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Event shapes and azimuthal correlations in Z + jets events in pp collisions at sqrt(s) = 7 TeV

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CMS Collaboration

High Energy Physics - Experiment · Submitted 8 Jan 2013 · 42 references

Pith's verdict: A competent CMS measurement whose genuinely new ingredient is the transverse thrust in Z+jets; the unfolding prior is the one soft spot worth pressing on.

T0 · AI review

How this review was made: An AI referee (selfhosted/deepseek-v4.1-flash) read the paper on November 10, 2026 and answered a fixed list of questions about it. T0 is the starting level every paper gets. The levels describe how deeply Pith has checked the paper, never the authors.

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  4. T3 Formal statement The claims written out in a formal math language a computer can read.
  5. T4 Proof Those claims proved, and the proof checked by computer.

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arXiv ID1301.1646 (version 2)
Pith ID46SJGRXD, Pith's permanent record of this paper and its review
CitationCMS Collaboration. "Event shapes and azimuthal correlations in Z + jets events in pp collisions at sqrt(s) = 7 TeV." arXiv:1301.1646 (2013). https://pith.science/paper/46SJGRXD
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The paper

The paper claims

CMS tests whether the standard simulations of a Z boson plus jets get the shape of the event right, not just the rate.

The reading

The CMS collaboration asks whether the Monte Carlo tools used to simulate a Z boson produced together with jets actually reproduce the topology of those events, not merely how often they occur. Using 5.0 fb$^{-1}$ of 7 TeV proton-proton collisions, the analysis measures the azimuthal angles between the Z and the leading jets, between pairs of jets, and the transverse thrust $-$ a single variable, written $1-|1-2/|$, that runs toward $-1$ for a spherical many-jet event and toward $-|$ for a clean back-to-back Z plus one jet. All distributions are unfolded back to particle level so that any model can be compared with them directly, and they are split by jet multiplicity and by whether the Z carries more than 150 GeV of transverse momentum, the boosted regime that dominates missing-energy searches. The payoff is a shape-level stress test: a generator that gets the inclusive rate right can still fail here, and the paper reports which ones do and by how much.

What carries the argument

The load-bearing object is the transverse thrust, $1-|1-2/|$, built from the transverse momenta of the Z and every accepted jet, with the thrust axis chosen to maximize the projected sum; it compresses an entire event topology into one number. Paired with it are the azimuthal separations $|(Z,j_i)$ and $|(j_i,j_k)$ for the three leading jets, which isolate the same physics channel by channel. The measurements are made comparable to theory by a singular-value-decomposition unfolding, driven by a response matrix from MADGRAPH and cross-checked against SHERPA, which maps detector-level counts onto particle-level distributions and absorbs migrations of up to 30% between jet-multiplicity bins.

Extended reading notes

Core claim

The unfolded, normalized distributions agree with MADGRAPH, SHERPA and POWHEG (Z + 1 jet at NLO) at roughly the 10% level over most of the measured range, while stand-alone PYTHIA disagrees by more than 20% in $|$ and predicts too large a share of back-to-back Z plus one-jet events at small $|$. In the azimuthal variable with the largest systematic uncertainty, $|(Z,j_1)$, MADGRAPH sits closest to the data, SHERPA falls about 10% low and POWHEG about 10% high, and these differences shrink as the inclusive jet multiplicity rises to two and three jets. The most striking result is that POWHEG, which is next-to-leading order only for the leading jet and generates everything beyond it from parton showers, still describes the $N_{jets}ge 3$ azimuthal distributions, including those between subleading jets. For $p_T^Z > 150$ GeV the angular distributions become more isotropic with increasing jet multiplicity, and PYTHIA improves there, consistent with the larger phase space available for parton emission.

Load-bearing premise

The correction that removes detector effects rests on a response matrix built from MADGRAPH and cross-checked only against SHERPA, so a jet response that both generators get wrong in the same way would pass into the unfolded distributions as an unaccounted bias.

