Hunting leptoquarks in monolepton searches

Abstract: We show that stringent limits on leptoquarks that couple to first-generation quarks and left-handed electrons or muons can be derived from the spectral shape of the charged-current Drell-Yan process ($p p \to \ell^\pm \nu$) at Run 2 of the LHC. We identify and examine all six leptoquark species that can generate such a monolepton signal, including both scalar and vector leptoquarks, and find cases where the leptoquark exchange interferes constructively, destructively or not at all with the Standard Model signal. When combined with the corresponding leptoquark-mediated neutral-current ($p p \to \ell^+ \ell^-$) process, we find the most stringent limits obtained to date, outperforming bounds from pair production and atomic parity violation. We show that, with 3000 fb$^{-1}$ of data, combined measurements of the transverse mass in $p p \to \ell^\pm \nu$ events and invariant mass in $p p \to \ell^+ \ell^-$ events can probe masses between 8 TeV and 18 TeV, depending on the species of leptoquark, for electroweak-sized couplings. In light of such robust sensitivities, we strongly encourage the LHC experiments to interpret Drell-Yan (dilepton and monolepton) events in terms of leptoquarks, alongside usual scenarios like $Z'$ bosons and contact interactions.