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Search for top quark decays $t\rightarrow qH$ with $H\to\gamma\gamma$ using the ATLAS detector

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ATLAS Collaboration

High Energy Physics - Experiment · Submitted 25 Mar 2014 · 79 references

Pith's verdict: A competent, honestly reported ATLAS null result: B(t→qH) < 0.79% at 95% CL, first diphoton-channel FCNC top search with the full 7+8 TeV dataset, with the usual statistics-limited caveats.

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arXiv ID1403.6293 (version 1)
Pith IDTTRO6OKJ, Pith's permanent record of this paper and its review
CitationATLAS Collaboration. "Search for top quark decays $t\rightarrow qH$ with $H\to\gamma\gamma$ using the ATLAS detector." arXiv:1403.6293 (2014). https://pith.science/paper/TTRO6OKJ
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The paper

The paper claims

ATLAS sets a 95% confidence-level upper limit of 0.79% on the flavour-changing decay of a top quark into a Higgs boson and a light quark, using the Higgs-to-two-photons channel in 25 fb⁻¹ of LHC data.

The reading

In the Standard Model a top quark essentially never turns into a lighter up-type quark plus a Higgs boson: the $t$-$q$-$H$ coupling is loop-suppressed by the GIM mechanism, and the predicted branching ratio for $t$-$qH$ sits near $3$ in $10^{15}$. Many extensions of the Standard Model, especially two-Higgs-doublet models without explicit flavour conservation, put that coupling at tree level and raise the branching ratio by many orders of magnitude, so a search reaching the $10^{-3}$ level tests a broad class of new-physics scenarios at once. This paper looks for the decay inside top-quark pairs, where the second top decays to $bW$, and uses the clean $H$ to two photons signature in both the all-hadronic and the single-lepton top decays. No significant excess appears, and the result is turned into a 95% confidence-level bound of 0.79% on the branching ratio and 0.17 on the combined coupling $

What carries the argument

The load-bearing device is a sideband fit to the diphoton invariant mass $m_{

Extended reading notes

Core claim

The collaboration reports a null result and converts it into an upper limit. A likelihood fit to the diphoton mass spectrum, combining the hadronic 7 TeV and 8 TeV selections and the leptonic 8 TeV selection, gives a fitted branching ratio $B = 0.22^{+0.31}_{-0.26}$ percent, corresponding to $3.1^{+4.3}_{-3.7}$ signal events; the probability that the background alone fluctuates to at least the observed excess is 18%. The observed 95% confidence-level upper limit on $B(t

Load-bearing premise

The shape of the hadronic background is taken from a smoothed simulation of diphoton-plus-jets rather than from the data, because the sidebands contain only 38 events, so the extracted signal yield and the resulting 0.79% limit depend on that simulated shape being correct.

Editorial extensions

If this is right

  • The combined coupling bound $\sqrt{\lambda_{tcH}^{2}+\lambda_{tuH}^{2}} < 0.17$ (expected 0.14) becomes the reference number against which future $t\to qH$ searches, and the tree-level two-Higgs-doublet scenarios that predict visible $t\to cH$ rates, must be compared.
  • Because the analysis deliberately does not separate $t\to cH$ from $t\to uH$, the limit applies to the sum in quadrature of the two couplings; a future measurement favouring one flavour would need an analysis that keeps the two channels apart.
  • The same event topology and diphoton machinery transfers directly to other rare top decays with a Higgs or a $Z$ in the final state, so the selection and background model are reusable for a broader flavour-changing-neutral-current programme.
  • The result is a constraint rather than an exclusion: the largest branching ratio quoted for any of the models discussed, about 0.15% in a type-III two-Higgs-doublet model, lies below the 0.79% bound, so that scenario survives and would need roughly a fivefold improvement in sensitivity to be reached.

Reading between the lines

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

  • The natural next step is to combine this diphoton channel with the $t\to qZ$ and $t\to q\gamma$ channels in a single fit, since all three share the same underlying coupling structure and a joint limit would be tighter than any one channel alone.
  • A data-driven background template built from a much larger diphoton sample would be the cheapest way to harden the hadronic limit, because the current template is a smoothed simulation and the sidebands hold only 38 events.
  • If a future excess appears near 125.5 GeV in this channel, the shape of the diphoton peak and the reconstructed top mass windows together give a handle for separating a real $t\to qH$ signal from a standard-model Higgs produced in association with top quarks, which this analysis already treats as a resonant background.
Original abstract

