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Search for top quark flavor-changing neutral currents in multilepton final states using an effective field theory approach in proton--proton collisions at $\sqrt{s}$ = 13 TeV

Using 138 fb $^{-1}$ of proton-proton collision data at $\sqrt{s}$ = 13 TeV collected by the CMS detector, this study presents the first global simultaneous search for top quark flavor-changing neutral currents within the Standard Model effective field theory framework, establishing the most stringent limits to date on relevant Wilson coefficients and decay branching fractions without observing any evidence for such interactions.

Original authors: CMS Collaboration

Published 2026-08-21

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CC BY 4.0

Original authors: CMS Collaboration

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the universe of subatomic physics, there is a set of rules known as the Standard Model that describes how the smallest building blocks of matter interact. For decades, this model has been remarkably successful, yet it leaves some questions unanswered. One of the most persistent mysteries involves the top quark, the heaviest known elementary particle. According to the Standard Model, a top quark should almost never change its identity into a lighter quark, such as an up or a charm quark, without swapping partners with a charged particle. This restriction is so strict that such an event is predicted to happen so rarely that it is effectively impossible to observe. However, many physicists suspect that this rule might be broken by new, undiscovered forces or particles that exist beyond our current understanding. If a top quark were ever seen to change its flavor without a charge exchange, it would be a clear sign that the Standard Model is incomplete and that new physics is waiting to be discovered.

A team of scientists working with the CMS detector at the Large Hadron Collider in Switzerland has now conducted the most comprehensive search yet for these rare events. They analyzed a massive collection of data from proton-proton collisions, representing a total exposure of 138 inverse femtobarns, which corresponds to the amount of data collected over several years of operation at an energy level of 13 tera-electronvolts. Instead of looking for just one specific type of new interaction, the researchers cast a wide net using a framework called the Standard Model Effective Field Theory. This approach allows them to test a vast number of possible new interactions simultaneously, treating them as small adjustments to the known laws of physics rather than assuming a single new particle exists. They focused their search on a specific and challenging signature: collisions that produced three or more charged particles known as leptons, which include electrons and muons, often carrying the same electric charge.

The researchers sifted through billions of collision events to find the few that matched their criteria. They were looking for a very specific pattern where a top quark might have interacted with a boson, such as a Higgs or Z boson, or directly with a pair of leptons, causing it to decay in an unusual way. To distinguish these rare signals from the overwhelming background of ordinary collisions, the team used sophisticated computer algorithms trained to recognize the subtle differences in the energy and direction of the particles produced. They organized the data into different categories based on the number of jets, which are sprays of particles created by quarks, and the number of b-tagged jets, which are jets identified as likely coming from a bottom quark. This careful sorting allowed them to isolate the regions where a new signal would be most visible.

After running their analysis, the scientists found no evidence of the flavor-changing neutral currents they were hunting for. The data matched the predictions of the Standard Model perfectly, with no significant deviations that would suggest new physics. This result is significant because it places the strictest limits yet on how often a top quark can change its flavor through these specific interactions. By testing twenty-four different theoretical parameters at the same time, the team established a new global benchmark for future experiments. While they did not find the new particles they hoped for, their work effectively rules out a wide range of possibilities for how new physics might manifest in the top quark sector. The study confirms that, within the precision of their measurements, the top quark behaves exactly as the Standard Model predicts, keeping the door closed on many theories that suggested otherwise.

Technical Summary: Search for Top Quark Flavor-Changing Neutral Currents in Multilepton Final States

Problem and Motivation
In the Standard Model (SM), flavor-changing neutral current (FCNC) processes involving the top quark are highly suppressed by the Glashow–Iliopoulos–Maiani mechanism, with predicted branching fractions at the level of $O(10^{-12})$ or smaller. However, various extensions of the SM, such as R-parity violating supersymmetry and models with extended Higgs sectors, predict enhanced FCNC rates that could be observable at the Large Hadron Collider (LHC). While previous searches by the ATLAS and CMS collaborations have probed top quark FCNC interactions mediated by specific particles (Higgs, gluon, photon, or Z boson) individually, these studies typically neglect correlations or interference effects between different mediators and do not account for FCNCs arising directly from heavy Beyond-the-Standard-Model (BSM) states. This analysis addresses these limitations by performing a global search within the framework of the Standard Model Effective Field Theory (SMEFT), simultaneously constraining all relevant operators.

