{"id":"1abed831-9208-4f3d-b7d9-fab8a8dc95ab","arxiv_id":"2609.14604","paper_version":1,"verdict":"ACCEPT","confidence":"HIGH","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":2,"one_line_summary":"STAR reports a negative second-order modulation A_2^phot = -0.39 +/- 0.11 (stat.) +/- 0.04 (sys.) for J/psi at low transverse momentum in Ru+Ru and Zr+Zr collisions, indicating that the J/psi polarization is correlated with the event plane.","lead":"In hadronic heavy-ion collisions, the STAR Collaboration measured the decay angular distribution of photon-induced J/psi particles relative to the collision event plane. The result shows a negative modulation consistent with linearly polarized photoproduction, providing a new direct probe of collision geometry.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek/deepseek-v4-flash","headline":"The hadronic-background subtraction assumes the Tsallis fit extrapolation is accurate and that A_had_2 is constant across p_T; systematic checks are partial and the potential photoproduction contamination in the fit region is not quantified.","rationale":"The reader's verdict is ACCEPT with medium correctness risk, and the reader's weakest_assumption concerns the hadronic subtraction — which is precisely the most load-bearing concern I identify. The paper includes systematic checks that show robustness to some variations (e.g., changing the Tsallis functional form or setting A_had_2 = 0), indicating that the result is not trivially driven by the assumption. However, the potential photoproduction contamination in the fit region and the assumption of constant A_had_2 are not fully quantified. The proposed test would provide a data-driven validation and reduce model dependence. Since the concern does not invalidate the result and the paper already acknowledges limitations, the verdict remains unchanged.","tokens_in":11718,"tokens_out":4441,"duration_ms":43081,"concrete_test":"Recompute A2_phot using an alternative method: (1) Determine f_had from a two-component fit to the J/ψ p_T spectrum (0–5 GeV/c) using a photoproduction template (e.g., from UPC data or a model) and a hadronic template (e.g., from p_T > 1 GeV/c where photoproduction is negligible). (2) Measure A_had_2 from J/ψ in a p_T interval where photoproduction is negligible, e.g., 0.5–1.5 GeV/c. If the resulting A2_phot changes by more than 1σ (stat.) from the published -0.39, the extraction is model-dependent and the central claim is weakened.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central extraction of A2_phot relies on subtracting a hadronic component characterized by fraction f_had and modulation A_had_2. The fraction f_had is obtained by fitting the J/ψ p_T spectrum in 0.2–5.0 GeV/c with a Tsallis function and extrapolating below 0.2 GeV/c. This assumes the hadronic region is pure and the Tsallis form holds down to zero p_T, but photoproduction may contaminate the fit region (0.2–0.5 GeV/c). The paper does not quantify this contamination; if non-zero, the extrapolated f_had is biased. The alternative fit form tested only changes the functional shape, not the purity assumption. The modulation A_had_2 is taken from 0.2–1.5 GeV/c and assumed constant at lower p_T without a data-driven cross-check in a region where photoproduction is negligible (e.g., p_T > 0.5 GeV/c). The systematic check of setting A_had_2 = 0 changes the central value by ~0.04, but the actual value could differ more if the hadronic modulation has p_T dependence. Together, these assumptions constitute the weakest link in the claim for a negative photoproduction modulation that is significantly different from zero.