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arxiv: 2606.00171 · v1 · pith:ASPFPVCNnew · submitted 2026-05-29 · ⚛️ nucl-ex · hep-ex· nucl-th

Dependence of two-particle azimuthal correlations on the forward rapidity gap width in pPb collisions at sqrt{s_NN} = 8.16 TeV

CMS Collaboration This is my paper

Pith reviewed 2026-06-28 19:55 UTC · model grok-4.3

classification ⚛️ nucl-ex hep-exnucl-th
keywords pPb collisionsazimuthal correlationsrapidity gapV_nDeltacollective flowphoton-lead interactionspomeron-lead interactionssmall systems
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The pith

pPb events selected by forward rapidity gaps exhibit non-zero V_nΔ coefficients with standard dependence on pT and multiplicity.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper measures the Fourier coefficients V_nΔ of two-particle azimuthal correlations in pPb collisions at 8.16 TeV. Events are selected by requiring no particles in the proton-going forward region, which enriches the sample for photon-lead and pomeron-lead interactions. The coefficients are reported as functions of the forward rapidity gap width, particle transverse momentum, and event multiplicity, and compared to earlier results from pp, pPb, and γp+IPp collisions plus modern generators.

Core claim

The measurements establish that V_nΔ remain non-zero in the gap-selected pPb sample and display similar dependence on gap width, pT, and multiplicity as seen in other small collision systems.

What carries the argument

The forward rapidity gap width, the rapidity interval containing no detected particles, used to enrich the pPb sample for γPb and IP Pb interactions.

If this is right

  • V_nΔ dependence on gap width, pT, and multiplicity can be compared directly to results from hadronic and photon-induced collisions.
  • Similarity to previous small-system data suggests the correlations are not unique to standard hadronic pPb interactions.
  • Discrepancies with event generators point to needed refinements in modeling azimuthal correlations in photon- and pomeron-induced processes.
  • The data constrain models that attribute the correlations to collective flow versus other mechanisms.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • If the gap selection isolates γPb and IP Pb events as intended, collective-like azimuthal correlations can appear even when one participant is a photon or pomeron rather than a full proton.
  • Varying the gap width offers a handle on the longitudinal extent of the interaction region that could be tested in future analyses.
  • Repeating the measurement at different center-of-mass energies would test whether the observed patterns scale with available phase space.

Load-bearing premise

Requiring no particles in the proton-going region successfully enriches the pPb sample in photon-lead and pomeron-lead interactions.

What would settle it

If V_nΔ values in the largest-gap events fall to zero while remaining finite in smaller-gap or inclusive events, the claim of persistent correlations in the enriched sample would be falsified.

Figures

Figures reproduced from arXiv: 2606.00171 by CMS Collaboration.

Figure 1
Figure 1. Figure 1: Sketch of a single-diffractive pPb event (left). The interaction proceeds via the ex [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: The Noffline trk spectra for inclusive pPb events (labeled “pPb”, from Ref. [45]) and events from the nominal sample in different ∆η F bins. The data points are located in the center of the bin of their corresponding Noffline trk category. The vertical bars indicate the size of the statistical uncertainties. the γp case [45]. In [PITH_FULL_IMAGE:figures/full_fig_p007_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: The Noffline trk spectra for inclusive pPb events (labeled “pPb”, from Ref. [45]) and inclu￾sive diffraction-enhanced events (labeled “Data (diff. enh.)”). The distribution for the inclusive diffraction-enhanced events is the same as that shown in orange in [PITH_FULL_IMAGE:figures/full_fig_p008_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: The 2D correlation distribution for multiplicity (left) and its projection on the [PITH_FULL_IMAGE:figures/full_fig_p009_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Summary of measurements of V1∆ (left) and V2∆ (right) as a function of ∆η F . The results are for tracks with 0.3 < p trig T < 3.0 GeV. The filled circles indicate the data. The open triangles indicate the simulation, in blue for PYTHIA 8.3.10 and red inverse-triangles for EPOS￾LHC. The data points are plotted at the center of the corresponding ∆η F bin. The height of the shaded bands indicates the systema… view at source ↗
Figure 6
Figure 6. Figure 6: Summary of measurements of V1∆ (left) and V2∆ (right) for the diffraction-enhanced events for different classes of N |η|<2.4, pT>0.3 GeV ch . The results are for tracks with 0.3 < p trig T < 3.0 GeV. The filled circles indicate the data. The open triangles indicate the simulation, in blue for PYTHIA 8.3.10 and red inverse triangles for EPOS-LHC. The data points are plotted at the mean of N |η|<2.4, pT>0.3 … view at source ↗
read the original abstract

