⚛️ phenomenology

Searching for charged Higgs bosons in top decays via the $t^*b$ channel

This paper presents a model-independent search for light charged Higgs bosons (110–165 GeV) produced via rare top decays ( $t \to H^\pm b$ ) followed by the off-shell decay $H^\pm \to t^*b$ , establishing new constraints on this previously unconstrained channel by reinterpreting ATLAS dileptonic measurements and demonstrating its competitive sensitivity in specific two-Higgs-doublet scenarios near the top-quark mass threshold.

Original authors: Saiyad Ashanujjaman, Guglielmo Coloretti, Andreas Crivellin, Benjamin Fuks, Yeonsu Heo

Published 2026-07-08

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CC BY 4.0

Original authors: Saiyad Ashanujjaman, Guglielmo Coloretti, Andreas Crivellin, Benjamin Fuks, Yeonsu Heo

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

Imagine the universe is a giant, high-speed particle collider, like a massive cosmic racetrack where protons smash into each other at nearly the speed of light. Inside these collisions, the heaviest known particle, the top quark, is born. Usually, this top quark is a "good citizen" of the Standard Model (our current rulebook for physics); it decays in a very predictable way, breaking apart into a W boson and a bottom quark.

But what if the top quark is actually a "rebel"? What if, instead of following the rules, it occasionally takes a secret detour to produce a charged Higgs boson—a new, undiscovered particle that physicists have been hunting for decades?

The Mystery of the "Ghost" Particle

For a long time, scientists have been looking for this charged Higgs boson. They've checked for it in two main ways:

The Heavy Hunt: Looking for heavy Higgs bosons that decay into a real, on-shell top quark.
The Light Hunt: Looking for light Higgs bosons that decay into things like tau particles or charm quarks.

However, there was a blind spot. If the charged Higgs is lighter than the top quark but heavier than a charm quark, it can't decay into a real top quark. Instead, it has to decay into a "ghost" top quark (an off-shell top, written as $t^*$ ). This ghost top is so short-lived and unstable that it immediately falls apart into a W boson and a bottom quark.

Because this "ghost" top is so fleeting, existing searches missed it. It's like trying to find a specific type of bird that only appears for a split second inside a storm; if you aren't looking for that specific split-second behavior, you won't see it.

The New Strategy: The "Double-Decker" Bus

The authors of this paper realized that when a top quark decays into this charged Higgs, and the Higgs then decays into the "ghost" top, the whole event leaves a very specific fingerprint.

Think of a standard top-antitop collision as a double-decker bus dropping off two passengers (a W boson and a bottom quark) at a stop.
Now, imagine a "rebel" event where:

One top quark decays normally (dropping off its passengers).
The other top quark takes the secret detour: it drops off a charged Higgs.
That charged Higgs immediately drops off a "ghost" top.
The "ghost" top immediately drops off its own passengers (a W boson and a bottom quark).

The result? Instead of a simple double-decker bus, you have a chaotic convoy with four bottom quarks (b-jets) and two W bosons. It looks like a standard top-antitop event, but with two extra bottom quarks tagging along.

The Detective Work

The researchers didn't build a new machine; they acted like digital detectives. They went back to data already collected by the ATLAS experiment at the Large Hadron Collider (LHC). ATLAS had already been measuring events with extra bottom quarks to test the Standard Model.

The team asked: "Could some of these 'extra' bottom quarks actually be the signature of our rebel top quarks and ghost Higgs bosons?"

They used a clever statistical method (called an R-fit) to account for the fact that their computer simulations of the "normal" background noise aren't perfect. They essentially said, "Even if our background prediction is off by 30%, could the data still fit our new theory?"

The Findings

Here is what they found:

The Search: They scanned for charged Higgs bosons with masses between 110 and 165 GeV.
The Limit: They found no evidence of this new particle. However, they set a very strict rule: If this particle does exist, the chance of a top quark turning into it (and then the Higgs turning into a ghost top) cannot be higher than 1.9% to 2.9%.
The Sweet Spot: The search was most sensitive when the Higgs mass was around 130–150 GeV. If the Higgs was too light, the "ghost" particles were too weak to be seen. If it was too heavy (close to the top quark's mass), there wasn't enough room for the decay to happen.

Why This Matters

The paper highlights that this new way of looking at the data is crucial for specific types of physics theories:

The "Top-Philic" Scenario: In some theories, the charged Higgs only talks to the top quark and ignores everyone else. In these cases, the old searches (looking for tau particles or charm quarks) are useless because the Higgs doesn't decay into those things. This new method is the only way to find it.
The "Low-Tan Beta" Corner: In other theories, there is a specific region of parameters (low "tan beta") where the old searches fail. This new method covers that blind spot.

The Bottom Line

This paper doesn't claim to have found the charged Higgs boson. Instead, it successfully closed a gap in the net. It showed that by re-examining old data with a fresh perspective—looking for the specific "chaotic convoy" of four bottom quarks—scientists can now rule out certain possibilities for how this new particle might behave.

The authors conclude that while this reinterpretation is a powerful tool, the LHC collaborations (ATLAS and CMS) should build dedicated searches specifically for this "ghost top" channel. With more data and better analysis tools, they could either find this elusive particle or rule it out even more definitively.

