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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.

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arXiv IDhep-ph/9710333 (version 1)
Pith IDLQGEMIPI, Pith's permanent record of this paper and its review
CitationB.R. Webber, S. Catani. "Infrared Safe but Infinite: Soft-Gluon Divergences Inside the Physical Region." arXiv:hep-ph/9710333 (1997). https://pith.science/paper/LQGEMIPI
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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

The Sterman–Weinberg criteria of infrared and collinear safety are usually read as a promise that the fixed-order perturbative coefficients of an observable are finite. Catani and Webber argue that this stronger reading is false. What safety actually guarantees is only that soft and collinear divergences cancel when the soft-gluon probability, a plus-distribution, acts on a sufficiently smooth test function. If the distribution at some order $n$ is not smooth at an accessible interior point $C_0$ — a step, or a discontinuous derivative — then the next order inherits a $+a/2$ times $[1/(1-z)]_+$ type convolution that produces a $+a/2$ times $[1/(1-z)]_+$ divergence, in practice a term going like $+a/2$ times $[1/(1-z)]_+$ evaluated at $C-C_0$, i.e. a double logarithm $+a/2$ times $[1/(1-z)]_+$ of $C-C_0$, and higher orders raise the power. They exhibit two real cases — the $C$-parameter in $e^+e^-$ annihilation, where the three-parton phase-space boundary sits at $C=3/4$, and the jet profile in hadron collisions, where the cone radius $R$ is a critical point — and then argue that summing soft-gluon terms to all orders removes the divergence and leaves a smooth, infinitely differentiable structure they name the Sudakov shoulder. If they are right, a whole class of observables that pass the standard safety test cannot be compared to fixed-order theory near their critical points, and the safety argument itself has to be restated at the level of resummed rather than fixed-order predictions.

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

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

  • 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.

Compiled by classifier-1.13-20260917 via OpenRouter, question bank abstract-p1-20260923, 2026-09-23: 1 request, 2,944 input tokens, $0.0001 reported.

What the abstract itself states, as probabilities: claims first 0.07 · states limitation 0.40 · human participants 0.01 · controlled comparison 0.02 · randomized assignment 0.01 · states quantitative result 0.06 · compares with prior results 0.44 · mentions code or data availability 0.02.

The abstract, sentence by sentence

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

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

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 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 kind of paper the classifier reports this is: theoretical result 1.00 · evidence from proof or derivation 0.99.

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. Predictions of the C-parameter distribution for αS = 0.12. Dashed: O(αS). Solid: O(α 2 S ). Dot-dashed: resummed. the three-parton upper limit C = 3 4 : A(C) C ∼ 3/4 ≃ A( 3 4 ) Θ( 3 4 − C) , (3.8) A( 3 4 ) = 32 243 π √ 3 M0 = 256 243 π √ 3 CF . (3.9) The derivative of A(C) is also finite at C = 3 4 − : in fact A ′ ( 3 4 ) = −8 3A( 3 4 ) . (3.10) The resulting first-order prediction in the vicinity of C = 3 4 is shown by the dashed curve in
Figure 2. Second-order prediction of the C-parameter distribution for C → 3 4 + . Points: EVENT Monte Carlo. Curve: Eqs. (3.12-3.15). Using Eq. (3.10), we see that the first term in the square bracket on the right-hand side of Eq. (3.12) is in complete agreement with Eq. (2.9) with a = (2CF + CA)αS(Q)/π. The second term in the square bracket is due to collinear but non-soft parton splitting: single￾logarithmic contributions of this type were neglected in the simplified discussion of Sect. 2, which was valid to double-logarithmic (DL) accuracy only. Comparing our analytic calculation with…

Figures reproduced from arXiv:hep-ph/9710333 by the authors.

References
How later papers used it

63 later papers cite it

Evidence: some. 62 of the citing papers share no author with it, and 4 of those carry a confident label from Pith's citation classifier.

Pith read 37 of the 87 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 data or values 3 papers
  • 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

  • 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

  • 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

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 · 73 sentences without a confident label

Uses its data or values 3 papers
  • We use the value predicted by exponentiation in Ref. [ref] .

    Accuracy and Precision in Collider Event Shapes · Nonsingular Terms · 0.87

  • 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

  • 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
Cites it as background 6 papers

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