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek/deepseek-v4-flash","summary":"This paper reports the first event-plane-resolved measurement of the decay angular distribution of photon-induced J/ψ in hadronic heavy-ion collisions (Ru+Ru and Zr+Zr at √s_{NN}=200 GeV). After correcting for the event-plane resolution and subtracting the residual hadronic background, the collaboration extracts a negative second-order modulation A^{phot}_2 = −0.39 ± 0.11 (stat.) ± 0.04 (sys.) for |y^{ee}|<0.8, p_T^{ee}<0.2 GeV/c, and 30-80% centrality. The result is interpreted as evidence that the J/ψ polarization is correlated with the collision reaction plane, as expected for linearly polarized quasi-real photons. The measurement is compared to EPA-VMD calculations and establishes polarized photoproduction as a direct probe of the transverse orientation of the initial collision geometry.","tokens_in":12143,"tokens_out":1940,"duration_ms":19778,"significance":"If the result holds, this measurement provides a novel way to connect the linear polarization of photons in heavy-ion collisions directly to the collision geometry via the event plane. It opens a new observable for studying initial electromagnetic fields and could eventually serve as a polarization-based reference for geometry studies in small or low-multiplicity systems. The analysis is careful, systematic uncertainties are evaluated from multiple sources, and the statistical significance (~3.5σ) is credible. The work builds on previous observations of polarization in UPCs and extends them to hadronic collisions, which is a nontrivial experimental achievement.","major_comments":[{"comment":"The extraction of A^{phot}_2 depends critically on the hadronic background subtraction. The hadronic fraction f_had is obtained by fitting the J/ψ p_T spectrum in 0.2–5.0 GeV/c with a Tsallis function and extrapolating to p_T<0.2 GeV/c. This fit region is assumed to be 'hadronic-dominated', but photoproduction contributions at these p_T values are not quantified. Even a small photoproduction contamination in the fit region could bias the extrapolated f_had. The authors should provide an estimate of the maximum possible photoproduction yield in the fit region (e.g., using the EPA-VMD model or data-driven sidebands) and discuss how such contamination would affect f_had and the final A^{phot}_2.","section":"§ 'Hadronic background subtraction' and Fig. 3"},{"comment":"The hadronic modulation A^{had}_2 is taken from the measured modulation in the p_T range 0.2–1.5 GeV/c and assumed to be constant for p_T<0.2 GeV/c. The systematic check of setting A^{had}_2=0 changes the central value by ~0.04, but the actual p_T dependence could be stronger. The paper should discuss the possibility that A^{had}_2 varies with p_T and, if possible, test the assumption by measuring A^{had}_2 in narrower p_T bins above 0.2 GeV/c where photoproduction is negligible. At minimum, the uncertainty associated with the constant-A^{had}_2 assumption should be explicitly quantified, not just replaced by the A^{had}_2=0 extreme.","section":"§ 'Hadronic background subtraction' and caption of Fig. 3"}],"minor_comments":[{"comment":"The notation p_T^{ee} is clear, but the superscript 'ee' is sometimes missing or used inconsistently (e.g., 'p_T^{ee}' vs 'p_T'). Please standardize throughout.","section":"Abstract and §1"},{"comment":"The caption says 'folded into [0,π/2]' but the figure x-axis is labeled in radians as multiples of π/5, which is slightly awkward. Consider labeling with fractions of π/2 for clarity.","section":"Figure 2 caption"},{"comment":"The description of the exclusion of electron tracks near the TPC central membrane (|η|<0.05) to avoid split tracks is good, but the justification for the η range (−1<η<−0.05 and 0.05<η<1) could be stated more explicitly.","section":"§ 'Event plane reconstruction'"},{"comment":"Ref. [35] is listed as 'in preparation' (arXiv:2512.02865). If it has been published or accepted by now, please update the reference status.","section":"References"},{"comment":"The sentence 'the most central interval has a larger uncertainty' in the discussion of Fig. 3 (left) is vague; please state the central value and uncertainty for the 0-30% bin so readers can compare.","section":"General"}],"recommendation":"minor_revision","confidential_remarks":"The paper is well within the scope of Physics Letters B or similar journals. The measurement is novel and carefully executed. The major concerns about the hadronic subtraction assumptions are addressable with additional quantification; I do not see a need for full reanalysis. I recommend minor revision rather than major because the central result is robust against reasonable variations, and the authors have already performed several systematic checks. The addition of a contamination estimate and a more explicit handling of the p_T dependence of A^{had}_2 would strengthen the manuscript considerably."