Technical Summary: Searching for Charged Higgs Bosons in Top Decays via the $t^*b$ Channel

Problem Statement
While extensive searches for light charged Higgs bosons ( $H^\pm$ ) with masses below the top quark mass ( $m_{H^\pm} < m_t$ ) have been conducted at the LHC, they primarily target conventional decay modes: $H^\pm \to \tau\nu$ , $cs$, and $cb$. The decay channel $H^\pm \to tb$ has historically been restricted to heavy charged Higgs scenarios where the top quark is on-shell. Consequently, the low-mass, off-shell channel $H^\pm \to t^*b$ remains largely unconstrained. This is a phenomenological gap because, below the top-quark threshold, the large top Yukawa coupling can make $H^\pm \to t^*b$ the dominant decay mode in specific models (e.g., top-philic or top-specific scenarios). Existing searches for standard decays do not cover this regime, leaving a potential signature of non-minimal scalar sectors unexplored.

Methodology
The authors propose a search strategy based on the rare top-quark decay $t \to H^\pm b$ , followed by the off-shell decay $H^\pm \to t^*b \to W^\pm b \bar{b}$ . In a $t\bar{t}$ event, if one top decays via this chain while the other decays via the Standard Model (SM) mode $t \to Wb$ , the final state resembles $t\bar{t}b\bar{b}$ (specifically $W^\mp b H^\pm b \to W^\mp b W^\pm b \bar{b}$ ).

The study employs the following methodology:

Signal Modeling: The signal process is simulated using MadGraph5_aMC@NLO at Leading Order (LO) with 2HDM UFO libraries. The charged Higgs mass is scanned between 100 and 165 GeV. The other top quark decays via the SM.
Reinterpretation of Data: The authors reinterpret recent ATLAS fiducial measurements of dileptonic $t\bar{t}$ production in association with additional $b$ -jets (based on 140 fb $^{-1}$ of 13 TeV data).
Validation: SM $t\bar{t}$ and $t\bar{t}b\bar{b}$ events are generated at Next-to-Leading Order (NLO) in QCD and interfaced with Pythia 8 for parton showering. The simulation is validated against ATLAS data and Powheg+Pythia8 predictions across four fiducial signal regions defined by jet multiplicities: $\ge 3b$ , $\ge 3b \ge 1c/j$ , $\ge 4b$ , and $\ge 4b \ge 1c/j$ .
Statistical Analysis: Model-independent limits are derived by comparing measured fiducial cross-sections with SM predictions supplemented by the charged Higgs signal. The analysis neglects interference between SM and BSM amplitudes, justified by the distinct resonant structures and kinematic mismatches (off-shell vs. on-shell top systems). Theoretical uncertainties on the SM $t\bar{t}b\bar{b}$ prediction (approx. 30%) are treated using the R-fit prescription, allowing the SM prediction to vary within its uncertainty band to maximize agreement with data.

Key Contributions

Identification of a New Channel: The paper establishes the $t^*b$ channel as a viable and previously unconstrained probe for light charged Higgs bosons, particularly in models where conventional decays are suppressed.
Model-Independent Limits: The authors derive the first direct constraints on the product of branching ratios $\text{Br}(t \to H^\pm b) \times \text{Br}(H^\pm \to t^*b)$ in the mass range 110–165 GeV.
Model-Dependent Interpretations: The limits are interpreted in four specific 2HDM scenarios:
- Top-philic: Where $H^\pm$ couples only to the top quark.
- Up-type Aligned: Where $H^\pm$ couples to all up-type quarks.
- Top-specific: A hybrid scenario with specific coupling patterns.
- Canonical $Z_2$ -symmetric (Type-I, X, Y): Standard benchmark models.

Results

Branching Ratio Limits: For charged Higgs masses between 110 and 165 GeV, the analysis sets 95% Confidence Level (CL) upper limits on $\text{Br}(t \to H^\pm b) \times \text{Br}(H^\pm \to t^*b)$ ranging from 1.9% to 2.9%. The sensitivity peaks around $m_{H^\pm} \simeq 130\text{--}150$ GeV, where the off-shell top decay products are energetic enough to satisfy jet multiplicity requirements without being phase-space suppressed.
Model Constraints:
- Top-philic 2HDM: The reinterpretation excludes values of the top Yukawa coupling $|\xi_t| \gtrsim 0.27$ in the most sensitive mass range. Since conventional searches for $H^\pm \to cs, cb, \tau\nu$ do not apply here, this provides the primary direct constraint.
- Up-type Aligned 2HDM: The $t\bar{t}b\bar{b}$ reinterpretation becomes competitive with dedicated $H^\pm \to cs$ searches near the top-quark threshold ( $m_{H^\pm} \approx 160$ GeV), where the $t^*b$ mode dominates over the $cs$ mode.
- Top-specific 2HDM: The analysis excludes a low- $\tan\beta$ region near the upper mass limit that is not covered by conventional $H^\pm \to \tau\nu$ searches, which lose sensitivity at low $\tan\beta$ due to suppressed leptonic branching fractions.
- Canonical Models: In Type-I, X, and Y models, the $t\bar{t}b\bar{b}$ channel offers no advantage over existing dedicated searches for $cs$, $cb$, and $\tau\nu$ modes, as those channels remain dominant and are already tightly constrained.

Significance and Claims
The paper claims that $t\bar{t}$ production with additional $b$ -jets serves as a complementary probe for light charged Higgs bosons and non-standard top decays. The authors emphasize that while dedicated searches dominate in canonical models, the $t^*b$ reinterpretation is crucial for "top-philic" and "top-specific" scenarios where standard decay modes are absent or suppressed.

The work demonstrates that recasting existing SM measurements can yield meaningful sensitivity to non-standard top decays without requiring new data collection. The authors conclude that these results motivate dedicated analyses by LHC collaborations (ATLAS and CMS) to specifically target the $t \to H^\pm b$ with $H^\pm \to t^*b$ channel. They suggest that such analyses, utilizing differential distributions and optimized multivariate discriminants, could significantly improve upon current reinterpretation limits, especially with the increased statistics of LHC Run 3 and the High-Luminosity phase.