CMS-PAS-TOP-25-015

CMS-PAS-TOP-25-015
Search for top quark FCNCs in multilepton final states using an effective field theory approach in proton-proton collisions at $ \sqrt{s}= $ 13 TeV
CMS Collaboration
2026-05-22

Abstract: We present a search for flavour-changing neutral current (FCNC) interactions involving the top quark in production and decay. This analysis establishes the first global search in the top-quark FCNC sector within the standard model effective field theory where all relevant operators are constrained simultaneously in a single fit to data, providing a framework for future measurements and combinations at the LHC. The analysis uses proton--proton collision data at $ \sqrt{s}= $ 13 TeV, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. Signal contributions are probed in final states with two same-sign leptons or three leptons. Multivariate discriminants are used to separate signal from background, and their distributions enter a likelihood fit to determine 95% confidence intervals for 24 Wilson coefficients. No evidence for FCNC interactions is observed. The results provide the first simultaneous constraints on the complete operator set and set the most stringent limits to date on FCNC top-quark decay branching fractions via two-lepton-two-quark interactions, $ \mathrm{BR}(\mathrm{t}\to \ell^{+}\ell^{-}\mathrm{q}) $, where $ \mathrm{q}=\mathrm{u} $ or c.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Representative Feynman diagrams for single top quark production (left) and top decays in $ \mathrm{t} \overline{\mathrm{t}} $ events (right) via FCNC interactions. The vertex with FCNC contributions is marked with a red circle.
png pdf	Figure 2: Postfit distributions of the final discriminant are shown for the (2 $ \ell \textrm{ss} $), (3 $ \ell\text{ on-}\mathrm{Z} $), and (3 $ \ell\text{ off-}\mathrm{Z} $) channels in the 1bLj, 1bHj, and (>1b) categories. Signal predictions for three representative WCs ($c_{t\phi}$, $c_{tZ}$, and $c_{te)^{(\ell)}$) are overlaid for illustration.
png pdf	Figure 3: 68 (thick line) and 95% (thin line) confidence intervals for the $ \mathrm{t}\mathrm{u} $ and $ \mathrm{t}\mathrm{c} $ FCNC WCs, obtained from profiled (solid lines) and fixed (dashed lines) fits. The limits on the WCs (left) are translated into the corresponding limits on the branching fractions (right).

Tables
png pdf	Table 1: Operators that can generate FCNC vertices at order 1 $ /\Lambda^2 $ and are considered in this analysis. Details on the operators, conventions, and symbol definitions are provided in Ref. [21].
png pdf	Table 2: Summary of the signal region (SR) and control region (CR) categorization requirements.

Summary

In summary, a search for flavor-changing neutral current (FCNC) interactions involving the top quark is performed using proton--proton collision data at $\sqrt{s}=13$ TeV recorded by the CMS experiment in 2016--2018, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. Events with same-sign dilepton or trilepton final states and exactly one b-tagged jet are selected, and boosted decision trees are used to enhance the separation of signal from background. A binned maximum-likelihood fit to the discriminant distributions is used to extract constraints on the underlying effective couplings. No significant deviation from the standard model prediction is observed. Confidence intervals at 68 and 95% confidence level are set on 24 Wilson coefficients. These results constitute the first simultaneous constraints on the complete set of relevant top-quark FCNC operators in the standard model effective field theory and establish a global benchmark for future top-quark FCNC measurements and their combination at the LHC. In addition, they provide the first direct experimental constraints on FCNC processes mediated by two-quark-two-lepton operators.

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