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Probing wrong-sign Yukawa couplings at the LHC and a future linear collider

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P.M. Ferreira, John F. Gunion, Howard E. Haber, Rui Santos

High Energy Physics - Phenomenology · Submitted 19 Mar 2014 · 78 references

Pith's verdict: A careful, honest quantification of a known mechanism: the wrong-sign b Yukawa survives 10% LHC agreement but not 5%, driven by a nondecoupling charged-Higgs loop that pins kappa_gamma near 0.94.

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arXiv ID1403.4736 (version 4)
Pith IDPW44MOQ6, Pith's permanent record of this paper and its review
CitationHoward E. Haber, John F. Gunion, P.M. Ferreira, Rui Santos. "Probing wrong-sign Yukawa couplings at the LHC and a future linear collider." arXiv:1403.4736 (2014). https://pith.science/paper/PW44MOQ6
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The paper

The paper claims

The sign of the Higgs coupling to bottom quarks is not yet fixed, and the diphoton rate may be the tell.

The reading

In the Standard Model the Higgs couples to fermions and to W and Z bosons with the same sign. This paper asks whether the observed Higgs could instead couple to down-type fermions — chiefly $b$ quarks — with the opposite sign, while every other tree-level coupling stays at its Standard Model value. Working inside the type-II two-Higgs-doublet model, the authors show that such a 'wrong-sign' scenario survives all present LHC Higgs-rate measurements taken at roughly 20% precision, but that it cannot survive a future measurement of the diphoton rate at about 5% precision, nor a future $e^+e^-$ linear collider measuring the gluon-initiated width through $e^+e^-$ collisions. The reason is that the wrong-sign regime forces a charged-Higgs loop into a nondecoupling mode that persistently suppresses $h$ decay to two photons by roughly 6%, an effect that does not shrink as the charged-Higgs mass grows. If the argument holds, the sign of the bottom-quark Yukawa is a decidable question with the next generation of Higgs measurements rather than a permanent ambiguity.

What carries the argument

The load-bearing object is the $|{+1}|$ coupling in the Higgs basis, $|{+1}|$, together with the mass relation $|{+1}|$. In the ordinary decoupling limit both conspire so that $|{+1}|$ stays of order $v$ and the charged-Higgs loop is suppressed by $|{+1}|$, exactly as a heavy particle not fed by the vacuum expectation value should be. In the wrong-sign limit $|{+1}|$ and $|{+1}|$, the angular factor $|{+1}|$ grows with the heavy mass scale and cancels the $|{+1}|$ in the loop denominator, leaving a fixed contribution $|{+1}|$ that adds coherently with the top loop. That is the mechanism that converts a sign choice in the fermion sector into a fixed, observable number in the diphoton width, and it is why the wrong-sign scenario is not simply a relabelling of the Standard Model.

Extended reading notes

Core claim

The paper establishes that the wrong-sign down-type Yukawa regime of the type-II two-Higgs-doublet model — where the normalized coupling $hDD$ is close to $-1$ while $hVV$, $hUU$ and the triple-Higgs coupling remain close to their Standard Model values — is a viable and internally consistent corner of parameter space, reached when $|{+1}|$ and $|{+1}|$ with large $|{+1}|$. Two loop-level handles distinguish it from the Standard Model. First, the interference between the bottom-quark and top-quark loops in the $ggh$ coupling reverses sign, raising the leading-order ratio $|{+1}|$ to about 1.27; at next-to-next-to-leading order the production ratio drops to about 1.06, so the LHC cannot exploit it directly, but a linear collider measuring $|{+1}|$ through $|{+1}|$ can, since every allowed wrong-sign point lies above about 1.12. Second, and decisively for the LHC, the charged-Higgs contribution to the $|{+1}|$ amplitude does not decouple: the coupling combination $|{+1}|$ tends to about $-2$ in the wrong-sign limit, so the loop adds with the same sign as the top loop and against the $W$ loop, driving $|{+1}|$ down to roughly 0.94 and the diphoton rate down by at least 5% to 7%. Scanning the parameter space with all rates required to sit within 20%, 10% or 5% of unity, the authors find the wrong-sign branch survives at 20% and 10% but is empty at 5%, with $|{+1}|$ never rising above about 0.925. Tree-level unitarity caps the charged-Higgs mass at roughly 650 GeV in this branch, beyond which the scenario stops being perturbatively trustworthy.

Load-bearing premise

The exclusion statement treats a uniform per-channel tolerance of 20%, 10% or 5% applied to each Higgs rate, with all production modes summed, as a faithful stand-in for a genuine correlated global fit; the paper itself calls this a rough approximation, so the location of the 5% threshold depends on that shortcut.

