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J/Psi Elliptic Flow in Pb-Pb Collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV

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ReviewThe objections and verdict, as JSON Paper breakdownThe authors' claims, conditions and measurements, as JSON Signed receiptProof this review has not been altered
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ALICE Collaboration

Nuclear Experiment · Submitted 23 Mar 2013 · 28 references

Pith's verdict: First J/ψ v2 at the LHC, honestly labeled a 2.7–2.9σ indication; the background-shape systematic is the real soft spot, and the paper handles it about as well as these data allow.

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arXiv ID1303.5880 (version 4)
Pith ID5IPCST4W, Pith's permanent record of this paper and its review
CitationALICE Collaboration. "J/Psi Elliptic Flow in Pb-Pb Collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV." arXiv:1303.5880 (2013). https://pith.science/paper/5IPCST4W
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The paper

The paper claims

ALICE reports the first measurement of J/ψ elliptic flow in Pb-Pb collisions at the LHC, finding a 2.7σ indication that the meson's azimuthal anisotropy is non-zero at intermediate transverse momentum.

The reading

The paper asks whether the J/ψ meson, a bound charm-anticharm pair, participates in the collective azimuthal motion of the medium created in Pb-Pb collisions. In the standard picture there are two ways to make an observed J/ψ: it can survive from the initial hard scattering, in which case it picks up only a small anisotropy from the different path lengths in and out of the reaction plane, or it can be regenerated from charm quarks that have themselves thermalized in the quark-gluon plasma, in which case it inherits the charm quarks' large elliptic flow $v_2$. ALICE measures inclusive J/ψ $v_2$ at forward rapidity with the muon spectrometer and the VZERO-A event plane, and finds a maximum of $v_2 = 0.116$ for $2 < p_T < 4$ GeV/$c$ in 20%–40% central collisions, with a combined significance of about 2.7σ over $2 < p_T < 6$ GeV/$c$ and 2.9σ when the 5%–20% and 20%–40% centrality bins are combined. If this reading holds, a sizeable fraction of the observed J/ψ at low $p_T$ must come from charm quarks that have lost their memory of the original production direction, which is exactly what regeneration models predict and what the earlier ALICE nuclear modification factor alone could not distinguish.

What carries the argument

The measurement rests on two pieces of machinery. The first is the event-plane method: the second-harmonic plane angle $Ψ$ is reconstructed from the VZERO-A scintillator array only, deliberately excluding VZERO-C because it overlaps in acceptance with the muon spectrometer and would introduce autocorrelations; the resulting resolution, from 0.185 in the 60%–90% class to 0.610 in 20%–40%, is corrected for and carries a 1.3% correlated systematic. The second is the fit of $v_2$ as a function of dimuon invariant mass, $v_2(m_{μμ}) = v_2^{sig} α(m_{μμ}) + v_2^{bkg}(m_{μμ})[1 - α(m_{μμ})]$, in which the signal fraction $α = S/(S+B)$ comes from a Crystal Ball plus continuum fit to the opposite-sign mass spectrum and $v_2^{bkg}$ is parametrized by a polynomial. The physics mechanism the numbers are weighed against is the competition between primordial J/ψ, which acquire only a small anisotropy from in-plane versus out-of-plane path lengths, and regenerated J/ψ, which inherit the full elliptic flow of thermalized charm quarks.

Extended reading notes

Core claim

The paper reports the first measurement of inclusive J/ψ elliptic flow in heavy-ion collisions at the LHC, using $17$ million dimuon-triggered Pb-Pb events at $2.76$ TeV in the rapidity range $2.5 < y < 4.0$. The elliptic flow is extracted as $v_2 = 2.7$–$2.9$σ non-zero in semi-central collisions: $v_2 = 0.116$ for $2 < p_T < 4$ GeV/$c$ in the 20%–40% class, with the broader $1.5 < p_T < 10$ GeV/$c$ sample giving $0.101$ (5%–20%) and $0.116$ (20%–40%). Peripheral collisions (40%–60% and 60%–90%) are consistent with zero, and the reconstructed mean transverse momentum stays in the narrow band 3.0–3.3 GeV/$c$ across centralities, so the centrality trend is not a $p_T$ migration artefact. The authors read this pattern as favouring a regeneration component: transport calculations in which about 30% of the observed J/ψ in the 20%–60% range come from deconfined charm quarks reproduce both the shape of $v_2(p_T)$ and the previously published nuclear modification factor, whereas a purely primordial population would give $v_2$ near 0.02 at $p_T ≈ 2.5$ GeV/$c$.

Load-bearing premise

The extraction assumes that the azimuthal anisotropy of the dimuon background as a function of invariant mass is captured by the three parametrizations tried, a second-order polynomial, a first-order polynomial and its inverse, and that the largest spread among them is a conservative enough systematic; the authors themselves identify this background-shape uncertainty as the dominant one, so a true background anisotropy outside that family would shift the fitted signal and move the significance with it.