Editorial extensions

If this is right

  • Multiparton leading-order matrix elements merged with parton showers, and next-to-leading-order Z + 1-jet matched to showers, are adequate shape-level descriptions of Z + jets at the 10% level, so they can be trusted as background models for searches in this kinematic region.
  • Stand-alone parton-shower PYTHIA is not adequate for the thrust and leading-jet azimuthal distributions at low Z transverse momentum, which sets a concrete limit on where shower-only simulations may be used for background estimation.
  • The agreement of POWHEG at $N_{jets}ge 3$ indicates that, for these observables, the parton shower is doing the multi-jet job well enough that a full multi-leg NLO calculation is not required.
  • The measured distributions are particle-level and generator-independent, so they can be used directly to tune or discriminate any future QCD model without redoing the detector correction.

Reading between the lines

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

  • Because the same shapes are the ones that control the $Z ightarrow uu$ background in missing-transverse-momentum searches, the quoted 10% generator agreement can be read as an estimate of the systematic floor on that background when it is modeled with these tools.
  • The variables measured here are defined from the Z plus jets system alone, so the same machinery extends naturally to W + jets and to dijet events, where no leptonically decaying boson is available to fix the reference axis.
  • A generator that reproduces the thrust distribution but misses $|(j_i,j_k)$ would point to a defect in the treatment of soft radiation rather than in the hard matrix element, which makes the joint set of distributions a sharper diagnostic than any single one.
Original abstract

Measurements of event shapes and azimuthal correlations are presented for events where a Z boson is produced in association with jets in proton-proton collisions. The data collected with the CMS detector at the CERN LHC at sqrt(s) = 7 TeV correspond to an integrated luminosity of 5.0 inverse femtobarns. The analysis provides a test of predictions from perturbative QCD for a process that represents a substantial background to many physics channels. Results are presented as a function of jet multiplicity, for inclusive Z boson production and for Z bosons with transverse momenta greater than 150 GeV, and compared to predictions from Monte Carlo event generators that include leading-order multiparton matrix-element (with up to four hard partons in the final state) and next-to-leading-order simulations of Z + 1-jet events. The experimental results are corrected for detector effects, and can be compared directly with other QCD models.

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, 4,388 input tokens, $0.0002 reported.

What the abstract itself states, as probabilities: claims first 0.03 · states limitation 0.10 · human participants 0.01 · controlled comparison 0.13 · randomized assignment 0.01 · states quantitative result 0.36 · compares with prior results 0.84 · mentions code or data availability 0.02.

The abstract, sentence by sentence

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

Measurements of event shapes and azimuthal correlations are presented for events where a Z boson is produced in association with jets in proton-proton collisions. The data collected with the CMS detector at the CERN LHC at sqrt(s) = 7 TeV correspond to an integrated luminosity of 5.0 inverse femtobarns. The analysis provides a test of predictions from perturbative QCD for a process that represents a substantial background to many physics channels. Results are presented as a function of jet multiplicity, for inclusive Z boson production and for Z bosons with transverse momenta greater than 150 GeV, and compared to predictions from Monte Carlo event generators that include leading-order multiparton matrix-element (with up to four hard partons in the final state) and next-to-leading-order simulations of Z + 1-jet events. The experimental results are corrected for detector effects, and can be compared directly with other QCD models.

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 Measurements of event shapes and azimuthal correlations are presented for events where a Z boson is produced in association with jets in proton-proton collisions. contribution 0.99 0.94 0.06 asserted 1.00
s1 The data collected with the CMS detector at the CERN LHC at sqrt(s) = 7 TeV correspond to an integrated luminosity of 5.0 inverse femtobarns. method 1.00 0.79 0.02 asserted 1.00
s2 The analysis provides a test of predictions from perturbative QCD for a process that represents a substantial background to many physics channels. contribution 0.38 0.52 0.09 asserted 0.96
s3 Results are presented as a function of jet multiplicity, for inclusive Z boson production and for Z bosons with transverse momenta greater than 150 GeV, and compared to predictions from Monte Carlo event generators that include leading-order multiparton matrix-element (with up to four hard partons in the final state) and next-to-leading-order simulations of Z + 1-jet events. result 0.41 0.90 0.12 asserted 0.99
s4 The experimental results are corrected for detector effects, and can be compared directly with other QCD models. method 0.93 0.89 0.06 asserted 1.00

What kind of paper the classifier reports this is: empirical study 1.00 · evidence from observation or experiment 0.86.

