign leptons. This captures events from both pair and single productions. Our strategy can significantly enhance the LHC discovery potential especially in the high-mass region where single productions become more prominent. Our estimation shows that a scalar leptoquark as heavy as $\sim1.7$ TeV can be discovered at the $14$ TeV LHC with 3 ab$^{-1}$ of integrated luminosity in the $t\ell\ell+X$ channel for $100\%$ branching ratio in the $\phi\to t\ell$ decay mode. However, in some scenarios, the discovery reach can increase beyond $2$ TeV even though the branching ratio comes down to about $50\%$.