Dijet Azimuthal Decorrelations in pp Collisions at sqrt(s) = 7 TeV

CMS Collaboration

REVIEW

Dijet Azimuthal Decorrelations in pp Collisions at sqrt(s) = 7 TeV

CMS Collaboration

High Energy Physics - Experiment · Submitted 26 Jan 2011 · 28 references

Pith's verdict: A careful, useful 7 TeV extension of the D0 decorrelation measurement; the ISR-sensitivity claim is a hedged PYTHIA parameter scan, not an independent measurement, and the paper is honest about that.

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arXiv ID1101.5029 (version 2)

Pith IDVIQCXMJC, Pith's permanent record of this paper and its review

CitationCMS Collaboration. "Dijet Azimuthal Decorrelations in pp Collisions at sqrt(s) = 7 TeV." arXiv:1101.5029 (2011). https://pith.science/paper/VIQCXMJC

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Editor's note

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This is the CMS dijet azimuthal decorrelation measurement at 7 TeV, 2.9 pb^-1, five leading-jet pT bins from 80 GeV up. What's genuinely new is the energy and the kinematic reach: the observable, the normalization, and the comparison strategy come straight from D0 at 1.96 TeV, and the paper says so. The pT scan is wider than D0's, the jets are particle-flow with small residual corrections, and the normalized distributions cancel luminosity and blunt the jet-energy-scale sensitivity. The systematic budget is itemized per source with numbers per region (11% at pi/2 down to 3% at pi), the unfolding is cross-checked between two generators, and the NLO comparison in Fig. 4 shows scale, PDF, and non-perturbative uncertainties separately rather than folding them into one band. That is the right way to present this. Data went to HEPData, which matters for a tuning input. Credit earned. The soft spots are real but bounded. First, the unfolding corrections come from PYTHIA 6 D6T and HERWIG++, the same generator class used in the Fig. 2 comparisons, and the corrections are largest (-19%) exactly at small delta-phi where the interesting physics lives. The paper bounds this at 8% at pi/2, but that bound is estimated from the same models, so it is a consistency check, not an independent one. Second, the abstract's ISR claim [abstract:s3] rests on Fig. 5, a kISR scan in PYTHIA 6 D6T whose default value of 2.5 was itself fitted to the D0 dijet decorrelation data. Showing that kISR moves delta-phi is partly a restatement of how the tune was built. The stress-test note is right about this, but it overstates the damage: the paper's own wording is 'could be used to tune parameters,' which is appropriately modest, and the 30% model spread against an 11% data uncertainty does suggest real discriminating power. What's missing is a best-fit kISR or a Delta-chi-squared against the CMS data, which would have converted an implicit argument into an explicit one. That is a modest interpretive gap, not a flaw in the measurement. The reliance on ref. [27] for 'kISR is the only relevant PYTHIA 6 parameter' is a cited published report, so that's fine, though MPI and underlying-event parameters are never scanned. Who should read it: anyone tuning or validating generators for LHC analyses, and anyone wanting a clean example of a normalized QCD measurement done properly. It deserves referees. I'd send it out, expect minor revision at most, and cite it.

Referee report

Significance

The measurement is a useful and timely addition to the 7 TeV QCD programme: it is the first of its kind at the LHC, it uses a normalized observable that cancels the luminosity uncertainty and substantially reduces jet-energy-scale sensitivity, and it spans a wide range of jet multiplicities without requiring explicit multijet reconstruction. The five-region pTmax scan, the explicit enumeration and quadrature combination of five systematic sources (11% at Delta-phi ~ pi/2 falling to 3% near pi), the cross-check of the unfolding between two generators, and the honest presentation of the large renormalization/factorization scale spread in the NLO comparison are all strengths. The comparison to four event generators plus NLO pQCD, and the demonstration that the shape responds strongly to kISR while being nearly insensitive to the final-state radiation parameter PARP(71), make the result directly useful to the tuning community. The forward-looking wording (the data 'could be used to tune parameters') is appropriately hedged rather than overclaimed.