One of the most striking features of relativistic heavy ion collisions is the presence of collective flow of thousands of produced particles. This flow can be characterized by the Fourier coefficients (${V_{n\Delta}}$) of the azimuthal angular distributions of charged particles, and its existence can be explained by the formation of a quark gluon plasma, which behaves as a fluid. Surprisingly, the angular distributions of particles from very small systems such as proton-lead (pPb), proton-proton (pp), electron-positron, and photon-proton ($\gamma$p) collisions also exhibit non-zero Fourier coefficients, raising the question of whether collective flow is present. This paper presents measurements of $V_{n\Delta}$ from a sample of pPb events at $\sqrt{s_\mathrm{NN}}$ = 8.16 TeV that are enriched in photon-lead ($\gamma$Pb) and pomeron-lead ($\mathrm{{\!I\!P}}$Pb) interactions by requiring no particles in the proton-going region. Measurements are made as a function of the forward rapidity gap width (the rapidity range in which no particles are found), the transverse momentum of the particles, and the multiplicity of particles in the event. The results are compared to previous measurements of pp, pPb, and $\gamma$p+$\mathrm{\!I\!P}$p events as well as modern event generators.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 0 minor

Summary. The manuscript reports measurements of two-particle azimuthal Fourier coefficients V_nΔ in pPb collisions at √s_NN = 8.16 TeV. Events are selected by requiring no particles in the proton-going region to enrich the sample for photon-lead (γPb) and pomeron-lead (IP Pb) interactions. Results are shown versus forward rapidity gap width, particle p_T, and event multiplicity, with comparisons to prior pp, pPb, and γp+IPp data plus modern event generators.

Significance. If the enrichment procedure is validated, the measurements would help clarify whether azimuthal correlations observed in small systems arise in photon-induced or diffractive processes, thereby constraining models of collectivity. Direct comparisons to other collision systems and generators provide additional context for the field.

major comments (1)
  1. [Event selection and sample enrichment (abstract and corresponding methods section)] The central analysis relies on the assumption that the 'no particles in the proton-going region' requirement successfully enriches the sample in γPb and IP Pb interactions. However, no purity, efficiency, or residual hadronic pPb contamination fractions are reported as a function of gap width (or versus multiplicity). This directly affects the interpretation of the reported V_nΔ(p_T, multiplicity, gap width) values and the comparisons to γp+IPp data.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their careful reading of the manuscript and for identifying this key point regarding sample enrichment. We address the comment below.

read point-by-point responses

  1. Referee: [Event selection and sample enrichment (abstract and corresponding methods section)] The central analysis relies on the assumption that the 'no particles in the proton-going region' requirement successfully enriches the sample in γPb and IP Pb interactions. However, no purity, efficiency, or residual hadronic pPb contamination fractions are reported as a function of gap width (or versus multiplicity). This directly affects the interpretation of the reported V_nΔ(p_T, multiplicity, gap width) values and the comparisons to γp+IPp data.

    Authors: We agree that explicit quantification of purity, efficiency, and residual hadronic contamination would strengthen the interpretation. The manuscript does not currently report these fractions. The forward rapidity gap selection is presented as a function of gap width to show the evolution of the measured V_nΔ, serving as an indirect indicator of enrichment. In the revised manuscript, we will add Monte Carlo-based estimates of the sample composition, including residual contamination levels as a function of gap width (and, where feasible, multiplicity) to the methods section, along with a discussion of how this affects the comparisons to γp+IPp data. revision: yes

Circularity Check

0 steps flagged

No circularity: direct experimental measurements only

full rationale

This is a pure experimental measurement paper reporting V_nΔ from selected pPb events. No derivation chain, fitted parameters, or predictions exist that reduce by the paper's own equations to prior inputs. The gap-based event selection is an experimental cut whose purity is not claimed to be derived internally; comparisons to other data and generators are external benchmarks. No self-citation load-bearing theorems or ansatze are invoked. The result is self-contained against external data.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Experimental measurement paper; relies on standard detector corrections and background subtraction procedures typical of the field rather than new parameters or entities.

axioms (1)
  • domain assumption Standard assumptions in heavy-ion collision data analysis such as detector efficiency corrections and background subtraction hold.
    Invoked implicitly for any V_nΔ extraction; not detailed in abstract.

pith-pipeline@v0.9.1-grok · 5787 in / 1213 out tokens · 27008 ms · 2026-06-28T19:55:13.103685+00:00 · methodology

discussion (0)

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