Editorial extensions

If this is right

  • A high-luminosity LHC run at $\sqrt{s}=14$ TeV with $L\ge 300$ fb$^{-1}$ can either confirm or exclude the wrong-sign bottom Yukawa through the diphoton channel alone, since the predicted suppression exceeds the projected $\kappa_\gamma$ uncertainty.
  • At a linear collider, a measurement of $\mu_h^{gg}(\mathrm{ILC})$ at the 7% to 8% level expected for $\sqrt{s}=250$ to $350$ GeV would exclude every wrong-sign point surviving current LHC constraints.
  • If the bottom Yukawa is confirmed to have the wrong sign, the charged Higgs must lie below roughly 650 GeV; a heavier charged Higgs in that branch would place the theory outside the perturbative regime the calculation assumes.
  • The same nondecoupling logic identifies a region of the ordinary-sign parameter space with $v^2 g_{hH^+H^-}/m_{H^\pm}^2\lesssim -2$ that would be excluded in exactly the same way, giving the diphoton channel a second target.
  • Within the minimal supersymmetric Standard Model, the paper's appendix argues that a wrong-sign $hb\bar b$ coupling is not achievable for generic parameters, and only becomes marginally possible in extreme large-$\tan\beta$ corners with $m_A$ near 200 GeV.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The argument suggests a general lesson beyond this model: whenever a new heavy scalar's coupling to the Higgs grows with its own mass, the loop-induced Higgs rates become a discovery channel even when the new particle is too heavy to produce directly.
  • Because the effect is a fixed few-percent suppression rather than a mass-dependent one, the wrong-sign scenario is a natural benchmark for calibrating how much per-channel precision a future global fit actually needs before it can claim to have measured a coupling sign.
  • The paper's own flat-tolerance treatment implies that the real discriminating power lives in the correlations between channels; a genuine global fit including the separately measured production modes would likely tighten the 5% threshold and could shift it in either direction.
  • The $\kappa_g$ enhancement of about 1.27 at leading order but only 1.06 at next-to-next-to-leading order is a reminder that loop-induced interference effects can be badly misestimated by leading-order intuition, which matters for any future model reinterpretation of gluon-fusion rates.
Original abstract

We consider the two-Higgs-doublet model as a framework in which to evaluate the viability of scenarios in which the sign of the coupling of the observed Higgs boson to down-type fermions (in particular, $b$-quark pairs) is opposite to that of the Standard Model (SM), while at the same time all other tree-level couplings are close to the SM values. We show that, whereas such a scenario is consistent with current LHC observations, both future running at the LHC and a future $e^+ e^-$ linear collider could determine the sign of the Higgs coupling to $b$-quark pairs. Discrimination is possible for two reasons. First, the interference between the $b$-quark and the $t$-quark loop contributions to the $ggh$ coupling changes sign. Second, the charged-Higgs loop contribution to the $\gamma \gamma h$ coupling is large and fairly constant up to the largest charged-Higgs mass allowed by tree-level unitarity bounds when the $b$-quark Yukawa coupling has the opposite sign from that of the SM (the change in sign of the interference terms between the $b$-quark loop and the $W$ and $t$ loops having negligible impact).

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13-20260917 via OpenRouter, question bank abstract-p1-20260923, 2026-09-23: 1 request, 4,536 input tokens, $0.0002 reported.

What the abstract itself states, as probabilities: claims first 0.03 · states limitation 0.22 · human participants 0.01 · controlled comparison 0.40 · randomized assignment 0.01 · states quantitative result 0.15 · compares with prior results 0.44 · mentions code or data availability 0.01.

The abstract, sentence by sentence

arXiv listing abstract v4. Results and the main claim are underlined; .

We consider the two-Higgs-doublet model as a framework in which to evaluate the viability of scenarios in which the sign of the coupling of the observed Higgs boson to down-type fermions (in particular, $b$-quark pairs) is opposite to that of the Standard Model (SM), while at the same time all other tree-level couplings are close to the SM values. We show that, whereas such a scenario is consistent with current LHC observations, both future running at the LHC and a future $e^+ e^-$ linear collider could determine the sign of the Higgs coupling to $b$-quark pairs. Discrimination is possible for two reasons. First, the interference between the $b$-quark and the $t$-quark loop contributions to the $ggh$ coupling changes sign. Second, the charged-Higgs loop contribution to the $\gamma \gamma h$ coupling is large and fairly constant up to the largest charged-Higgs mass allowed by tree-level unitarity bounds when the $b$-quark Yukawa coupling has the opposite sign from that of the SM (the change in sign of the interference terms between the $b$-quark loop and the $W$ and $t$ loops having negligible impact).

