REVIEW
Infrared Safe but Infinite: Soft-Gluon Divergences Inside the Physical Region
High Energy Physics - Phenomenology · Submitted 13 Oct 1997 · 26 references
Pith's verdict: The fixed-order mechanism is correct and cleanly derived; the resummation half is a double-logarithmic sketch, and the abstract's 'to all orders' slightly outruns Sect. 4.
T0 · AI review
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Referee report
The paper
The paper claims
Infrared- and collinear-safe QCD observables can still be infinite at ordinary interior points of phase space, order by order, whenever a lower-order distribution has a step or kink there.
The reading
What carries the argument
Three pieces carry the argument. First, the plus-prescription of Eq. (2.4), $dw/dz = a[ln(1/(1-z))/(1-z)]_+$, which is the only cancellation that safety guarantees and which is well behaved only on smooth test functions — this is the precise sense in which safety is weaker than finiteness. Second, the convolution of Eq. (2.6), $hat sigma^{(n+1)}(C) = int_0^C dy, hat sigma^{(n)}(C-y), (dw/dz)_{z=1-y} + ...$, together with its evaluated form Eq. (2.9), which converts a step or a kink in the lower-order distribution into a logarithmically enhanced, integrable singularity at the critical point. Third, the exponentiation of Eq. (4.2), $hat sigma^{(infty)}_+(C) - hat sigma^{(infty)}_-(C_0) = exp[-(a/2)ln^2(C-C_0)],[hat sigma^{(n_0)}_+(C_0) - hat sigma^{(n_0)}_-(C_0)]$, which turns the divergent series into a smooth interpolation across $C_0$ — the Sudakov shoulder. The name is chosen by analogy with the Sudakov form factor that suppresses the exclusive boundary $C to 0$ in Eq. (4.1); the difference is that at the boundary the exponential kills the cross section, whereas at an interior critical point it merely rounds off a step.
Extended reading notes
Core claim
The paper's central claim is that infrared and collinear safety, as formulated by Sterman and Weinberg, does not imply that the coefficients of the perturbative expansion are non-singular functions. The cancellation mechanism that safety guarantees is the plus-prescription of Eq. (2.4): real and virtual soft-gluon emissions combine into a distribution that is well behaved only when integrated against a test function that is smooth at $z=1$. The $(n+1)$-st order distribution is obtained by convolving the $n$-th order one with that distribution, Eq. (2.6), so if the $n$-th order has a step at an interior point $C_0$ the convolution diverges. Eq. (2.9) makes this quantitative: a jump $[hat sigma^{(n)}_+(C_0) - hat sigma^{(n)}_-(C_0)]$ generates a term $-a/2 ln^2(C-C_0)$ at order $n+1$, and further orders multiply the logarithm. For the $C$-parameter the step is at $C=3/4$, the largest value three partons can reach (the symmetric $x_1=x_2=x_3=2/3$ configuration), and the authors verify the predicted second-order behaviour $B_+(C) propto A(3/4)[(2C_F+C_A)(1-delta)ln^2 delta + ...]$ against the EVENT Monte Carlo, finding $B_-(3/4)=230 plus or minus 10$ and $h(3/4)=146 plus or minus 3$. For jet shapes the critical point is the cone radius $r=R$, giving single-logarithmic divergences in the cone algorithm and a double-sided singularity in the $k_perp$ algorithm. Finally, iterating the soft-gluon convolution and exponentiating gives Eq. (4.2), in which the step is suppressed by $exp[-(a/2)ln^2(C-C_0)]$ rather than the divergence being enhanced: the resummed distribution is finite, continuous and infinitely differentiable at $C_0$, the Sudakov shoulder.
Load-bearing premise
The resummation argument assumes the observable always increases when a soft gluon is emitted, so that only the step across the critical point needs exponentiating and the lower side carries no soft-gluon singularities; for the non-monotonic case, which the paper itself identifies in the $k_perp$ jet algorithm, no analytic resummation is given and the smoothing is asserted by analogy to a numerical figure in a cited preprint.
Editorial extensions
If this is right
- Fixed-order predictions for the $C$-parameter distribution cannot be trusted in a neighbourhood of $C=3/4$; the second-order result already diverges as $ln^2(C-3/4)$ above the point, and each further order raises the power of the logarithm.
- Jet-shape and jet-profile distributions in hadron collisions are unreliable near the cone radius $r=R$, with a single-logarithmic divergence in cone algorithms and a two-sided singularity in $k_perp$ algorithms.
- Phase-space slicing calculations acquire an unphysical $ln s_{min}$ dependence, Eq. (3.27), that cancels only when the slicing parameter is taken sufficiently small.
- Any use of an apparently safe observable to measure $alpha_S$ or test QCD requires first locating its critical points and either avoiding them or resumming the soft-gluon terms.
- Because the fixed-order divergence is integrable, hadronization smearing could hide it — but only at the cost of non-perturbative corrections that scale logarithmically with $Q$ rather than as inverse powers, which would undermine the safety argument itself.
Reading between the lines
- The argument generalizes into a practical recipe: for any candidate observable, compute the lowest order at which the distribution has a step or a discontinuous derivative, locate that point, and check whether it lies inside the accessible phase space. The paper effectively proposes this as a routine step before any fixed-order comparison with data.