Major objections (3)

[Unfolding corrections; Fig. 2] The unfolding corrections are derived from PYTHIA 6 tune D6T and HERWIG++ 2.4.2 and the average of the two is applied to data, yet Fig. 2 then concludes that precisely these two generators describe the data well while MADGRAPH and PYTHIA 8 do not. The generator-spread uncertainty of 8% at Delta-phi ~ pi/2 partially covers this, but a spread is not a closure test, and the ranking of generators is one of the paper's headline results. A concrete and inexpensive check would be to unfold separately with each of the four generators (or at least with MADGRAPH and PYTHIA 8) and show that the ordering of the ratio curves in Fig. 2 is unchanged; if it is, the conclusion is robust and the point is settled. As written, the reader cannot exclude that part of the stated preference for PYTHIA 6 and HERWIG++ is inherited from the unfolding choice.
[ISR sensitivity; Fig. 5 and the paragraph on kISR] The central claim of the abstract (abstract:s3) rests on Fig. 5, which shows that varying kISR by plus or minus 0.5 about its default changes the PYTHIA 6 D6T prediction by about 30% at Delta-phi ~ pi/2. The paper itself states that the default kISR = 2.5 in D6T was determined from the D0 dijet azimuthal decorrelation results [1], i.e. from the same observable at a different energy, so part of the demonstration is a restatement of how the tune was constructed. What is missing is a direct statement of the CMS data's discriminating power: the 30% model variation visibly exceeds the 11% total data uncertainty at pi/2, but this comparison is left implicit. The authors should say so explicitly, and ideally quote a chi-squared or Delta-chi-squared between the data and the kISR variants in at least the two lowest pTmax regions, so the reader can see whether values away from the default are excluded. Relatedly, the assertion that kISR is the only PYTHIA 6 parameter with significant impact rests on ref. [27], a 2006 Tevatron-for-LHC working-group report; the authors should state whether that conclusion was established at Tevatron energies and whether multiple-parton-interaction and underlying-event parameters, which are not scanned here, could mimic the effect.
[Fig. 1 and Fig. 2; data-point placement] The data points are plotted at the abscissa value for which the PYTHIA 6 D6T prediction equals the bin average (the Lafferty-Wyatt procedure, ref. [14]), and the same abscissa is then used for the ratios to all generators in Fig. 2. Because the distributions are steepest near Delta-phi = pi, this procedure shifts the horizontal positions in a way that depends on the model being tested and mildly favors PYTHIA 6 D6T in the region where the comparison is most sensitive. The authors should either quantify the size of the shift (for example by showing the ratio plots with points at the bin center as a cross-check) or state explicitly that the effect is small compared with the systematic band. This is a modest point but it bears directly on the generator-ranking conclusion.

Minor objections (8)

[Abstract and event selection] The abstract quotes an integrated luminosity of 2.9 inverse picobarns, but the lowest pTmax region (80-110 GeV) uses only 0.3 inverse picobarns; this should be noted in the abstract or at first mention so that the quoted figure is not misread.
[Event selection and luminosity] The 11% luminosity uncertainty is quoted even though the distributions are normalized to the integrated dijet cross section; the text should state explicitly that this uncertainty cancels in the normalized observable.
[Trigger paragraph] The sentence 'The corresponding offline corrected pTmax values are 80 GeV (110 GeV)...' is missing a word and reads awkwardly; please correct it.
[Introduction] The statement that three-jet topologies dominate 2 pi/3 < Delta-phi < pi while smaller angles are populated by four-jet events is a leading-order parton-level expectation; it should be qualified as such, since the measured distributions include all orders.
[Results presentation] Given the stated intent that these data could be used to tune generator parameters, the numerical values of the normalized distributions (and, if possible, the bin-to-bin covariance) should be provided in a table or through HepData rather than only in figures.
[Fig. 3 and Fig. 4 captions] The captions should state that the region near Delta-phi = pi is excluded from the pQCD comparison and why, so that the figures are self-contained.
[Systematic uncertainties] The unfolding uncertainty is estimated from the spread among generators and tunes; please state whether the two-generator average is an arithmetic mean and how the spread is propagated into the quoted 8% figure.
[Fig. 2 and Fig. 5] The shaded systematic band appears identical across panels; please clarify whether the uncertainties are treated as fully correlated across Delta-phi bins and across pTmax regions, since ratios of normalized distributions are being plotted.

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The paper

The paper claims

The angle between the two leading jets at the LHC measures radiation emitted before the hard collision, and CMS shows the data can distinguish models on that basis.

The reading

CMS measures how far the two highest-momentum jets in a proton-proton collision at $7$ TeV deviate from being exactly back-to-back in azimuth, using $2.9$ inverse picobarns of data split into five leading-jet $p_T$ regions. At the simplest level of QCD the two jets are emitted exactly opposite, so any measured deviation, $1 - (1/2)(1-2) = 2$ in the notation the paper uses, is a direct count of extra radiation: soft gluon emission, and three-jet or four-jet topologies. The paper's aim is to turn that count into a calibrated test of how well theory and simulation describe radiation, and specifically to show that the small-angle part of the distribution is controlled by radiation emitted before the hard scatter rather than after it. If that holds, the measurement becomes a tuning handle for the event generators that searches for new physics depend on, and a benchmark where fixed-order QCD can be checked in the region where it is weakest.