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. Topology of Z + jets events: (a) for ln τT → −∞ and ∆φ(Z, j1) → π; (b) for ln τT → −1 and ∆φ(Z, j1) π. To investigate the dependence of the topological properties on the complexity of the final state, the events are categorized as a function of jet multiplicity. In particular, the azimuthal distri￾butions are reported in inclusive bins of one, two, or three jets. Furthermore, the phase space is characterized according to the pT of the Z boson, and measurements performed either for all p Z T or in the region of p Z T > 150 GeV
Figure 2. Distributions for Z → µµ candidate events in data, compared with expectations from simulated signal and background contributions using MADGRAPH simulations normalized to the integrated luminosity of the data: (a) as a function of associated jet multiplicity Njets, and (b) as a function of pT of the dimuon pair (p µµ T ) for Njets ≥ 1. The dibosons WW, WZ, ZZ and W + jets backgrounds are collectively denoted as EW in the legends. The plots in (c) and (d) show the ratios of the data to predictions from MC. The error bars on the data points represent only their statistical uncertainties
Figure 3. Distributions for Z → ee candidate events in data, compared with expectations from simulated signal and background contributions using MADGRAPH simulations normalized to the integrated luminosity of the data: (a) as a function of associated jet multiplicity Njets, and (b) as a function of pT of the dielectron pair (p ee T ) for Njets ≥ 1. The dibosons WW, WZ, ZZ and W + jets backgrounds are collectively denoted as EW in the legends. The plots in (c) and (d) show the ratios of the data to predictions from MC. The error bars on the data points represent only their statistical uncertain
Figure 4. Normalized ∆φ(Z, j1) distributions for the leading jet in the inclusive jet-multiplicity bins Njets ≥ 1, ≥ 2, and ≥ 3: (a) all p Z T and (b) p Z T > 150 GeV. Plots in (c) and (d) show the corresponding ratios of the data (solid points), and of other MC predictions, relative to MADGRAPH. The ratio for PYTHIA MC is not included in these plots. The error bars on the data points represent their statistical uncertainties, the solid yellow shaded band around the points represents the sum of statistical and systematic uncertainties taken in quadrature, while the cross-hatched (cyan) bands
Figure 5. Normalized ∆φ(Z, ji) distributions for the inclusive Njets ≥ 3 jet-multiplicity bin: (a) all p Z T and (b) p Z T > 150 GeV. Plots in (c) and (d) show the ratios of the data and other MC predictions, relative to MADGRAPH, as described in
Figure 6. Normalized ∆φ(ji , jj) distributions for the inclusive Njets ≥ 3 jet-multiplicity bin: (a) all p Z T and (b) p Z T > 150 GeV. Plots in (c) and (d) show the ratios of the data and other MC predictions, relative to MADGRAPH, as described in
Figure 7. Normalized distributions in ln τT for (a) all p Z T and Njets ≥ 1 data, and (b) for p Z T > 150 GeV and Njets ≥ 1. Plots in (c) and (d) show the ratios of the data and other MC predictions, relative to MADGRAPH, as described in

Figures reproduced from arXiv:1301.1646 by the authors.

References
How later papers used it

45 later papers cite it

Evidence: substantial. 35 of the citing papers share no author with it, and 5 of those carry a confident label from Pith's citation classifier.

Pith read 14 of the 61 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

Compares with it 2 of 5 papers with this label

  • Compared its theoretical calculation of the azimuthal angle decorrelation between the boson and the leading jet with the experimental result at sqrt(s)=7 TeV in this paper.

    Resummation of Boson-Jet Correlation at Hadron Colliders (2019) · 0.99 · paraphrased: the licence does not allow quoting

  • In this section we present our results for the resummed distribution $1/σ σ̣/δ̣ϕ$ and compare our findings with parton shower results obtained with various MC event generators and with experimental data from the CMS collaboration [ref] .

    Azimuthal decorrelation between a jet and a Z boson at hadron colliders (2022) · Comparison to parton showers and experimental data · 0.96

How the literature uses it

44 arXiv papers cite this in 60 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 · 32 sentences without a confident label

Compares with it 5 papers
Cites it as background 18 papers

What it cites · 2 sentences without a confident label

Uses the method or tool of 1 paper
  • The unfolding of data to the particle level is performed using the Singular Value Decomposition method [ref] , implemented in the RooUnfold package [ref] .

    Unfolding algorithms and tests using RooUnfold · Analysis procedure and systematic uncertainties · 1.00

Uses data or values from 1 paper

Discussion

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