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 We consider the two-Higgs-doublet model as a framework in which to evaluate the viability of scenarios in which the sign of the coupling of the observed Higgs boson to down-type fermions (in particular, $b$-quark pairs) is opposite to that of the Standard Model (SM), while at the same time all other tree-level couplings are close to the SM values. contribution 0.73 0.46 0.06 asserted 0.38
s1 We show that, whereas such a scenario is consistent with current LHC observations, both future running at the LHC and a future $e^+ e^-$ linear collider could determine the sign of the Higgs coupling to $b$-quark pairs. result 0.95 0.89 0.85 hedged possibility 0.66
s2 Discrimination is possible for two reasons. interpretation 0.48 0.39 0.03 asserted 0.52
s3 First, the interference between the $b$-quark and the $t$-quark loop contributions to the $ggh$ coupling changes sign. result 0.69 0.59 0.05 asserted 1.00
s4 Second, the charged-Higgs loop contribution to the $\gamma \gamma h$ coupling is large and fairly constant up to the largest charged-Higgs mass allowed by tree-level unitarity bounds when the $b$-quark Yukawa coupling has the opposite sign from that of the SM (the change in sign of the interference terms between the $b$-quark loop and the $W$ and $t$ loops having negligible impact). result 0.86 0.79 0.09 asserted 1.00

What kind of paper the classifier reports this is: theoretical result 0.98 · evidence from argument 0.46.

What was examined, and what was not

sources read
  • arXiv listing abstract v4
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. FIG. 1. Ratio of the lightest Higgs couplings to down quarks in the 2HDM relative to the SM as a function of tan
Figure 2. FIG. 2. Allowed regions for 2HDM type-II with all
Figure 3. FIG. 3. Left panel: The Yukawa coupling ratio
Figure 4. FIG. 4. For the 2HDM type-II, we show regions in
Figure 5. FIG. 5. Assuming that the
Figure 6. FIG. 6. Allowed regions for 2HDM type-II with all
Figure 8. FIG. 8. Left panel
Figure 9. indicate that, as in the κD < 0 scenario, the points in the case of κD > 0 with v 2 ghH+H− /m2 H± <∼ −2 will not survive if all the µ h f (LHC) are measured to be within 5% of the SM value of unity. We have already noted that in the κD < 0 scenario there will be a limitation on m2 H± coming from perturbativity and unitarity. The relevant constraints are incorporated in all of our plots. Once mH± becomes too large, the theory becomes perturbatively unreliable and insisting on tree level unitarity will then imply that only the κD > 0 possibility 22
Figure 10. FIG. 10. We show points in the
Figure 11. Note that |a +| is hitting the tree level unitarity bound of 0.5 for both the κD < 0 and κD > 0 scenarios. However, there is no limit on the associated mH± value in the latter case, whereas there is the already quoted limit of ∼ 650 GeV in the former case. We now show that for the type-II 2HDM with κD < 0, where v 2 ghH+H− /mH± ∼ −2, and with κD > 0, where v 2 ghH+H− /mH± <∼ −2 (cf
Figure 12. FIG. 12. In the left panel we plot

Showing 11 of the paper's 12 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:1403.4736 by the authors.

References
How later papers used it

81 later papers cite it

Evidence: some. 67 of the citing papers share no author with it, and 3 of those carry a confident label from Pith's citation classifier.

Pith read 25 of the 143 citing sentences, choosing only those whose citation markers point at this paper alone, and wrote the paraphrases from them. The summary, the counts, the evidence level, the links and the quotes come from code; the summary counts the papers listed below. A later paper is listed under a label when Pith's citation classifier gives one of its sentences that label at probability 0.8 or more (0.95 for support), Pith's reading finds that the citing paper itself did it, and a second check confirms the paraphrase against the sentence. This section lists uses only. It does not list criticism or disagreement, so their absence here says nothing about how the paper has been received. Papers that share an author with this one are listed apart, under "By the same authors". A quote is the citing authors' own sentence, shown when their paper's licence is CC BY, CC BY-SA or CC0. JSON

Uses its method or tool 1 paper
How the literature uses it

81 arXiv papers cite this in 144 sentences

Each citing sentence is taken from the citing paper's LaTeX source and typed by Pith's classifier. A label appears at probability 0.8 or more (0.95 for support), a contest at 0.7 or more; the quoted words are the citing authors'. How each label was checked · JSON

Papers citing it · 113 sentences without a confident label

Uses its method or tool 1 paper
Compares with it 3 papers
Cites it as support 2 papers
Cites it as background 18 papers

What it cites · 35 sentences without a confident label

Uses the method or tool of 1 paper
Uses data or values from 4 papers
Compares with 2 papers
Cites as background 7 papers

Discussion

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