REVIEW
Top-Quark Decay at Next-to-Next-to-Leading Order in QCD
High Energy Physics - Phenomenology · Submitted 10 Oct 2012 · 25 references
Pith's verdict: The differential NNLO distributions are the real new content, and they rest on a τ0 slicing whose residual is asserted rather than quantified.
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Referee report
The paper
The paper claims
The top quark's decay width and its decay-product distributions can now be computed to next-to-next-to-leading order in QCD, with electroweak, finite bottom-mass and finite W-width effects included.
The reading
What carries the argument
The load-bearing device is the SCET factorization formula for the jet invariant-mass distribution in the singular limit $tau$ to 0, $frac{1}{Gamma_t^{(0)}} frac{dGamma_t}{d tau} = H(x, mu) int dk, dm^2 J(m^2, mu) S(k, mu) delta(tau - (m^2 + 2 E_J k)/m_t^2)$, written in Laplace-transformed form as a product of the hard, jet and soft functions with the $tau$-dependent plus-distribution factor that exponentiates the large logarithms of $tau_0$. This is what turns the hardest part of the double-real radiation — the region where the jet is soft or collinear — into known NNLO ingredients, leaving only a numerically tractable remainder computed with one-loop and tree-level amplitudes and dipole subtraction. The authors note the close analogy to the $q_T$ subtraction method: both exploit the universality of infrared divergences plus knowledge of the resummed singular limit.
Extended reading notes
Core claim
The central claim is a complete NNLO QCD calculation of the top-quark decay width together with the first fully differential NNLO QCD results for the semileptonic decay $t$ to $W^+(l^+nu)b$. Numerically, at $m_t = 173.5$ GeV the LO width is 1.5109 GeV and the corrections, in percent, are $-0.26$ from finite bottom mass, $-1.49$ from finite $W$ width, $+1.69$ from NLO electroweak, $-8.58$ from NLO QCD and $-2.09$ from NNLO QCD. The NNLO piece is obtained by splitting the jet-invariant-mass variable $tau = (p_b + p_X)^2/m_t^2$ at a cutoff $tau_0$: below the cutoff the rate follows a SCET factorization formula built from the NNLO hard, jet and soft functions, above it the rate comes from NLO $t$ to $W b$ plus one jet and tree-level $t$ to $W b$ plus two jets with dipole subtraction. The three pieces individually grow to roughly twice the LO width as $tau_0$ runs from $10^{-3}$ to $10^{-6}$, while their sum stays near 2.1 percent of the LO width — the authors present this cancellation as the validation of the method. The paper also gives normalized distributions in charged-lepton energy, lepton-hardest-jet invariant mass, $cos(theta^*)$ and $cos(theta_{lj})$, and fits the $cos(theta^*)$ distribution to obtain $W$ helicity fractions $F_L : F_+ : F_- = 0.689 : 0.0017 : 0.309$.
Load-bearing premise
The claim rests on the assumption that the neglected remainder of the $tau_0$ slicing extrapolates to zero as advertised: each of the three NNLO pieces grows to roughly twice the LO width as the cutoff shrinks, so the quoted $-2.09$ percent is a cancellation of large logarithms whose residual is asserted to be of order $tau_0$ but is not given a quantitative error band.
Editorial extensions
If this is right
- The total top-quark width can be quoted with a residual renormalization-scale uncertainty near 0.8 percent rather than the NLO figure of about 1.6 percent, since the NNLO correction removes part of the running-coupling dependence.
- Because the output is fully differential, arbitrary infrared-safe cuts can be imposed at parton level, so the same calculation can supply acceptance corrections for top-mass and $tWb$-coupling measurements rather than only a single number.
- The extracted helicity fractions $F_L : F_+ : F_- = 0.689 : 0.0017 : 0.309$, now including finite $b$ mass, finite $W$ width, NLO electroweak and NNLO QCD, agree with earlier self-energy-based determinations, which gives an independent cross-check on the $V-A$ structure of the $tWb$ vertex.