What carries the argument

The carrier is the differential dijet cross section in $1-2$, normalized by the dijet cross section integrated over the whole azimuthal range, so that many experimental and theoretical uncertainties cancel in the ratio. Jets are built with the anti-$k_T$ algorithm at $R=0.5$ on particle-flow objects, both leading jets are required to have $p_T>30$ GeV and $|y|<1.1$, and the distributions are unfolded for jet $p_T$, rapidity and azimuthal resolution using correction factors derived from PYTHIA 6 D6T and HERWIG++, averaged between the two. The sensitivity argument is carried by the PYTHIA 6 initial-state shower cutoff parameter, whose variation moves the prediction most strongly exactly where multi-gluon radiation dominates.

Extended reading notes

Core claim

The measured normalized distributions in the dijet azimuthal angle are strongly peaked at $1-2$ and steepen as the leading-jet $p_T$ rises. Compared against generators, PYTHIA 6 tune D6T and HERWIG++ reproduce the shape across the full range, MADGRAPH (leading-order matrix elements up to four partons) predicts too little decorrelation, and PYTHIA 8 predicts too much. Against next-to-leading-order QCD from NLOJET++ within FASTNLO, the shape agrees over much of the range, but below $2-1/3$ the prediction is effectively leading-order only, since tree-level four-parton final states dominate there, and it both under-predicts the decorrelation and shows large renormalization and factorization scale sensitivity. The paper's own headline claim is that these distributions are sensitive to initial-state gluon radiation: varying the PYTHIA 6 parameter $k_{1-2}$ by $0.5$ about its default changes the prediction by about $30$ percent near $1-2$, while varying the final-state radiation parameter PARP(71) from $2.5$ to $8$ changes it by less than about $10$ percent in every $p_T$ region.

Load-bearing premise

The corrections that map the raw detector-level distributions back to particle level are derived from PYTHIA 6 and HERWIG++ simulations and then averaged, so if the real detector migration differs from what those two models produce, the corrected shapes shift most strongly in the small-angle region where the initial-state radiation claim is made.

Editorial extensions

If this is right

The measured shapes provide a target that generator tunes must hit at small azimuthal angles, not only near the back-to-back peak where most dijet data have been used for tuning.
MADGRAPH with up to four partons and PYTHIA 8 fail in opposite directions, so the data discriminate between matrix-element-plus-shower matching schemes rather than merely ranking generators.
The residual disagreement with NLO QCD below $2-1/3$ is localized to a kinematic region where the calculation is effectively leading order, which points at a specific place to extend the theory rather than a diffuse failure.
Because the small-angle region responds strongly to the initial-state shower cutoff and weakly to the final-state one, the measurement can be used to constrain initial-state radiation in the generators used for background modelling in searches.

Reading between the lines

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

If the initial-state attribution is correct, an independent handle on initial-state radiation, such as the transverse momentum spectrum of Drell-Yan pairs or underlying-event observables, should shift in the same direction as the dijet decorrelation when the same tune is varied.
The same measurement at higher collision energy would widen the initial-state phase space and should increase the separation between PYTHIA 8 and MADGRAPH, making the discrimination sharper than at $7$ TeV.
A generator that matches the data near $1-2$ but misses near $5-1/6$ would suggest the decorrelation is being absorbed into the wrong parameter, which is a testable failure mode of the tuning programme the paper proposes.
Extending the fixed-order calculation to four-parton final states at next-to-leading order should reduce the scale sensitivity below $2-1/3$; if the disagreement with data survives that extension, the missing physics is genuinely non-perturbative rather than a truncation artifact.

Original abstract

Measurements of dijet azimuthal decorrelations in pp collisions at sqrt(s) = 7 TeV using the CMS detector at the CERN LHC are presented. The analysis is based on an inclusive dijet event sample corresponding to an integrated luminosity of 2.9 inverse picobarns. The results are compared to predictions from perturbative QCD calculations and various Monte Carlo event generators. The dijet azimuthal distributions are found to be sensitive to initial-state gluon radiation.

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, 3,564 input tokens, $0.0001 reported.

