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REVIEW 3 major objections 5 minor 46 references

The ratio of χc2 to χc1 production in proton-lead collisions is flat across multiplicity, rapidity, and pT, and matches proton-proton data, showing no relative modification of the two P-wave charmonium states.

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2026-08-01 16:47 UTC pith:DYPDKXG2

load-bearing objection First midrapidity pPb χc2/χc1 ratio: a careful null result with one systematic worth tightening before publication. the 3 major comments →

arxiv 2607.17847 v1 pith:DYPDKXG2 submitted 2026-07-20 nucl-ex hep-ex

Study of chi_(c) production in pPb collisions at sqrt{s_(NN)} = 8.16 TeV

CMS Collaboration This is my paper
classification nucl-ex hep-ex
keywords charmoniumχc statesproton-lead collisionscold nuclear matterquarkonium suppressionmultiplicity dependenceP-wave charmoniumLHC
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

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

The paper reports the first measurement of the ratio of χc2 to χc1 production cross sections in proton-lead (pPb) collisions at 8.16 TeV. The ratio is found to be independent of event charged-particle multiplicity, rapidity, and transverse momentum, and consistent with pp measurements. This indicates that the cold nuclear medium does not differentiate the two P-wave charmonium states, in contrast to S-wave ψ(2S) suppression. If correct, it implies that quarkonium dissociation in pPb depends primarily on the spatial size of the state, not its spin configuration or fine-structure binding energy.

Core claim

The central result is the χc2-to-χc1 ratio, defined as [σ(χc2)B(χc2→J/ψγ)]/[σ(χc1)B(χc1→J/ψγ)], measured in the rapidity range |y(J/ψ)|<2.4 and pT(J/ψ) between 6.5 and 30 GeV. The ratio is constant within uncertainties as a function of event multiplicity, rapidity, and pT, with constant fits yielding values around 0.22–0.23. It is consistent with existing pp measurements, indicating an absence of strong relative modification of the χc states in pPb collisions. This stands in contrast to the ψ(2S)/J/ψ ratio, which shows a significant multiplicity-dependent suppression.

What carries the argument

The analysis uses the radiative decay χc→J/ψγ, with J/ψ reconstructed via its muon pair and the photon via conversion to an e+e− pair, which provides superior momentum resolution for the soft photon. The ratio is extracted from a fit to the χc invariant mass spectrum using double-sided crystal-ball functions for the two signal peaks and a threshold background function. Acceptance and efficiency corrections are based on Monte Carlo simulation, with the key assumption that the χc states follow the polarization scenario Jz=0 for both states (λθ=+1 for χc1 and −0.6 for χc2), an assumption the paper notes has a noticeable effect on the measured ratio.

Load-bearing premise

The results assume that the χc1 and χc2 polarization in pPb collisions is the same as measured in pp collisions; if the polarization differs, the reported flat ratio could be an artifact of the acceptance correction.

What would settle it

If a future measurement of the χc2/χc1 ratio in pPb collisions using a different polarization assumption (e.g., unpolarized) showed a multiplicity or pT dependence, or if a direct measurement of χc polarization in pPb found values inconsistent with Jz=0, the claim of a flat ratio would be undermined.

Watch this falsifier — get emailed when new claim-graph text bears on it.

If this is right

  • If the flat ratio is confirmed at higher precision, cold nuclear matter effects are effectively flavor-blind between the two P-wave charmonium states.
  • The result provides a new baseline for interpreting χc production in heavy-ion collisions, where a hot medium could differentiate the states.
  • The multiplicity independence constrains models of comover dissociation, which would generally predict a stronger effect for the larger, more weakly bound state.
  • The absence of relative modification between states with nearly identical radii supports size-driven suppression as the dominant mechanism.