},"author_rebuttal":null,"desk_editor":{"model":"deepseek/deepseek-v4-flash","letter":"Bottom line: this is the first measurement of J/psi decay anisotropy referenced to the collision event plane in hadronic heavy-ion collisions, and it delivers what it claims. The extracted A2_phot of about -0.39 at very low pT is negative, consistent with linearly polarized photoproduction tied to impact-parameter geometry. The experimental side is careful: signal extraction, resolution correction, efficiency weighting, and the standard systematic checks are described well. The event-plane resolution correction is honest, and the non-flow caveat pushes the result toward conservatism. The EPA-VMD comparison is post-hoc but reasonable, and the centrality and pT trends broadly track the model.\n\nThe genuinely soft spot is exactly where the stress-test note points: the hadronic subtraction. The hadronic fraction f_had comes from a Tsallis fit to the 0.2–5.0 GeV/c spectrum, extrapolated below 0.2 GeV/c. The paper tests an alternative functional form, but not the more important issue of photoproduction contamination in the fit region. At 0.2–0.5 GeV/c, photoproduction may not be negligible, so the purity assumption is an unquantified bias source. Likewise, A_had_2 is measured in 0.2–1.5 GeV/c and assumed constant at lower pT. The A_had_2 = 0 check shifts the result by only 0.04, but that is a bracketing test, not a bound on pT-dependent hadronic modulation.\n\nThat said, the size of the effect makes the conclusion robust to plausible variations of these inputs. To kill the signal you would need f_had substantially wrong in a direction that reduces A2_phot, or a hadronic modulation that is strongly negative and varying. There is no indication either is happening, and the negative sign aligns with the decay kinematics of a transversely polarized J/psi. This is evidence, not discovery: about 3.5 sigma including systematics, on a first measurement. Referees should ask for a quantitative estimate of photoproduction in the Tsallis fit region and for any available pT differential information on A_had_2, but those requests are strengthening exercises, not fatal objections.\n\nCitations are appropriate and current; I see no citation-pattern red flags. This deserves serious peer review and, assuming the authors can quantify or defend the extrapolation, publication. I would cite it and bring it to a reading group.","headline":"First event-plane-resolved J/psi photoproduction anisotropy measurement; analysis is careful, result is plausible evidence, and the main caveat is an unquantified hadronic-subtraction bias that is unlikely to flip the sign.","tokens_in":12760,"tokens_out":2291,"would_cite":true,"duration_ms":21839,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"pith_extraction":{"msc":[],"pacs":["25.75.-q","13.60.Le"],"model":"deepseek/deepseek-v4-flash","headline":"Photoproduced J/psi mesons show a negative second-order decay modulation relative to the event plane (A2^phot = -0.39 ± 0.11 ± 0.04), establishing their linear polarization as a direct probe of collision geometry.","keywords":["coherent photoproduction","J/psi polarization","event plane","collision geometry","heavy-ion collisions","linear polarization","equivalent-photon approximation","isobar collisions"],"falsifier":"Remeasure the modulation in the narrower interval $p_T^{ee}<0.1$ GeV/$c$, where the hadronic fraction drops to $0.14\\pm0.02$: the polarization–geometry picture requires $A_2^{\\rm phot}$ to stay clearly negative there rather than drifting toward the near-zero hadronic value. A data-driven cross-check would replace the Tsallis extrapolation with a hadronic $p_T$ template taken from an event class or collision system where photoproduction is negligible, and require the same negative $A_2^{\\rm phot}$ to survive.","tokens_in":13493,"feed_emoji":"⚛️","tokens_out":16418,"duration_ms":124001,"temperature":0.7,"pith_summary":"The paper reports the first measurement of the decay angular distribution of photon-induced $J/\\psi$ mesons relative to the event plane in heavy-ion collisions with hadronic overlap, using Ru+Ru and Zr+Zr collisions at $\\sqrt{s_{\\mathrm{NN}}}=200$ GeV recorded by STAR. After correcting for the finite event-plane resolution and subtracting the residual hadronic component, the extracted second-order modulation is $A_2^{\\rm phot} = -0.39 \\pm 0.11~(\\mathrm{stat.