Methodology
The analysis utilizes proton–proton collision data collected by the CMS detector at $\sqrt{s} = 13$ TeV, corresponding to an integrated luminosity of 138 fb $^{-1}$ . The search targets multilepton final states containing either two same-sign charged leptons ( $2\ell_{ss}$ ) or three leptons ( $3\ell$ ), requiring exactly one $b$ -tagged jet.

Theoretical Framework: The study employs the SMEFT formalism, extending the SM Lagrangian with dimension-six operators. It focuses on a reduced set of 24 Wilson coefficients (WCs) that directly mediate FCNC interactions in the top quark sector ($tu$ and $tc$ transitions). These include operators involving two quarks and a boson (Higgs, W, Z, photon, gluon) and operators involving two quarks and two leptons. To reduce free parameters, the analysis assumes equal coupling strengths for left- and right-handed top quark fields and a common WC for the three charged-lepton flavors in two-quark–two-lepton operators.
Event Selection and Categorization: Events are categorized based on lepton multiplicity and invariant mass. The $3\ell$ channel is further subdivided into "on-Z" (containing an opposite-sign same-flavor lepton pair with invariant mass near $m_Z$ ) and "off-Z" (invariant mass far from $m_Z$ ) categories to distinguish between Z-mediated processes and two-quark–two-lepton operators. Signal regions (SR) are defined by exactly one $b$ -jet, while control regions (CR) with more than one $b$ -jet are used to constrain backgrounds.
Signal Extraction: Multivariate discriminants are constructed using Boosted Decision Trees (BDT). Input features include kinematic variables of leptons, jets, and $b$ -jets, as well as reconstructed top quark and boson (Higgs or Z) kinematics derived via the "jigsaw" reconstruction algorithm. The BDTs are trained to separate FCNC signals from irreducible SM backgrounds (e.g., $t\bar{t}W$ , $t\bar{t}Z$ , diboson) and reducible backgrounds (non-prompt leptons, charge misidentification, photon conversions).
Statistical Analysis: A simultaneous binned maximum likelihood fit is performed across all categories and data-taking periods. The expected signal yield is parametrized as a quadratic function of the 24 WCs. Systematic uncertainties, including experimental effects (luminosity, lepton/jet efficiencies) and theoretical uncertainties (PDFs, scale variations, background cross-sections), are treated as nuisance parameters. Confidence intervals are derived using profile likelihood scans.

Key Contributions

Global SMEFT Fit: This work presents the first global search for top quark FCNCs within the SMEFT framework where all relevant operators are constrained simultaneously in a single fit to data.
Direct Probe of Two-Quark–Two-Lepton Operators: The analysis provides the first direct experimental constraints on FCNC processes mediated by two-quark–two-lepton operators, a class of interactions previously unprobed in this context.
Unified Framework: By treating production and decay modes simultaneously and accounting for interference and correlations, the analysis establishes a framework for future measurements and combinations at the LHC.

Results
No evidence for FCNC interactions was observed; the data are consistent with the SM prediction. The analysis sets 68% and 95% confidence intervals for all 24 Wilson coefficients.

Wilson Coefficients: Limits are placed on coefficients associated with $tu$ and $tc$ transitions, including those mediated by the Higgs ( $c_{t\phi}$ ), Z boson ( $c_{tZ}$ , $c_{\phi Q}$ , $c_{\phi t}$ ), photon ( $c_{tA}$ ), gluon ( $c_{tG}$ ), and four-fermion operators ( $c_{t\ell}$ , $c_{te}$ , etc.).
Branching Fractions: The limits on Wilson coefficients are translated into constraints on top quark FCNC decay branching fractions. The results place the most stringent limits to date on FCNC top quark decay branching fractions via two-quark–two-lepton interactions. The upper limits on $B(t \to Hq)$ and $B(t \to Zq)$ within the EFT framework are found to be similar to or more restrictive than prior results based on anomalous couplings.

Significance
The paper claims that these results constitute the first simultaneous constraints on the complete set of relevant top quark FCNC operators in the SMEFT. By providing a global benchmark, the analysis facilitates future combinations of top quark FCNC measurements at the LHC. Furthermore, it offers the first direct experimental constraints on FCNC processes mediated by two-quark–two-lepton operators, significantly expanding the scope of the search for physics beyond the Standard Model in the top quark sector.

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