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Search for charged Higgs bosons decaying into top and bottom quarks in lepton+jets final states in proton-proton collisions at $\sqrt{s}$ = 13 TeV

Using 138 fb $^{-1}$ of 13 TeV proton-proton collision data collected by the CMS detector, this study presents the first search for charged Higgs bosons decaying into top and bottom quarks within the generalized two-Higgs-doublet model (g2HDM), observing a 2.4 $\sigma$ local excess at 600 GeV and setting 95% confidence level limits on the model's Yukawa couplings $\rho_{\mathrm{tt}}$ and $\rho_{\mathrm{tc}}$ .

Original authors: CMS Collaboration

Published 2026-10-14

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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

For decades, the most successful theory of how the universe works has been the Standard Model. It is a precise map of the fundamental particles that make up everything we see and the forces that push and pull them. In 2012, scientists at the Large Hadron Collider in Switzerland confirmed the final piece of this map: a particle called the Higgs boson. This discovery was a triumph, but it left a lingering question. The Standard Model predicts that the Higgs boson comes in a single form, a solitary particle. Yet, many physicists suspect the universe is more complex, perhaps hiding a second, heavier version of this particle, or even a whole family of them. If such particles exist, they could explain mysteries the current map cannot, such as why the universe is made of matter rather than antimatter. The search for these extra particles is not just about finding something new; it is about testing whether our understanding of reality is complete or merely a sketch of a deeper truth.

A researcher using the CMS detector at the Large Hadron Collider has taken a fresh look at this question, hunting for a specific type of heavy particle known as a charged Higgs boson. Unlike the familiar Higgs boson, which has no electric charge, this hypothetical particle would carry an electric charge, much like an electron or a proton. The researcher analyzed a massive collection of data from proton-proton collisions, corresponding to an integrated luminosity of 138 fb⁻¹, recorded between 2016 and 2018. They were looking for a very specific signature: a collision that produces a single electron or muon, a burst of invisible energy, and several jets of particles containing bottom quarks. This specific combination of debris would be the smoking gun if a charged Higgs boson had been created and then decayed into a top quark and a bottom quark.

The search was guided by a theoretical framework called the generalized two-Higgs-doublet model. In simple terms, this model suggests that the Higgs boson interacts with other particles in ways that the standard theory does not allow. Specifically, it proposes that the Higgs boson can talk to the top quark and the charm quark with a strength that is not limited by the usual rules of particle physics. The researcher tested this idea by searching for charged Higgs bosons with masses ranging from 200 billion electronvolts to one trillion electronvolts. They used sophisticated computer algorithms to sift through the data, distinguishing the faint signal of a new particle from the overwhelming background noise of ordinary particle collisions.

The analysis revealed a small but intriguing hint. In the data corresponding to a charged Higgs boson with a mass of 600 billion electronvolts, the researcher observed a slight excess of events compared to what the Standard Model predicts. This excess appeared with a statistical significance of 2.4 standard deviations. In the language of particle physics, this is not enough to claim a discovery, which requires a much higher level of certainty, but it is a noticeable fluctuation that warrants further attention. It suggests that nature might be behaving slightly differently than expected in this specific mass range, though it could also be a rare statistical fluke.

Despite this hint, the study primarily serves to set strict boundaries on what is possible. The researcher used their data to rule out large regions of the theoretical landscape. They determined that if the generalized model is correct, the strength of the interaction between the Higgs boson and the top and charm quarks cannot be too large. Specifically, they excluded values for this interaction strength that are greater than 0.11 to 0.38, depending on the mass of the particle and other assumptions. For the entire mass range they tested, from 200 billion to one trillion electronvolts, they were able to rule out interaction strengths above 0.38. These limits are the first of their kind for charged Higgs bosons within this specific theoretical model, providing a crucial constraint for theorists trying to build a more complete picture of the universe.

The work represents a significant step forward in the quest to understand the Higgs sector. By focusing on a channel that was previously difficult to study, the researcher has opened a new window into the properties of potential new particles. While the slight excess at 600 billion electronvolts remains an open question, the exclusion of so many theoretical possibilities brings the scientific community closer to knowing whether the universe holds a second Higgs boson or if the Standard Model stands alone. The search continues, with the data from this analysis serving as a vital reference point for future experiments that will probe even deeper into the heart of matter.

Technical Summary: Search for Charged Higgs Bosons in the g2HDM Framework

Problem and Motivation
Following the 2012 discovery of the 125 GeV Higgs boson, which is consistent with Standard Model (SM) predictions, the existence of a second scalar doublet remains a compelling theoretical possibility. While traditional Two-Higgs-Doublet Models (2HDM) impose a $Z_2$ symmetry to suppress flavor-changing neutral currents (FCNCs), the Generalized Two-Higgs-Doublet Model (g2HDM) relaxes this constraint. In the g2HDM, additional Yukawa couplings ( $\rho_{tt}$ , $\rho_{tc}$ , $\rho_{tu}$ ) allow for unsuppressed FCNC interactions involving the top quark. Specifically, the coupling $\rho_{tc}$ can significantly enhance the production cross-section of charged Higgs bosons ( $H^\pm$ ) via the $pp \to (b)H^\pm$ process, where $H^\pm$ decays into a top and bottom quark ($tb$). Unlike the 2HDM-II, where such channels are suppressed, the g2HDM predicts dominant production in these channels, motivating a dedicated search in the single-lepton final state.