A search is performed for flavour-changing neutral currents in the decay of a top quark to an up-type ($c, u$) quark and a Higgs boson, where the Higgs boson decays to two photons. The proton-proton collision data set used corresponds to 4.7 fb$^{-1}$ at $\sqrt{s}$ = 7 \tev\ and 20.3 fb$^{-1}$ at $\sqrt{s}$ = 8 \tev\ collected by the ATLAS experiment at the LHC. Top quark pair events are searched for in which one top quark decays to $qH$ and the other decays to $bW$. Both the hadronic and the leptonic decay modes of the $W$ boson are used. No significant signal is observed and an upper limit is set on the $t\rightarrow qH$ branching ratio of 0.79% at the 95% confidence level. The corresponding limit on the $tqH$ coupling combination $\sqrt{\lambda_{tcH}^{2} + \lambda_{tuH}^{2}}$ is 0.17.

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, 5,127 input tokens, $0.0002 reported.

What the abstract itself states, as probabilities: claims first 0.03 · states limitation 0.56 · human participants 0.01 · controlled comparison 0.07 · randomized assignment 0.01 · states quantitative result 0.99 · compares with prior results 0.05 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

A search is performed for flavour-changing neutral currents in the decay of a top quark to an up-type ($c, u$) quark and a Higgs boson, where the Higgs boson decays to two photons. The proton-proton collision data set used corresponds to 4.7 fb$^{-1}$ at $\sqrt{s}$ = 7 \tev\ and 20.3 fb$^{-1}$ at $\sqrt{s}$ = 8 \tev\ collected by the ATLAS experiment at the LHC. Top quark pair events are searched for in which one top quark decays to $qH$ and the other decays to $bW$. Both the hadronic and the leptonic decay modes of the $W$ boson are used. No significant signal is observed and an upper limit is set on the $t\rightarrow qH$ branching ratio of 0.79% at the 95% confidence level. The corresponding limit on the $tqH$ coupling combination $\sqrt{\lambda_{tcH}^{2} + \lambda_{tuH}^{2}}$ is 0.17.

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 A search is performed for flavour-changing neutral currents in the decay of a top quark to an up-type ($c, u$) quark and a Higgs boson, where the Higgs boson decays to two photons. contribution 1.00 0.85 0.03 asserted 0.81
s1 The proton-proton collision data set used corresponds to 4.7 fb$^{-1}$ at $\sqrt{s}$ = 7 \tev\ and 20.3 fb$^{-1}$ at $\sqrt{s}$ = 8 \tev\ collected by the ATLAS experiment at the LHC. method 1.00 0.65 0.02 asserted 0.99
s2 Top quark pair events are searched for in which one top quark decays to $qH$ and the other decays to $bW$. method 0.83 0.86 0.02 asserted 0.90
s3 Both the hadronic and the leptonic decay modes of the $W$ boson are used. method 1.00 0.88 0.01 asserted 1.00
s4 No significant signal is observed and an upper limit is set on the $t\rightarrow qH$ branching ratio of 0.79% at the 95% confidence level. result 1.00 0.97 0.94 asserted 1.00
s5 The corresponding limit on the $tqH$ coupling combination $\sqrt{\lambda_{tcH}^{2} + \lambda_{tuH}^{2}}$ is 0.17. result 0.94 0.95 0.15 asserted 1.00

What kind of paper the classifier reports this is: empirical study 1.00 · evidence from observation or experiment 0.54.