Where Pith is reading between the lines

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

  • The fixed-polarization assumption is the main caveat: if χc polarization in pPb differs from the assumed pp-based scenario, the corrected ratio and its differential dependence could be distorted. A direct polarization measurement in pPb would remove this ambiguity.
  • The current systematic uncertainty (16–17%) is dominated by photon conversion efficiency; with more data or improved conversion reconstruction, a genuine few-percent multiplicity dependence could still emerge.
  • A natural extension is to measure the χc0 state, or the bottomonium P-wave states, to test whether the flat ratio reflects a general property of P-wave quarkonia in pPb collisions.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Referee Report

3 major / 5 minor

Summary. The paper reports the ratio R = [σ(χc2)B(χc2→J/ψγ)]/[σ(χc1)B(χc1→J/ψγ)] for prompt χc mesons in pPb collisions at √sNN = 8.16 TeV, using 175 nb−1 of CMS data. The χc states are reconstructed via χc→J/ψγ with J/ψ→μ+μ− and converted photons γ→e+e−. The ratio is measured differentially in N_tracks, y_lab,p(J/ψ), and pT(J/ψ) within |y(J/ψ)|<2.4 and 6.5<pT(J/ψ)<30 GeV. The multiplicity and pT dependences are fitted with constants giving C=0.23±0.03 and C=0.22±0.05, respectively; the rapidity dependence is shown to be flat within uncertainties. The results are compared with LHCb pPb measurements and with ATLAS/CMS pp measurements. The authors conclude that the χc2/χc1 ratio is independent of multiplicity, rapidity, and pT, and consistent with pp, indicating no strong relative cold-nuclear-matter modification of the two P-wave states, in contrast to the ψ(2S)/J/ψ behavior. The analysis includes a detailed systematic assessment (total 16–17%) and uses the Jz=0 polarization scenario for acceptance corrections.

Significance. If the central result survives scrutiny, this is a valuable first CMS midrapidity measurement of the χc2/χc1 ratio in pPb collisions and an important test of models that distinguish quarkonium states by size and binding energy. The paper has clear strengths: the ratio is extracted as a free fit parameter with correlated uncertainties; the photon-conversion reconstruction is cross-checked with a tag-and-probe method; nonprompt contamination is treated with an extrapolation; and the results are provided in HEPData. The main caveat is that the acceptance correction depends on the assumed χc polarization, and no polarization systematic is propagated. Because the adopted λθ values for χc1 and χc2 have opposite signs (+1 and −0.6), a change of the polarization scenario has a state-dependent, noticeable effect on the ratio. This issue is load-bearing for the flatness claim and requires additional quantification before the central conclusion can be regarded as fully supported.

major comments (3)
  1. [Section 6, Table 2] The Jz(χc1)=0, Jz(χc2)=0 polarization scenario (λθ=+1 and −0.6) is used to reweight MC and correct the acceptance, but no polarization uncertainty is included in Table 2. The paper itself states that this assumption 'has a noticeable effect on the χc2-to-χc1 ratio result' because the λθ values have opposite signs. If the true pPb polarization differs from the pp-based assumption, or varies with N_tracks, y, or pT, the corrected ratio—and therefore the observed flatness—could be biased. The unpolarized green line/band in Figs. 2 and 3 is only an alternative endpoint; it is not propagated as a systematic. I request that the authors either (a) constrain the χc polarization in this data sample, e.g., by fitting the dimuon angular distribution in the χc signal region, or (b) assign a polarization systematic from a scan over the λθ values consistent with Refs. [40,41] and show its magnitude in
  2. [Section 7, final paragraph] The statement that the flat trend 'remains robust under various polarization scenarios' is stronger than what is actually shown: only two scenarios (Jz=0 and unpolarized) are presented. A range of intermediate or opposite-sign λθ combinations, including the uncertainty bands of the external polarization measurements, should be evaluated. In addition, the rapidity panel in Fig. 2 is not accompanied by a constant-fit p-value, so the flatness in y is asserted from visual inspection. Please quantify all three differential flatness statements with fit probabilities and include the polarization scan in the paper.
  3. [Section 5.2] The 13% conversion-selection systematic is derived from a single looser alternative selection. Since this is the dominant systematic and the flatness claim concerns differences across bins, the authors should show that the alternative selection yields consistent shapes, not only an overall normalization offset, in the differential distributions. If the 13% is a fully correlated scale uncertainty, that should be stated explicitly; if not, the bin-to-bin variation should be reflected in the systematic uncertainties for the N_tracks, y, and pT points. This is important because a bin-dependent conversion efficiency could, in principle, create or hide a slope in the corrected ratio.
minor comments (5)
  1. [Section 7] Report the p-value of the constant fit for the rapidity dependence, as is done for the N_tracks and pT panels.
  2. [Fig. 3 caption] The phrase 'with the same polarization assumption' is ambiguous: specify whether ATLAS and CMS pp points have been recast to the Jz=0 scenario or are shown under their own stated assumptions.
  3. [Section 5.4] Clarify how the two lifetime selections bracket the nonprompt J/ψ contamination and whether the assumed linear dependence of the χc2/χc1 ratio on contamination was validated with MC; a two-point linear extrapolation is an assumption that should be stated as such.
  4. [Eq. (2)] Define n_L and n_H and specify the normalization convention for the double-sided crystal-ball function; the continuity conditions at ±α are not stated.
  5. [Abstract and Section 7] The pp comparison is made at √s=7 TeV while the pPb measurement is at 8.16 TeV; this energy and phase-space difference should be explicitly acknowledged in the comparison discussion.