Editorial extensions

If this is right

  • The inclusive J/ψ nuclear modification factor measured earlier by ALICE, which is larger than at RHIC and shows no strong centrality dependence, gains a companion observable that points the same way: low-$p_T$ J/ψ are not simply the survivors of suppression.
  • Charm quarks must be substantially thermalized in the medium at LHC energies, since regeneration can only transfer flow that the charm quarks themselves carry.
  • The qualitative difference from the STAR measurement at $\sqrt{s_{NN}} = 200$ GeV, which is compatible with zero for $p_T \geq 2$ GeV/$c$, becomes a genuine energy dependence of the charm-sector dynamics rather than a statistical fluctuation.
  • Transport models with regeneration, which already described the nuclear modification factor, are now constrained on a second, independent axis and survive that test.
  • Non-prompt J/ψ from b-hadron decays, at roughly 7%–11% of the sample, could themselves carry flow if bottom quarks thermalize, so the measured value is a lower bound on the prompt component's anisotropy under that hypothesis.

Reading between the lines

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

  • If the effect is real, the natural next discriminant is the $p_T$ at which $v_2$ turns over: regeneration models place the maximum near $2.5$ GeV/$c$ because primordial J/ψ take over above it, so a measured turnover position that moves with centrality would be hard to accommodate without tuning the regeneration fraction.
  • The same event-plane machinery applied to the $\Upsilon$ family would test whether the mechanism scales with quark mass as thermalization arguments suggest, since bottom quarks are heavier and are expected to flow less at a given $p_T$.
  • A data-driven continuum anisotropy, taken from same-sign dimuons rather than from a fitted polynomial family, would shrink the dominant systematic and could turn a 2.7σ indication into a measurement; the paper already notes that same-sign pairs follow the opposite-sign continuum trend.
  • Because the flow is inherited rather than generated by the J/ψ itself, a correlated measurement of open-charm $D$-meson $v_2$ in the same centrality and $p_T$ bins would either confirm a common parent anisotropy or expose a mismatch that regeneration alone cannot explain.
Original abstract

We report on the first measurement of inclusive J/$\psi$ elliptic flow, $v_2$, in heavy-ion collisions at the LHC. The measurement is performed with the ALICE detector in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV in the rapidity range $2.5 < y < 4.0$. The dependence of the J/$\psi$ $v_2$ on the collision centrality and on the J/$\psi$ transverse momentum is studied in the range $0 < p_{\rm T} < 10$ GeV/$c$. For semi-central Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV, an indication of non-zero $v_2$ is observed with a maximum value of $v_2 = 0.116 \pm 0.046 {\rm (stat.)} \pm 0.029 {\rm (syst.)}$ for J/$\psi$ in the transverse momentum range $2 < p_{\rm T} < 4$ GeV/$c$. The elliptic flow measurement complements the previously reported ALICE results on the inclusive J/$\psi$ nuclear modification factor and favors the scenario of a significant fraction of J/$\psi$ production from charm quarks in a deconfined partonic phase.

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, 4,720 input tokens, $0.0002 reported.

What the abstract itself states, as probabilities: claims first 0.96 · states limitation 0.35 · human participants 0.01 · controlled comparison 0.07 · randomized assignment 0.01 · states quantitative result 0.99 · compares with prior results 0.85 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v4. Results and the main claim are underlined; .

We report on the first measurement of inclusive J/$\psi$ elliptic flow, $v_2$, in heavy-ion collisions at the LHC. The measurement is performed with the ALICE detector in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV in the rapidity range $2.5 < y < 4.0$. The dependence of the J/$\psi$ $v_2$ on the collision centrality and on the J/$\psi$ transverse momentum is studied in the range $0 < p_{\rm T} < 10$ GeV/$c$. For semi-central Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV, an indication of non-zero $v_2$ is observed with a maximum value of $v_2 = 0.116 \pm 0.046 {\rm (stat.)} \pm 0.029 {\rm (syst.)}$ for J/$\psi$ in the transverse momentum range $2 < p_{\rm T} < 4$ GeV/$c$. The elliptic flow measurement complements the previously reported ALICE results on the inclusive J/$\psi$ nuclear modification factor and favors the scenario of a significant fraction of J/$\psi$ production from charm quarks in a deconfined partonic phase.