What the abstract itself states, as probabilities: claims first 0.03 · states limitation 0.09 · human participants 0.01 · controlled comparison 0.19 · randomized assignment 0.01 · states quantitative result 0.45 · compares with prior results 0.88 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v2. Results and the main claim are underlined; show every role.

Measurements of dijet azimuthal decorrelations in pp collisions at sqrt(s) = 7 TeV using the CMS detector at the CERN LHC are presented. The analysis is based on an inclusive dijet event sample corresponding to an integrated luminosity of 2.9 inverse picobarns. The results are compared to predictions from perturbative QCD calculations and various Monte Carlo event generators. The dijet azimuthal distributions are found to be sensitive to initial-state gluon radiation.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#	sentence	role	the authors' own work	states the main conclusion	strength
s0	Measurements of dijet azimuthal decorrelations in pp collisions at sqrt(s) = 7 TeV using the CMS detector at the CERN LHC are presented.	contribution 0.99	0.94	0.05	asserted 0.99
s1	The analysis is based on an inclusive dijet event sample corresponding to an integrated luminosity of 2.9 inverse picobarns.	method 0.99	0.88	0.02	asserted 1.00
s2	The results are compared to predictions from perturbative QCD calculations and various Monte Carlo event generators.	method 0.87	0.57	0.06	asserted 0.94
s3	The dijet azimuthal distributions are found to be sensitive to initial-state gluon radiation.	result 0.50	0.93	0.69	asserted 1.00

What kind of paper the classifier reports this is: empirical study 1.00 · evidence from observation or experiment 0.93.

What was examined, and what was not

sources read

arXiv listing abstract v2

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.** Normalized ∆ϕdijet distributions in several pT max regions, scaled by the multiplicative factors given in the figure for easier presentation. The curves represent predictions from PYTHIA6, PYTHIA8, HERWIG++, and MADGRAPH. The error bars on the data points include statistical and systematic uncertainties

**Figure 2.** Ratios of measured normalized ∆ϕdijet distributions to PYTHIA6, PYTHIA8, HERWIG++, and MADGRAPH predictions in several pT max regions. The shaded bands indicate the total systematic uncertainty

**Figure 3.** Normalized ∆ϕdijet distributions in several pT max regions, scaled by the multiplicative factors given in the figure for easier presentation. The curves represent predictions from LO (dotted line) and NLO pQCD (solid line). Non-perturbative corrections have been applied to the predictions. The error bars on the data points include statistical and systematic uncertainties

**Figure 4.** Ratios of measured normalized ∆ϕdijet distributions to NLO pQCD predictions with non-perturbative corrections in several pT max regions. The error bars on the data points include statistical and systematic uncertainties. The effect on the NLO pQCD predictions due to µr and µf scale variations and PDF uncertainties, as well as the uncertainties from the nonperturbative corrections are shown

**Figure 5.** Ratios of measured normalized ∆ϕdijet distributions to PYTHIA6 tune D6T with various values of kISR in several pT max regions. The shaded bands indicate the total systematic uncertainty

Figures reproduced from arXiv:1101.5029 by the authors.

References

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Forward citations

Cited by 6 Pith papers

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

Azimuthal Angular Asymmetry of Soft Gluon Radiation in Jet Production
hep-ph 2021-06 conditional novelty 6.0 of 10

Resummed soft gluon radiation generates sizable cos(φ) and cos(2φ) azimuthal asymmetries in single-jet and dijet events, and the perturbative cos(2φ) can dominate over the intrinsic linearly polarized gluon signal.
Lepton-jet Correlations in Deep Inelastic Scattering at the Electron-Ion Collider
hep-ph 2018-12 conditional novelty 6.0 of 10

Lepton-jet angular correlations in deep inelastic scattering factorize into a single transverse-momentum-dependent quark distribution, making them a fragmentation-free probe of the quark Sivers function and of cold nu...
Studies of jet quenching using isolated-photon + jet correlations in PbPb and pp collisions at sqrt(s[NN]) = 2.76 TeV
nucl-ex 2012-05 accept novelty 6.0 of 10

In central PbPb collisions at 2.76 TeV, jets opposite an isolated photon carry on average 0.73 of the photon momentum instead of 0.86, and only 49% of photons have an above-threshold jet versus 69% in a no-energy-loss...
Observation and studies of jet quenching in PbPb collisions at nucleon-nucleon center-of-mass energy = 2.76 TeV
nucl-ex 2011-02 accept novelty 6.0 of 10

In central PbPb collisions at 2.76 TeV, dijets become strongly momentum-imbalanced, and the imbalance is carried by low-momentum particles emitted at large angles to the away-side jet.
ATLAS measurements of the properties of jets for boosted particle searches
hep-ex 2012-06 accept novelty 5.0 of 10

ATLAS publishes particle-level reference distributions of jet mass, width, eccentricity, planar flow and angularity for high-momentum jets, with pileup corrections, and finds Pythia and PowHeg describe them while Herw...