- The distinction the authors draw between a step (which diverges at the next order) and a kink (which diverges only in the derivative) suggests a hierarchy of severity that could be tabulated for the standard event-shape variables; thrust is the worked example of the milder case, with the cusp appearing at $C_0 = 1/3$.
- If the Sudakov shoulder is real and smooth, its width is set by $a$ and $alpha_S$, so measuring the shoulder's shape near $C=3/4$ would be an independent handle on $alpha_S$ and on the colour factor combination $C_F + C_A/2$ that enters $A(1)$ — the authors note the shoulder would be twice as steep in an abelian theory.
- The paper's own treatment of the non-monotonic $k_perp$ case is by analogy to a numerical figure in a cited preprint rather than by derivation, which leaves an opening: an analytic resummation of the double-sided singularity would test whether the smoothing mechanism really works when the observable is not monotonic in soft-gluon energy.
Original abstract
We show that QCD observables defined as infrared- and collinear-safe, according to the usual Sterman-Weinberg criteria, can nevertheless be infinite at accessible points inside phase space, to any finite order of perturbation theory. Infrared finiteness is restored after resummation of divergent terms to all orders. The resulting characteristic structure, which we call a Sudakov shoulder, represents an interesting new class of QCD predictions.
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 show that QCD observables defined as infrared- and collinear-safe, according to the usual Sterman-Weinberg criteria, can nevertheless be infinite at accessible points inside phase space, to any finite order of perturbation theory. Infrared finiteness is restored after resummation of divergent terms to all orders. The resulting characteristic structure, which we call a Sudakov shoulder, represents an interesting new class of QCD predictions.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | We show that QCD observables defined as infrared- and collinear-safe, according to the usual Sterman-Weinberg criteria, can nevertheless be infinite at accessible points inside phase space, to any finite order of perturbation theory. | result 0.92 | 0.92 | 0.90 | asserted 1.00 |
| s1 | Infrared finiteness is restored after resummation of divergent terms to all orders. | result 0.90 | 0.68 | 0.15 | asserted 1.00 |
| s2 | The resulting characteristic structure, which we call a Sudakov shoulder, represents an interesting new class of QCD predictions. | interpretation 0.88 | 0.85 | 0.17 | asserted 0.99 |
What was examined, and what was not
- arXiv listing abstract v1
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- figures and tables
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- relations to other papers
- P abstract
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- R cross-paper relations
Figures
Figures reproduced from arXiv:hep-ph/9710333 by the authors.
References
How later papers used it
63 later papers cite it
3 later papers by other authors are listed below: 3 used its data or values.
Uses its data or values 3 papers
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Used the values calculated in this paper, including b_02=43.3122 and b_12=-115.499, and fit the rest of the coefficients to EVENT2 output.
Accuracy and Precision in Collider Event Shapes (2016) · 0.98 · paraphrased: the licence does not allow quoting
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Used the known value of the leading-order differential cross section for the C-parameter at its maximum value from this paper.
Covariantizing Phase Space (2020) · 1.00 · paraphrased: the licence does not allow quoting
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Used the values calculated in this paper, including b_02=43.3122 and b_12=-115.499, and fit the rest of the coefficients to EVENT2 output.
C-parameter Distribution at N${}^3$LL$^\prime$ including Power Corrections (2014) · 0.99 · paraphrased: the licence does not allow quoting
How the literature uses it
61 arXiv papers cite this in 85 sentences
Papers citing it · 73 sentences without a confident label
Uses its data or values 3 papers
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We use the value predicted by exponentiation in Ref. [ref] .
Accuracy and Precision in Collider Event Shapes · Nonsingular Terms · 0.87
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In fact, using the known value of the leading-order differential cross section for the $C$-parameter at its maximum value [ref] , where [equation] the value of the differential cross section at a general value $C$ is [equation] Here, $Ω$ is the region of phase space where the $C$-parameter takes values between $C$ and $3/4$.
Covariantizing Phase Space · Introduction · 1.00
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We use the values calculated in Ref. [ref] , , $b_02=43.3122$, and $b_12 = - 115.499$ and fit the rest of the coefficients to EVENT2 output.
C-parameter Distribution at N${}^3$LL$^\prime$ including Power Corrections · Nonsingular Terms · 0.99
Compares with it 1 paper
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We also note that the Sudakov shoulder effect [ref] is observed around $p_T^A$ = 30 GeV from the distribution as in the scalar Higgs case [ref] .
Pseudoscalar Higgs plus jet production at Next-to-Next-to-Leading Order in QCD · Phenomenology · 0.89
Cites it as background 6 papers
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These logarithms were identified by Catani and Webber [ref] in 1997, and a systematically improvable method to compute and resum these logarithms was developed in 2022 [ref] .
NNLL Resummation of Sudakov Shoulder Logarithms in the Heavy Jet Mass Distribution · Conclusions · 0.93
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This is the Sudakov shoulder first identified by Catani and Webber in 1997 [ref] .
Heavy Jet Mass in Hadronic Higgs Decays · Introduction · 0.84
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It is well known [ref] that in the case of symmetric (or nearly-symmetric) cuts fixed-order calculations are affected by perturbative (soft-gluon) instabilities at higher orders.
Drell-Yan lepton-pair production: $q_T$ resummation at N$^3$LL accuracy and fiducial cross sections at N$^3$LO · Introduction · 0.80
Discussion
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