- The method is presented as reusable for other heavy-to-light decays, with $B$-meson semileptonic decay named explicitly as a target.
- The fully differential top decay result is the missing ingredient for a complete NNLO description of top-quark pair production followed by decay, matching the NNLO production calculations already available.
Reading between the lines
- The agreement between this distribution-level calculation and the older self-energy-based inclusive NNLO result is a nontrivial consistency test: the two routes share almost no technical machinery, so their matching at the $10^{-4}$ level is evidence that both are correct rather than that one reproduces the other's assumptions.
- A natural extension the authors do not pursue here is polarized top decay: they note that soft radiation does not change spin and that the jet function sums over $b$ spin, which suggests the same factorization could deliver NNLO predictions for the top's spin-analysing powers, not just the $W$ helicity fractions.
- If the $tau_0$ extrapolation is as benign as claimed, the same slicing strategy should transfer to charged-current $B$ decays, where the analogous singular region dominates and where the jet function would need a finite quark-mass treatment.
- The quoted helicity fractions are a place where new physics in the $tWb$ vertex would show up as a shift in $F_+$; because $F_+$ is predicted at 0.0017, even a modest anomalous tensor coupling would be visible against the theory error once the experimental acceptance is corrected with these spectra.
Original abstract
We present the complete calculation of the top-quark decay width at next-to-next-to-leading order in QCD, including next-to-leading electroweak corrections as well as finite bottom quark mass and $W$ boson width effects. In particular, we also show the first results of the fully differential decay rates for top-quark semileptonic decay $t\to W^+(l^+\nu)b$ at next-to-next-to-leading order in QCD. Our method is based on the understanding of the invariant mass distribution of the final-state jet in the singular limit from effective field theory. Our result can be used to study arbitrary infrared-safe observables of top-quark decay with the highest perturbative accuracy.
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.
The abstract, sentence by sentence
We present the complete calculation of the top-quark decay width at next-to-next-to-leading order in QCD, including next-to-leading electroweak corrections as well as finite bottom quark mass and $W$ boson width effects. In particular, we also show the first results of the fully differential decay rates for top-quark semileptonic decay $t\to W^+(l^+\nu)b$ at next-to-next-to-leading order in QCD. Our method is based on the understanding of the invariant mass distribution of the final-state jet in the singular limit from effective field theory. Our result can be used to study arbitrary infrared-safe observables of top-quark decay with the highest perturbative accuracy.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | We present the complete calculation of the top-quark decay width at next-to-next-to-leading order in QCD, including next-to-leading electroweak corrections as well as finite bottom quark mass and $W$ boson width effects. | contribution 0.95 | 0.96 | 0.78 | asserted 1.00 |
| s1 | In particular, we also show the first results of the fully differential decay rates for top-quark semileptonic decay $t\to W^+(l^+\nu)b$ at next-to-next-to-leading order in QCD. | result 0.61 | 0.96 | 0.20 | asserted 1.00 |
| s2 | Our method is based on the understanding of the invariant mass distribution of the final-state jet in the singular limit from effective field theory. | method 1.00 | 0.57 | 0.04 | asserted 1.00 |
| s3 | Our result can be used to study arbitrary infrared-safe observables of top-quark decay with the highest perturbative accuracy. | interpretation 0.88 | 0.63 | 0.20 | asserted 0.66 |
What was examined, and what was not
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Figures
Showing 5 of the paper's 6 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:1210.2808 by the authors.
References
How later papers used it
105 later papers cite it
7 later papers by other authors are listed below: 5 used its data or values, 1 compared its own results with it and 1 cited it as support. 2 more papers by the same authors are listed: 2 used its method or tool.
Uses its method or tool 2 of 10 papers with this label · 2 by the same authors
By the same authors
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Based its calculation of the NNLO decay width of the charged Higgs boson or top quark on this paper's phase space slicing method.