Circularity Check

0 steps flagged

No significant circularity: the χc2/χc1 ratio is a direct fit observable, and the cited inputs are external measurements or MC model choices that do not encode the target result.

full rationale

The paper's central quantity, R = [σ(χc2)B(χc2→J/ψγ)]/[σ(χc1)B(χc1→J/ψγ)] (Eq. 1), is extracted as a free parameter in an unbinned fit to the reconstructed m(χc) distribution (Sec. 4.2.1): 'The χc2/χc1 yield ratio is included as a free parameter in the fit, so its statistical uncertainty is obtained directly from the fit result'. No fitted parameter elsewhere is renamed as this prediction, and the flatness claim is simply a constant fit to the measured points. The MC samples are weighted to match pT(J/ψ) and N_tracks distributions observed in data, which is a mild data-to-correction feedback, but reweighting those one-dimensional distributions does not constrain the relative χc2/χc1 yield and therefore does not make the ratio equal to an input by construction. The polarization scenario Jz(χc1)=0, Jz(χc2)=0 is imported from external CMS/LHC measurements (Refs. [40,41]) and from a published angular-distribution procedure (Ref. [44]); it is an external physics assumption and a source of systematic uncertainty, explicitly discussed and compared with the unpolarized case, not a self-citation used to force the result. The reuse of the prior CMS χc selection [20] and the threshold background function is methodological and does not load-bear on the physics conclusion. Thus the derivation chain is self-contained: data → yield fit → efficiency/acceptance corrections → ratio; the main limitations (polarization assumption, 16–17% systematics) are assumptions/uncertainties, not circular reductions.

Axiom & Free-Parameter Ledger

2 free parameters · 4 axioms · 0 invented entities

No new particles, forces, or conserved quantities are introduced. The central claim depends on a direct data fit, standard external branching fractions, a chosen polarization scenario, and MC-based efficiency corrections. The two listed free parameters are model inputs affecting the correction factors, not fitted outputs of the measurement.

free parameters (2)
  • PYTHIA pTHatMin and associated generator settings = pTHatMin = 4.5 GeV (varied 3.0–6.0 GeV); c-quark mass = 1.5 GeV; renormalization/factorization scales = 2–3 mc
    Controls the χc pT spectrum in simulation; acceptance and efficiency corrections are derived from this MC. Nominal values are chosen by hand, not fitted to the measured ratio, and are varied only for systematic uncertainty (§5.3).
  • χc polarization parameters λθ = λθ(χc1) = 1, λθ(χc2) = −0.6 (Jz=0 scenario); unpolarized alternative λθ = 0
    Adopted from external pp polarization measurements [40,41]; the acceptance correction depends on these values and the paper states they shift the χc2/χc1 ratio noticeably (§6). They are not measured in this pPb dataset.
axioms (4)
  • domain assumption Branching fractions B(χc→J/ψγ) from PDG and cancellation of B(J/ψ→μ+μ−)
    The reported ratio is a σ·B ratio; B(χc→J/ψγ) values are external inputs (§1, §4.1).
  • domain assumption Prompt/b feed-down separation via cτ/σ(cτ) < 3 is unbiased
    The lifetime selection is optimized on prompt χc simulation; residual nonprompt contamination is estimated via a linear extrapolation and assigned an 8% systematic (§5.4).
  • domain assumption MC simulation (PYTHIA + EPOS LHC + GEANT4) reliably models acceptance, efficiency, and underlying event after reweighting
    Corrections depend on generator modeling; cross-checked with tag-and-probe conversion efficiency (§5.2) and PYTHIA parameter variations (§5.3).
  • domain assumption EPOS LHC underlying-event simulation adequately describes pPb event activity for N_tracks-dependent corrections
    Efficiency corrections as a function of charged-particle multiplicity rely on simulated multiplicity distributions (§3, §5.3).

