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.
Highlights
Leptoquarks—bosonic color triplets with baryon and lepton numbers—appear in theories of grand unification [1,2], supersymmetry with R-parity violation [3], dark matter [4], and explanations of anomalies in low-energy flavor experiments [5,6,7,8]
The LHC collaborations hunt them in pair production processes [9,10,11], with current limits having surpassed those placed by the Tevatron and HERA [12,13,14,15]
Leptoquarks coupling to electrons are strongly constrained by tests of atomic parity violation (APV) [17]
Summary
Leptoquarks—bosonic color triplets with baryon and lepton numbers—appear in theories of grand unification [1,2], supersymmetry with R-parity violation [3], dark matter [4], and explanations of anomalies in low-energy flavor experiments [5,6,7,8]. For both S3 and U3, the helicity structure of the monolepton amplitude again matches the SM; in this case, the interference is constructive, which will lead to strong bounds on these leptoquarks. Operator y2QLðu CPLeÞðd CPLνÞ y2QLðuγμPLνÞðdγμPLeÞ yuLyQeðu PLνÞðd PReÞ yQeyuLðu CγμPReÞðd CγμPLνÞ right handed), and the contribution of leptoquarks is neither enhanced nor suppressed by interference effects For these two species we will set the couplings yuL 1⁄4 yQe throughout our analysis for simplicity, and denote this common coupling by “yQL” to economize notation. Note that our monolepton analysis remains unchanged by the more complicated interference patterns in the dilepton signal, another argument for the importance of a monolepton analysis in searching for, and studying, leptoquarks
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