Decay of the charged Higgs boson and the top quark in two-Higgs-doublet model at NNLO in QCD (2022) (same authors) · 0.99 · paraphrased: the licence does not allow quoting
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Employed phase-space slicing at NNLO from this paper, a generalization of the qT-subtraction concept of Catani and Grazzini.
Charm-quark production in deep-inelastic neutrino scattering at NNLO in QCD (2016) (same authors) · 0.97 · paraphrased: the licence does not allow quoting
Uses its data or values 5 of 14 papers with this label
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The resulting branching ratios are given by [equation] with $Γ_t ≡Γ(t→bW) = 1.32$ GeV being the total width calculated at Next-to-next-to-LO in QCD [ref] and $Γ(t→qX)$ is the corresponding top FCNC partial width [ref] .
Top quark flavor changing neutral currents at Future Linear Colliders (2024) · Top quark FCNC decays triggered by dark matter · 1.00
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This can in turn be interpreted in terms of a limit on the branching fraction thanks to the relation [equation] with $Γ_=1.32$ at NNLO [ref] and $Γ_^=1 = Γ_^=1 = 0.19$.
Search for flavor-changing neutral current interactions of the top quark and the Higgs boson decaying to a bottom quark-antiquark pair at $\sqrt{s}$ = 13 TeV (2021) · Results · 1.00
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These upper limits on the branching fractions can be translated into upper limits on the coupling strengths using the relations: [equation] where the total top quark width is $Γ_ = 1.32$ [ref] , and the partial width for the FCNC decay process of the top quark is $Γ_ = Γ_ = 0.184$for $κ_ = κ_ = 1$.
Search for the flavor-changing neutral current interactions of the top quark and the Higgs boson which decays into a pair of b quarks at $\sqrt{s}=$ 13 TeV (2017) · Results · 0.99
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We calculate $Γ_t = Γ_SM + ∑_jΓ(t →H_1,2^+d_j)$, where the NLO (EW) + NNLO (QCD) Standard Model result $Γ_SM = 1.322$ GeV is taken from [ref] , while $∑_jΓ(t →H_1,2^+d_j)$ is computed from Eq. [ref] summed over all the relevant channels.
Constraining CP4 3HDM with top quark decays (2021) · Top quark width · 1.00
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The top-quark mass ($m_t$) and width were set to 172.5 and 1.32 [ref] , respectively, in all MC event generators.
Measurements of top-quark pair differential and double-differential cross-sections in the $\ell$+jets channel with $pp$ collisions at $\sqrt{s}=13$ TeV using the ATLAS detector (2019) · Signal simulation samples · 0.99
Compares with it 1 of 5 papers with this label
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Checked that its assembled fully differential spin-averaged top quark decay width agrees within numerical uncertainties with this paper's program NNTopDec.
Higher order corrections to spin correlations in top quark pair production at the LHC (2019) · 0.98 · paraphrased: the licence does not allow quoting
Cites it as support 1 of 2 papers with this label
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Treated the decay part at tree level only, citing this paper as evidence that radiative corrections at the level of the decay are very small in the Standard Model.
Probing anomalous $Wtb$ couplings at the LHC in single top quark production through $t$-channel (2018) · 0.97 · paraphrased: the licence does not allow quoting
How the literature uses it
104 arXiv papers cite this in 131 sentences
Papers citing it · 67 sentences without a confident label
Uses its method or tool 10 papers
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In this work we employ the $N$-jettiness subtraction scheme [ref] to perform the evaluation of the NNLO cross section.
NNLO predictions for Z-boson pair production at the LHC · Details of the calculation · 1.00
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Our calculation of the NNLO decay width of charged Higgs boson or top quark is based on the phase space slicing method [ref] .
Decay of the charged Higgs boson and the top quark in two-Higgs-doublet model at NNLO in QCD · Phase space slicing method · 0.99
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We can thus replace the zero-jet by a full $q_T$-vetoed NNLO calculation, using the cutoff method [ref] , and complete the cross section using the one-jet [ref] : Each event removed from the one-jet bin by means of the emission probability $1-Π_0(t,μ_Q^2)$ is added to the zero-jet bin.