pith-pipeline@v1.3.0-alltime-deepseek · 36671 in / 13811 out tokens · 135769 ms · 2026-08-01T16:47:38.088719+00:00 · methodology

0 comments
read the original abstract

Production of prompt P-wave charmonium states $\chi_\mathrm{c1}$(1P) and $\chi_\mathrm{c2}$(1P) is studied in proton-lead (pPb) collisions at a center-of-mass energy per nucleon pair of $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}$ = 8.16 TeV. The analysis is based on data corresponding to an integrated luminosity of 175 nb$^{-1}$ collected by the CMS experiment at the CERN LHC. The $\chi_\mathrm{c}$ states are measured via their decay $\chi_\mathrm{c}$ $\to$ J$/\psi\,\gamma$. The J$/\psi$ meson is reconstructed via its decay to a muon pair, while the photon is reconstructed through its conversion to an electron-positron pair. The ratio of production cross sections times branching fractions for the two charmonium states, [$\sigma(\chi_\mathrm{c2}) \mathcal{B}(\chi_\mathrm{c2}$ $\to$ J$/\psi \, \gamma$)] / [$\sigma(\chi_\mathrm{c1}) \mathcal{B}(\chi_\mathrm{c1}$ $\to$ J$/\psi \, \gamma)$], is reported in the rapidity range $\vert y($J$/\psi)\vert$ $\lt$ 2.4 for the transverse momentum range 6.5 $\lt$ $p_{\mathrm{T}}$(J$/\psi$) $\lt$ 30 GeV. The $\chi_\mathrm{c2}$-to-$\chi_\mathrm{c1}$ ratio is found to be independent of event charged-particle multiplicity, as well as of the rapidity and $p_\mathrm{T}$ of the J$/\psi$. The consistency of this ratio with proton-proton measurements at $\sqrt{s}$ = 7 TeV indicates an absence of strong relative modification for $\chi_\mathrm{c}$ states in pPb collisions, in contrast to the behavior observed in the $\psi$(2S)-to-J$/\psi$ ratio.

Figures

Figures reproduced from arXiv: 2607.17847 by CMS Collaboration.

Figure 1
Figure 1. Figure 1: Examples of fits for χc candidates, integrated over pT (J/ψ) and Ntracks. Left: midra￾pidity bin (0.0 < ylab,p(J/ψ) < 1.0), right: forward rapidity bin (1.6 < ylab,p(J/ψ) < 2.4). The vertical lines of data points represent the statistical uncertainties. The overall fit is shown in blue; the dashed blue line is the background. The individual peak DCB functions are shown in red for χc1 and green for χc2. 5 S… view at source ↗
Figure 2
Figure 2. Figure 2: The χc2-to-χc1 ratio as a function of Ntracks (left) and as a function of rapidity ylab,p (right). The red points are for the Jz (χc1) = 0, Jz (χc2) = 0 scenario, while the green line rep￾resents the unpolarized case. The light green band indicates the difference between the results obtained under the unpolarized and polarization hypotheses. The hashed boxes on the points represent the systematic uncertain… view at source ↗
Figure 3
Figure 3. Figure 3: The χc2-to-χc1 ratio as a function of pT (J/ψ), shown by red points for the Jz (χc1) = 0, Jz (χc2) = 0 polarization assumption. The results are integrated over all the detector accep￾tance |ylab(J/ψ)| < 2.4, corresponding to −2.9 < yCM < 1.9, and over the full charged-particle multiplicity range. The hashed boxes on the points represent the systematic uncertainties. The χc2-to-χc1 ratio is compared with ex… view at source ↗

discussion (0)

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