Methodology
The analysis utilizes proton-proton collision data collected by the CMS detector at a center-of-mass energy of $\sqrt{s} = 13$ TeV, corresponding to an integrated luminosity of 138 fb $^{-1}$ (2016–2018). The search targets the process $pp \to (b)H^\pm \to (b)tb$ , with the top quark decaying leptonically ( $t \to b\ell\nu$ , where $\ell = e, \mu$ ).

Event Selection: Events are required to contain exactly one "tight" isolated lepton (electron or muon), missing transverse momentum ( $p_T^{miss} > 35$ GeV), and at least two $b$ -tagged jets. Additional kinematic requirements include a scalar sum of jet transverse momenta ( $H_T > 200$ GeV) and the veto of events with additional loose leptons.
Signal Regions (SRs): To optimize sensitivity across different production modes and mass hypotheses, the analysis defines five signal regions based on the number of $b$ -tagged jets and total jets: 2b2j, 2b3j, 2b4j, 3b3j, and 3b4j.
Reconstruction: The $H^\pm$ mass ( $m_{H^\pm}$ ) is reconstructed by first inferring the neutrino momentum using the $W$ boson mass constraint, then pairing the reconstructed top candidate with a $b$ -jet to form the $H^\pm$ candidate.
Background Estimation: Dominant backgrounds include $t\bar{t}$ , single top, and $W$ +jets. Non-prompt lepton backgrounds (from heavy-flavor decays or misidentified hadrons) are estimated using a "tight-to-loose" data-driven method. Other backgrounds are modeled using Monte Carlo (MC) simulations normalized to higher-order cross-section calculations.
Signal Extraction: A parameterized Deep Neural Network (pDNN) is employed to discriminate signal from background. The pDNN is trained on kinematic variables (e.g., lepton $p_T$ , $b$ -jet angular separations, invariant masses) and is parameterized by the hypothesized $m_{H^\pm}$ to interpolate between mass points. The network is trained separately for 2b and 3b SRs and different data-taking periods.
Statistical Analysis: A simultaneous fit of the pDNN score distributions across all SRs and channels ( $e$ and $\mu$ ) is performed to extract signal strengths. Systematic uncertainties, including those from luminosity, detector performance, and theoretical modeling (PDFs, renormalization/factorization scales), are treated as nuisance parameters.

Key Contributions

First g2HDM Search in Single-Lepton Channels: This work presents the first search for charged Higgs bosons within the g2HDM framework specifically targeting single-lepton final states. Previous g2HDM searches focused on neutral Higgs bosons in multi-top final states.
Comprehensive Kinematic Modeling: The use of a pDNN allows for effective interpolation of signal hypotheses across the mass range (200 GeV to 1 TeV) and handles the complex kinematic dependencies introduced by the extra Yukawa couplings.
Interpretation of Extra Yukawa Couplings: The results are interpreted to constrain the real components of the extra Yukawa couplings $\rho_{tt}$ and $\rho_{tc}$ , assuming $\rho_{tu} = 0$ .

Results

Excess Observation: An excess of events over the SM expectation is observed for a signal mass hypothesis of $m_{H^\pm} = 600$ GeV. The local significance of this excess is 2.4 standard deviations.
Cross-Section Limits: Upper limits at 95% confidence level (CL) are set on the product of the production cross-section and branching fraction, $\sigma(pp \to (b)H^\pm) \times \mathcal{B}(H^\pm \to tb)$ .
Exclusion of Couplings: Depending on the assumed $m_{H^\pm}$ $m_{H^{\pm}}$ and $\rho_{tt}$ $ρ_{tt}$ values, the analysis excludes values of $\rho_{tc} \gtrsim 0.11–0.38$ $ρ_{t c} ≳ 0.11-0.38$ at 95% CL.
- The strongest exclusion occurs near $m_{H^\pm} = 400$ GeV, where $\rho_{tc} > 0.11$ is excluded for $\rho_{tt} = 1.0$ .
- For $\rho_{tt} = 1.0$ , values of $\rho_{tc} > 0.38$ are excluded across the entire mass range tested (200 GeV to 1 TeV).
Best Fit Values: For the observed excess at 600 GeV, the best-fit cross-section times branching fraction values are $\sigma(pp \to H^\pm)\mathcal{B} = 0.055^{+0.068}_{-0.055}$ pb and $\sigma(pp \to bH^\pm)\mathcal{B} = 0.032^{+0.035}_{-0.029}$ pb.

Significance
The paper claims that these results represent the first direct search for charged Higgs bosons within the g2HDM framework in single-lepton final states. The analysis complements existing constraints on neutral Higgs bosons in same-sign top, three-top, and four-top final states. By probing the $\rho_{tc}$ and $\rho_{tt}$ couplings up to values of unity, the study provides critical constraints on the parameter space of the g2HDM, particularly in regions where FCNC interactions could explain electroweak baryogenesis or where alignment mechanisms might hide new physics. The observed 2.4 $\sigma$ excess, while not constituting a discovery, highlights a potential deviation from the SM that warrants further investigation with increased luminosity.

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