Higgs-boson production through gluon fusion at NNLO QCD with parton showers · UN$^2$LOPS matching · 0.96
Uses its data or values 14 papers
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The resulting branching ratios are given by [equation] where $Γ_t ≡Γ(t→b W) = 1.32 GeV$ calculated at Next-to-Next-to-Leading Order in QCD including the finite mass and width effects and Next-to-Leading Order electroweak corrections [ref] .
Dark matter as the trigger of flavor changing neutral current decays of the top quark · Top quark FCNC decays · 1.00
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The resulting branching ratios are given by [equation] with $Γ_t ≡Γ(t→bW) = 1.32$ GeV being the total width calculated at Next-to-next-to-LO in QCD [ref] and $Γ(t→qX)$ is the corresponding top FCNC partial width [ref] .
Top quark flavor changing neutral currents at Future Linear Colliders · Top quark FCNC decays triggered by dark matter · 1.00
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For a top-quark mass of 172.5 GeV the predicted width is 1.322 GeV, including NNLO QCD, finite $b$-quark and $W$ masses, and NLO electroweak corrections [ref] .
Extracting the Top-Quark Width from Non-Resonant Production · Existing Calculations and Measurements · 0.92
Compares with it 5 papers
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The above results of $δ_b^(0),δ_W^(0),δ_EW^(1),δ_QCD^(1)$ and $δ_QCD^(2)$ agree with the ones in [ref] when using the same input parameters.
Analytic result for the top-quark width at next-to-next-to-leading order in QCD · Introduction · 1.00
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A comparison of our NLO QCD+EW prediction—with and without the additional NNLO correction of Refs. [ref] —reveals good agreement with the most recent measurement by the CMS experiment [ref] : [equation] However, the quoted experimental error is still one order of magnitude larger than the typical EW corrections of $∼ +1.3%$ addressed here.
Techniques for the treatment of IR divergences in decay processes at NLO and application to the top-quark decay · Conclusions · 0.81
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The comparison is performed with $m_t=172.5$, but the relative size of the corrections is somewhat insensitive to the precise top-quark mass [ref] .
Single-top-quark production in the $t$-channel at NNLO · Top-quark-decay corrections · 0.92
Cites it as support 2 papers
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The NNLO corrections lead to a softer spectrum due to both soften of the top-quark $p_T$ and additional radiations in top-quark decay [ref] .
Top-quark mass determination from t-channel single top production at the LHC · NNLO predictions · 0.98
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The full tree-level amplitude, anomalous amplitude, will be denoted $𝒫_0^ano.$, such that [equation] Since the radiative corrections at the level of the decay are very small in the SM (see e.g. [ref] ), the decay part will be considered at tree-level only.
Probing anomalous $Wtb$ couplings at the LHC in single top quark production through $t$-channel · Including anomalous $Wtb$ couplings · 0.97
Cites it as background 30 papers
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The NNLO corrections were calculated in the large top quark mass limit in Refs. [ref] , and the full NNLO corrections were given in [ref] .
Decay of the charged Higgs boson and the top quark in two-Higgs-doublet model at NNLO in QCD · Introduction · 0.87
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The corrections to the top-quark decay process $→$ are known at NLO [ref] and next-to-next-to-leading order (NNLO) [ref] in QCD and up to NLO for the electroweak (EW) corrections [ref] .
Techniques for the treatment of IR divergences in decay processes at NLO and application to the top-quark decay · Introduction · 0.90
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Later, the total and differential decay widths were calculated numerically [ref] , and polarized decay rates were also studied [ref] .
Analytic result for the top-quark width at next-to-next-to-leading order in QCD · Introduction · 0.96
What it cites · 4 sentences without a confident label
Compares with 1 paper
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Our calculations are complementary to the NNLO QCD predictions for top-quark pair production [ref] .
Percent level precision physics at the Tevatron: first genuine NNLO QCD corrections to q qbar -> t tbar + X · Introduction · 0.93
Discussion
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