Show 1 more citing papers

Measurement of Dijet Azimuthal Decorrelations in pp Collisions at sqrt(s)=7 TeV
hep-ex 2011-02 conditional novelty 5.0 of 10

The normalized dijet azimuthal decorrelation distribution is measured in nine leading-jet pT bins at sqrt(s)=7 TeV and compared with NLO QCD and the SHERPA, PYTHIA, and HERWIG generators.

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1 later paper by other authors is listed below: 1 compared its own results with it.

Compares with it 1 of 5 papers with this label

Showed its own predictions for the azimuthal de-correlation of high dijets for different regions using TMD parton densities with off-shell matrix elements, parton shower and hadronization in comparison with measurements at 7 TeV in pp collisions.

Calculations with off-shell matrix elements, TMD parton densities and TMD parton showers (2017) · 0.88 · paraphrased: the licence does not allow quoting

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45 arXiv papers cite this in 56 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 · 23 sentences without a confident label

Contests it 1 paper

The theory results published by and CMS for $< 2π/3$ and labeled “NLO” in references [ref] are inconsistent, because they mix relative orders for the numerator (LO) and denominator (NLO).

Dijet azimuthal decorrelations for $\Delta \phi_{\rm dijet} < 2\pi/3$ in perturbative QCD · Perturbative QCD calculations for cross section ratios · contest 0.94

Compares with it 5 papers

[ref] we show predictions for the azimuthal de-correlation $Δϕ$ for high dijets for different regions of $^leading$ using TMD parton densities with off-shell matrix elements, parton shower and hadronization in comparison with measurements at $√(s) = 7$ TeV in pp collisions at the LHC [ref] .

Calculations with off-shell matrix elements, TMD parton densities and TMD parton showers · Predictions including TMD parton showers · 0.88
These rules are listed in table [ref] and compared to the computational procedures applied in the experimental publications [ref] .

Dijet azimuthal decorrelations for $\Delta \phi_{\rm dijet} < 2\pi/3$ in perturbative QCD · Perturbative QCD calculations for cross section ratios · 0.94
Fig. [ref] shows predictions for the azimuthal de-correlation $Δϕ$ for high dijets for different regions of $^leading$ using TMD parton densities with off-shell matrix elements, parton shower and hadronization in comparison with measurements at $√(s) = 7$ TeV in pp collisions at the LHC [ref] .

QCD theory overview · Monte Carlo with Transverse Momentum Dependent shower · 0.96

Cites it as support 1 paper

Performance studies [ref] have shown that there is excellent agreement between the measured positions of clusters and tracks in data, indicating no systematic misalignment between the calorimeter and inner detector.

ATLAS measurements of the properties of jets for boosted particle searches · Uncertainties on the detector material description and cluster reconstruction · 0.96

Cites it as background 23 papers

QCD phenomenology and in particular jet physics are playing a central role in the physics programme of the first period of the LHC running [ref] .

The dijet cross section with a jet veto · Introduction · 0.96
Following the measurement, the quantity $P$ was also measured by the CMS and ATLAS collaborations [ref] .

Dijet azimuthal decorrelations for $\Delta \phi_{\rm dijet} < 2\pi/3$ in perturbative QCD · Introduction · 0.85
Experiments conducted at the Large Hadron Collider (LHC) involve precise measurements of azimuthal correlations within the context of bosons + jets [ref] and multi-jet [ref] final states.

High-$p_T$ Azimuthal Correlations of Z+jet and Multi-jet Production · Introduction · 0.99

What it cites · 4 sentences without a confident label

Uses the method or tool of 2 papers

The correction factors were determined using two independent MC samples: pythia 6.422 (pythia6) [ref] tune D6T [ref] , and herwig++ 2.4.2 [ref] .

Early LHC Underlying Event Data - Findings and Surprises · Introduction · 0.99
The madgraph generator is based on leading-order matrix element multiparton final-state predictions, using pythia6 for parton showering and hadronization, and the MLM method [ref] to map the parton-level event into a parton shower history.

Matching Parton Showers and Matrix Elements · Introduction · 0.97

Discussion

Disagree with the review, or know something it missed? Leave a comment, highlight any passage above to comment on it, or challenge this review formally.

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