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Search for a Higgs boson produced in association with a charm quark and decaying to a W boson pair 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 paper searches for Higgs bosons produced in association with a charm quark and decaying to a W boson pair, setting an observed 95% confidence level upper limit of $|\kappa_\mathrm{c}| < 211$ on the Higgs-charm Yukawa coupling strength modifier, which improves to $|\kappa_\mathrm{c}| < 47$ when combined with previous diphoton channel results.

Original authors: CMS Collaboration

Published 2026-08-24

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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 more than a decade, the scientific community has been peering into the fundamental building blocks of the universe, guided by a theoretical framework known as the Standard Model. This model acts as a periodic table for the subatomic world, cataloging the particles that make up matter and the forces that govern their interactions. At the heart of this framework sits the Higgs boson, a particle discovered in 2012 that is responsible for giving other particles their mass. While the Higgs boson interacts strongly with heavy particles like the top quark and the bottom quark, its relationship with lighter particles remains a mystery. One of the most elusive connections is the bond between the Higgs boson and the charm quark, a lighter particle that is surprisingly difficult to detect because it is so common in nature. Understanding this specific interaction is crucial because any deviation from the predictions of the Standard Model could hint at new, undiscovered physics that lies beyond our current understanding.

A researcher working with the Compact Muon Solenoid, or CMS, detector at the Large Hadron Collider in Switzerland has now taken a significant step toward solving this puzzle. They conducted a massive search for a specific event: a Higgs boson being created alongside a charm quark. In the high-energy collisions of protons inside the collider, the Higgs boson is usually produced in association with other heavy particles, but the team focused on the rare instances where it appears with a charm quark. To find this signal, they looked for a very specific decay pattern. When the Higgs boson is created, it almost immediately breaks apart into two W bosons, which are carriers of the weak nuclear force. These W bosons then decay further, with one turning into an electron and a neutrino, and the other into a muon and a neutrino. The researchers sifted through data corresponding to 138 billion billion collisions, collected between 2016 and 2018, looking for this unique signature of an electron, a muon, and a charm quark jet, all accompanied by missing energy carried away by the invisible neutrinos.

The challenge in this search was immense because the signal they were hunting for is incredibly faint, buried under a mountain of background noise from other common particle interactions. To separate the rare signal from the noise, they employed sophisticated computer algorithms trained to recognize the subtle differences between the desired events and the overwhelming background. They categorized the data based on how many charm quark jets were detected and used statistical methods to compare what they observed against what the Standard Model predicted. The analysis revealed that the number of events they found was consistent with the background noise alone; they did not find a clear excess of events that would indicate the presence of the Higgs boson produced with a charm quark.

Because no clear signal was found, the researcher set strict upper limits on how often this event could be happening. They determined that if this process occurs, it happens no more than about 1,065 times more frequently than the Standard Model predicts, with a 95 percent confidence level. When they combined these results with a previous search for the same phenomenon but in a different decay channel involving two photons, the constraints became even tighter. The combined analysis suggests that the strength of the interaction between the Higgs boson and the charm quark is less than 47 times the strength predicted by the Standard Model. While this does not yet prove the existence of new physics, it significantly narrows the window for where such new physics might hide. The study demonstrates that even with the most advanced detectors and analysis techniques, the charm quark remains a stubbornly difficult partner for the Higgs boson to reveal, requiring even larger datasets and more refined methods in the future to finally pin down the exact nature of their connection.

Technical Summary: Search for a Higgs boson produced in association with a charm quark decaying to a W boson pair

Problem and Motivation
Since the discovery of the Higgs boson, precise measurements of its couplings to Standard Model (SM) particles have become a primary objective. While couplings to third-generation fermions and bosons are well-established, direct constraints on the Yukawa coupling to the charm quark ( $\kappa_c$ ) remain challenging. Previous direct searches for $H \to c\bar{c}$ decays have improved significantly with advanced charm tagging techniques, yet the coupling to the charm quark is also accessible via the associated production of a Higgs boson with a charm quark ( $cH$ ). This channel probes $\kappa_c$ through the radiation of a Higgs boson from a charm quark in the initial state. Although the cross-section for $cH$ production is small ( $\sim 0.2\%$ of the total Higgs production rate), it offers a distinct event signature compared to $H \to c\bar{c}$ decays. This paper presents a search for $cH$ production where the Higgs boson decays to a pair of $W$ bosons ( $H \to WW$ ), with one $W$ decaying to an electron-neutrino pair and the other to a muon-neutrino pair ( $e\nu\mu\nu$ ). This specific final state is chosen to suppress the dominant $Z$ +jets background, which predominantly affects same-flavor dilepton channels.