What was examined, and what was not

sources read
  • arXiv listing abstract v1
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. (a) shows the distribution of m1 (four entries per event) for all selected events before mass cuts in the √ s = 8 TeV data set. [GeV] γγ j m 0 50 100 150 200 250 300 350 400 450 500 Entries/10 GeV 0 10 20 30 40 50 60 70 80 90 100 ATLAS -1 L dt = 20.3 fb ∫ s = 8 TeV, Hadronic Selection (a) Data SHERPA γγ j, norm. to data Signal, B = 5% [GeV] jjj m 0 50 100 150 200 250 300 350 400 450 500 Entries/10 GeV 0 2 4 6 8 10 12 14 16 18 20 22 ATLAS -1 L dt = 20.3 fb ∫ s = 8 TeV, Hadronic Selection < 191 GeV γγ j 156 < m (b) Data SHERPA γγ j, norm. to data Signal, B = 5%
Figure 2. Distribution of the invariant mass of the two photons, mγγ, for events passing the full hadronic selection (see text for details). The Sγγj background sample is normalised to data. [GeV] miss T E 0 20 40 60 80 100 120 140 160 180 200 Events/10 GeV 0 10 20 30 40 50 60 70 ATLAS -1 L dt = 20.3 fb ∫ s = 8 TeV, (a) Data t & W(t γ ), normalised to luminosity SHERPA γγ j → l, norm. to data - (t & W(t γ )) Signal, normalised to data [GeV] T m 0 20 40 60 80 100 120 140 160 180 200 Events/10 GeV 0 10 20 30 40 50 60 ATLAS -1 L dt = 20.3 fb ∫ s = 8 TeV, (b) Data t & W(t γ ), normalised to…
Figure 3. Distributions of (a) the missing transverse energy Emiss T and (b) the transverse mass mT of the W candidates for events with two high pT photons and one lepton. The tt&W(γ) and Sγγj→ℓ background samples are defined in the text. The two leading jets are considered. However, as for the hadronic selection, some priority is given to b-tagged jets: if the jet ranked second is not b-tagged and if there is a b-tagged third jet that passes all other requirements, the second jet is replaced by the third. The replacement procedure is repeated in case there is a 4th b-tagged jet and the second
Figure 4. (a) shows the invariant mass distribution of the Top1 combinations (two entries per event) for all selected events before the mass selections, and without the b-tagging re￾quirement. In the signal sample, normalised to the expectation for a 5% t → cH branching ratio, the narrow peak associated with the top quark is clearly visible, as well as a tail at higher masses corresponding to the wrong combination of final state objects. The Sγγj→ℓ sample, together with the tt and W(γ) contributions give a reasonable description of the data. The interval ∆m1 chosen for the m1 selection is…
Figure 5. Distribution of mγγ for the selected events in the hadronic channel. The result of a fit to the data of the sum of a signal component with the mass of the Higgs boson fixed to mH = 125.5 GeV and a background component (dashed) described by a second-order polynomial is superimposed. The small contribution from SM Higgs boson production, included in the fit, is also shown (difference between the dotted and dashed lines). The diphoton mass spectrum in the hadronic channel is shown in
Figure 6. Evolution of CLs as a function of the branching fraction B of the t → qH decay for the observation of a signal at 125.5 GeV (solid line) and the expectation in the absence of signal (dashed line). The 1 and 2 σ uncertainty bands around the expected curve are also shown. 6 Conclusions The FCNC t → qH decay, followed by H → γγ, has been searched for in a data set of proton-proton collisions recorded by the ATLAS experiment, consisting of 4.7 fb−1 at √ s = 7 TeV and 20.3 fb−1 at √ s = 8 TeV. Candidate events were selected by requiring the presence of two high-ET isolated pho￾tons. Furth

Figures reproduced from arXiv:1403.6293 by the authors.

References
How the literature uses it

39 arXiv papers cite this in 48 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 · 21 sentences without a confident label

Uses its method or tool 1 paper
Uses its data or values 15 papers
  • Currently, the strongest experimental limits on the off-diagonal elements of $y_u$ come from ATLAS [ref] and impose the 95% CL constraints [equation] on FCNC top quark decays, which translates into [equation] Here, we have used the leading order expression for the branching ratio [ref] , supplemented by a correction factor $η_QCD ≃1+0.97α_s=1.10$ [ref] accounting for NLO QCD contributions: [equation] Note that a refined analysis taking into account also the process $p p →t h$, which is relevant in the case of $tuh$ couplings (but not for $tch$ couplings) could improve the bounds on $(t →uh)$ and $√(|y^ut|^2 + |y^tu|^2)$ by about a factor 1.5 [ref] .

    New Signatures of Flavor Violating Higgs Couplings · Effective Field Theory Framework for Flavor Violating Higgs Couplings · 0.97

  • Both ATLAS using $h →γγ$ channel (7-TeV and 8-TeV) [ref] and CMS through inclusive multi-lepton channels (8-TeV with 19.5 fb$^-1$) [ref] searched for $t→ch$ and set the upper bounds on the branching ratio of 0.79% and 1.3%, respectively, which correspond to $√(|λ_tc|^2 + |λ_ct|^2) < 0.17$ and $< 0.21$, where $λ_tc$ and $λ_ct$ are the flavor-changing Yukawa coupling appearing in the interaction Lagrangian, $- (λ_tc t̅_R c_L + λ_ct c̅_R t_L)h +$ h.c.

    Flavor-Changing Neutral-Current Decays in Top-Specific Variant Axion Model · Top quark FCNC interactions · 0.99

  • The most recent limits are $(t→h c) < 0.56 %$ from CMS [ref] and $(t→h c) < 0.79 %$ from ATLAS [ref] and are shown in the plot as a red band.

    Flavor from the Electroweak Scale · Flavor Violating Top Decays · 0.89

Compares with it 2 papers
Cites it as background 7 papers

Discussion

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