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 report on the first measurement of inclusive J/$\psi$ elliptic flow, $v_2$, in heavy-ion collisions at the LHC. contribution 1.00 0.97 0.11 asserted 0.99
s1 The measurement is performed with the ALICE detector in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV in the rapidity range $2.5 < y < 4.0$. method 0.99 0.94 0.02 asserted 1.00
s2 The dependence of the J/$\psi$ $v_2$ on the collision centrality and on the J/$\psi$ transverse momentum is studied in the range $0 < p_{\rm T} < 10$ GeV/$c$. contribution 0.62 0.89 0.03 asserted 0.93
s3 For semi-central Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV, an indication of non-zero $v_2$ is observed with a maximum value of $v_2 = 0.116 \pm 0.046 {\rm (stat.)} \pm 0.029 {\rm (syst.)}$ for J/$\psi$ in the transverse momentum range $2 < p_{\rm T} < 4$ GeV/$c$. result 1.00 0.97 0.44 hedged possibility 0.94
s4 The elliptic flow measurement complements the previously reported ALICE results on the inclusive J/$\psi$ nuclear modification factor and favors the scenario of a significant fraction of J/$\psi$ production from charm quarks in a deconfined partonic phase. interpretation 1.00 0.59 0.74 hedged possibility 0.62

What kind of paper the classifier reports this is: empirical study 1.00 · evidence from observation or experiment 0.99.

What was examined, and what was not

sources read
  • arXiv listing abstract v4
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. (color online) Invariant mass distribution (a) and hcos2(φ − Ψ)i as a function of mµµ (b) of OS dimuons with 2 ≤ pT < 4 GeV/c and 2.5 < y < 4 in semi-central (20%–40%) Pb-Pb collisions. continuum parametrizations described before were tested to assess the signal and the related systematic uncertainties. The J/ψ v2 and its statistical uncertainty in each pT and centrality class were determined as the average of the v sig 2 obtained by fitting v2(mµµ) using Eq. 1 with the various α(mµµ), while the corresponding systematic uncertainties were defined as the RMS of these results
Figure 2. (color online) Inclusive J/ψ v2(pT) for semi-central (20%–40%) Pb-Pb collisions at √ sNN = 2.76 TeV (see text for details on uncertainties). The used pT ranges are: 0–2, 2–4, 4–6 and 6–10 GeV/c. the time projection chamber (TPC), with pseudo-rapidity gaps ∆ηV0A−TPC=1.9, ∆ηV0A−V0C=4.5 and ∆ηTPC−V0C=0.8; second, VZERO-A, ring 0 of VZERO-C and VZERO-C-3rd ring, with pseudo-rapidity gaps ∆ηV0A−V0C0=6.0, ∆ηV0C0−V0C3=1.0 and ∆ηV0A−V0C3=4.5. The differences between the EP reso￾lution for VZERO-A obtained from these two sets of sub-events are taken as systematic uncertainties. Since v2 is…
Figure 3. (color online) v2 (a) and hpTi uncor (b) of inclusive J/ψ with 1.5 ≤ pT < 10 GeV/c as a function of hNparti in Pb-Pb collisions at √ sNN = 2.76 TeV (see text for details on uncertainties)
Figure 4. (color online) Inclusive J/ψ v2(pT) for non-central (20%–60%) Pb-Pb collisions at √ sNN = 2.76 TeV (see text for details on uncertainties). The used pT ranges are: 0–2, 2–4, 4–6 and 6–10 GeV/c. Calculations from two transport models [26] and [14] are also shown (see text for details). all centralities. Thus, the observed centrality dependence of the v2 for inclusive J/ψ with 1.5 ≤ pT < 10 GeV/c does not result from any bias in the sampled pT distributions. For J/ψ with pT < 1.5 GeV/c (not shown), the v2 is compatible with zero within one standard deviation for the four centrality…

Figures reproduced from arXiv:1303.5880 by the authors.

References
How later papers used it

48 later papers cite it

Evidence: substantial. 34 of the citing papers share no author with it, and 6 of those carry a confident label from Pith's citation classifier.

Pith read 19 of the 53 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 1 of 4 papers with this label · 1 by the same authors

By the same authors

How the literature uses it

46 arXiv papers cite this in 51 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 whose main result relates to this one's

Found among the papers closest to this one by title and main claim. Pith's classifier compared each pair's main results and a second model family confirmed the relation; checked against two further families, shown conflicts agreed 118 times in 120 and shown same results 218 times in 239. The quoted sentence is the other paper's main claim.

States the same result 1 paper

Papers citing it · 40 sentences without a confident label

Uses its method or tool 1 paper
Uses its data or values 1 paper
Compares with it 3 papers
Cites it as support 3 papers
Cites it as background 3 papers

What it cites · 10 sentences without a confident label

Uses the method or tool of 6 papers
Uses data or values from 1 paper
Compares with 2 papers
  • This trend seems qualitatively different from that of the STAR measurement [ref] at lower collision energy, which is compatible with zero for $p_T ≥2$ GeV/$c$ albeit in somewhat different (10%–40% and 0%–80%) centrality ranges.

    Measurement of J/{psi} Azimuthal Anisotropy in Au+Au Collisions at {sqrt{s_{NN}}} = 200 GeV · Introduction · 1.00

  • The inclusive ȷ reported is larger than that measured at the SPS [ref] and at RHIC [ref] for central collisions and does not exhibit a significant centrality dependence.

    1103.6269 · Introduction · 0.99

Cites as background 1 paper

Discussion

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