Methodology
The analysis utilizes proton-proton collision data collected by the CMS detector at $\sqrt{s} = 13$ TeV during the 2016, 2017, and 2018 data-taking periods, corresponding to an integrated luminosity of 138 fb $^{-1}$ .

Event Selection: Events are selected requiring exactly one isolated electron and one isolated muon with opposite charges. Kinematic requirements include leading lepton $p_T > 25$ GeV, subleading lepton $p_T > 15$ GeV, missing transverse momentum ( $p_T^{miss}$ ) $> 20$ GeV, and a transverse mass of the subleading lepton $m_{T}^{\ell_2} > 30$ GeV.
Charm Jet Identification: Jets are reconstructed using the anti- $k_T$ algorithm. Charm jets ( $c$ -jets) are identified using the DEEPJET algorithm, a deep neural network-based tagger. Events must contain at least one jet satisfying $p_T > 20$ GeV, $|\eta| < 2.4$ , and specific discriminant thresholds ($CvsL > 0.12$ and $CvsB > 0.5$) to ensure charm tagging efficiency while suppressing light-flavor and $b$ -jet mistags.
Signal Regions (SRs) and Control Regions (CRs): Events are categorized into two SRs based on the number of identified charm jets ( $N_{c-j} = 1$ and $N_{c-j} > 1$ ) and two CRs (high-mass dilepton and top-enriched) to constrain background normalizations. The dominant background is $t\bar{t}$ production, contributing $\sim 90\%$ of the total background.
Multivariate Analysis: Two Boosted Decision Tree (BDT) classifiers are employed:
1. $D_{bkg}$ : Distinguishes the $cH$ signal from non-Higgs backgrounds (e.g., $t\bar{t}$ , single top, diboson, $V$ +jets).
2. $D_{H-bkg}$ : Separates the $cH$ signal from other Higgs boson production processes (e.g., $ggH$, $VBF$ , $VH$ , $t\bar{t}H$ , and $bH$ ).
  The output of these classifiers is combined using a $k$ -means clustering algorithm to create a 1D binned distribution for the final statistical fit.
Simulation and Background Modeling: Signal ( $cH$ ) and background processes are simulated using various generators (MADGRAPH5_aMC@NLO, POWHEG, SHERPA) with PYTHIA 8 for parton showering and hadronization. Theoretical uncertainties regarding heavy-flavor schemes (4-flavor vs. 3-flavor for $cH$ ) and heavy-flavor modeling in $H$ +jets are treated as significant shape uncertainties.

Key Contributions and Results
The analysis performs a simultaneous binned maximum likelihood fit across all signal and control regions. Two scenarios are evaluated:

1POI Fit: The normalization of the $H$ +charm background ( $H+c$ -bkg) is fixed to the SM prediction.
2POI Fit: The normalization of the $H+c$ -bkg is allowed to float, testing for potential correlations between the signal and the background component.

Results:

Signal Strength ( $\mu_{cH}$ ): In the 1POI scenario, the observed (expected) upper limit on the signal strength relative to the SM is 1065 (506). The measured signal strength is $\mu_{cH} = 390^{+380}_{-270}$ , consistent with the SM within 1.5 standard deviations. The sensitivity is dominated by the $N_{c-j} = 1$ category.
Coupling Modifier ( $\kappa_c$ ): Under the flat direction assumption (where other Higgs couplings adjust to keep other signal strengths at unity), the observed (expected) constraint on the charm Yukawa coupling modifier is $|\kappa_c| < 211$ (95) at 95% confidence level (CL).
Combined Analysis: When combined with the previous CMS search for $cH$ in the diphoton decay channel, the limits improve. The combined observed (expected) upper limit on $\mu_{cH}$ is 257 (281), translating to constraints on the coupling modifier of $|\kappa_c| < 47$ (51) at 95% CL.

Significance
This paper presents the first search for $cH$ production with the Higgs boson decaying to $WW$ . While the current limits are not yet competitive with direct $H \to c\bar{c}$ searches or indirect constraints, this channel provides an alternative and complementary probe of the Higgs-charm Yukawa coupling. The analysis demonstrates the capability to isolate $cH$ production in a complex final state dominated by $t\bar{t}$ background using advanced machine learning techniques and specific kinematic selections. The paper notes that the sensitivity is currently limited by statistical uncertainties in the data and significant theoretical uncertainties related to heavy-flavor modeling. Future improvements are anticipated with larger datasets from the High-Luminosity LHC and advances in heavy-flavor jet identification algorithms.

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