}) \\pm 0.04~(\\mathrm{sys.})$ for $|y^{ee}|<0.8$, $p_T^{ee}<0.2$ GeV/$c$, and 30–80% centrality. The negative sign is the signature expected if the $J/\\psi$ inherits the linear polarization of the quasi-real photons, whose electric-field direction is aligned with the impact-parameter vector that defines the reaction plane. A sympathetic reader should care because this connects a photon-induced final state directly to the initial collision geometry, establishing polarized photoproduction as a probe of the transverse orientation of the early electromagnetic field rather than of final-state collective behavior.","feed_headline":"A -0.39 modulation pins photon-made J/psi to the collision plane","feed_subtitle":"It ties a photon-made meson's spin to the early collision geometry, a first in hadronic heavy-ion collisions.","key_machinery":"The central observable is the second-order azimuthal modulation $A_2 = \\langle \\cos 2(\\phi_e - \\Psi_{\\mathrm{EP}})\\rangle$, the Fourier moment of the electron azimuthal angle in the $J/\\psi$ rest frame relative to the event-plane angle. The event plane is reconstructed from the hadronic final state with the $p_T$-weighted $Q$-vector method applied to TPC tracks, excluding tracks that could form a $J/\\psi$-mass dielectron pair to suppress self-correlations, and the raw modulation is divided by the event-plane resolution $R_2 = 0.46$ for the 30–80% class. Because very-low-$p_T$ $J/\\psi$ production is dominated by coherent photoproduction, the residual hadronic contribution is removed by fitting the $J/\\psi$ $p_T$ spectrum in the hadronic-dominated region 0.2–5.0 GeV/$c$ with a Tsallis function, extrapolating below 0.2 GeV/$c$ to obtain the hadronic fraction $f_{\\rm had}$, and correcting via $A_2^{\\rm phot} = (A_2^{\\rm incl} - f_{\\rm had} A_2^{\\rm had})/(1 - f_{\\rm had})$. The underlying physics is the equivalent-photon picture: the Lorentz-contracted Coulomb field of each nucleus acts as a beam of quasi-real photons whose electric-field vectors point radially outward, approximately aligned with the impact parameter, so the photon linear polarization inherited by the $J/\\psi$ is tied to the reaction plane.","core_discovery":"The central claim is that coherently photoproduced $J/\\psi$ mesons at very low transverse momentum are linearly polarized along the impact-parameter direction, so their dielectron decay products carry a $\\cos 2\\Delta\\phi$ modulation relative to the event plane whose sign is set by the $J/\\psi\\to e^+e^-$ decay structure. For the combined 30–80% centrality class the extracted photoproduction modulation is $A_2^{\\rm phot} = -0.39 \\pm 0.11~(\\mathrm{stat.}) \\pm 0.04~(\\mathrm{sys.})$ at $p_T^{ee}<0.2$ GeV/$c$; the central values are negative across all measured centrality intervals, there is no significant centrality dependence, and the result agrees with the EPA–VMD calculation used for comparison. The negative sign reflects the fact that spin-1/2 leptons are emitted preferentially perpendicular to the $J/\\psi$ polarization and hence to the event-plane direction. The paper concludes that the produced $J/\\psi$ retains a strong correlation with the reaction-plane direction, and that polarized photoproduction is a direct probe of the transverse orientation of the initial collision geometry.","pith_inferences":["Since the modulation strength is set by the spatial structure of the early electromagnetic field, which depends on the impact parameter and the nuclear size, a high-statistics version of this measurement could constrain nuclear size and deformation parameters; the Ru/Zr isobar pair is a natural place to look for such sensitivity.","The same reaction-plane-aligned $\\cos 2\\Delta\\phi$ argument should apply to $\\rho^0$ and $\\phi$ photoproduction, so measuring $A_2^{\\rm phot}$ for those mesons would test whether the alignment originates in the electromagnetic field or is specific to $J/\\psi$ decay kinematics.","At high statistics the $J/\\psi$ polarization axis offers an independent, electromagnetic cross-check of standard flow-plane reconstruction: agreement between the two directions would validate the $Q$-vector event plane against a reference that is free of final-state collective dynamics."],"forward_implications":["The $J/\\psi$ spin is shown to keep a strong correlation with the reaction-plane direction, confirming that the linear polarization of the quasi-real photon field is tied to the impact-parameter vector.","Polarized photoproduction becomes a direct experimental probe of the transverse orientation of the initial collision geometry, carrying information about the early electromagnetic field rather than about final-state collective flow.","Because it does not rely on hadronic final-state correlations, the polarization direction can serve as an estimator of the reaction plane in ultra-peripheral and low-multiplicity collisions where standard event-plane reconstruction is unavailable.","The observable is intrinsically insensitive to non-flow correlations, since coherent production probes the configuration-averaged nuclear geometry in the Good–Walker picture.","With larger data samples, the same polarization-based reference could enable geometry-sensitive measurements in small or low-multiplicity collision systems."],"supporting_citations":[{"why":"Proposes the reaction-plane-aligned polarization observable and supplies the EPA–VMD calculation that the measured A2 is compared against.","marker":"[36]"},{"why":"Establishes that coherent photoproduction dominates J/psi production at very low pT in hadronic collisions and supplies the Monte Carlo signal shape used in the invariant-mass fits.","marker":"[40]"},{"why":"Documents photoproduction of low-pT J/psi from peripheral to central Pb–Pb collisions, the cross-system evidence that the low-pT excess persists with hadronic overlap.","marker":"[42]"},{"why":"Supplies the pT-weighted Q-vector method used to reconstruct the event plane and to evaluate its resolution R2.","marker":"[43]"},{"why":"Provides the coherent J/psi photoproduction calculation in hadronic heavy-ion collisions that motivates treating the pT<0.2 GeV/c sample as photoproduction-dominated.","marker":"[47]"},{"why":"Prior STAR evidence for polarization-induced angular modulations in exclusive J/psi photoproduction, which this measurement extends by using the event plane instead of the pair momentum as reference.","marker":"[35]"},{"why":"Provides the Tsallis functional form used to fit the hadronic J/psi spectrum and to extrapolate the hadronic background below 0.2 GeV/c.","marker":"[58]"}],"fun_headline_variants":["Photon-made J/ψ spin tied to heavy-ion collision plane","Polarized photoproduction of J/ψ maps initial geometry","Dielectron decay reveals J/ψ aligned with event plane","Negative modulation links photon J/ψ to reaction plane","J/ψ photoproduction shows correlation with collision shape"],"cache_read_input_tokens":0,"weakest_assumption_plain":"The extraction assumes that the hadronic $J/\\psi$ yield below $p_T^{ee}=0.2$ GeV/$c$ follows the Tsallis shape extrapolated from the hadronic-dominated spectrum at higher $p_T$, and that the hadronic modulation measured at 0.2–1.5 GeV/$c$ remains representative at very low $p_T$; if either assumption fails, the subtracted $A_2^{\\rm phot}$ would be biased.","fun_headline_variants_meta":{"raw":{"variants":["Photon-made J/ψ spin tied to heavy-ion collision plane","Polarized photoproduction of J/ψ maps initial geometry","Dielectron decay reveals J/ψ aligned with event plane","Negative modulation links photon J/ψ to reaction plane","J/ψ photoproduction shows correlation with collision shape"]},"model":"deepseek/deepseek-v4-flash","effort":"low","cost_usd":0.000266,"raw_usage":{"cost":0.000266448,"is_byok":false,"cost_details":{"upstream_inference_cost":0.000266448,"upstream_inference_prompt_cost":0.000117488,"upstream_inference_completions_cost":0.00014896},"total_tokens":1576,"prompt_tokens":1044,"completion_tokens":532,"prompt_tokens_details":{"audio_tokens":0,"video_tokens":0,"cached_tokens":256,"cache_write_tokens":0},"completion_tokens_details":{"audio_tokens":0,"image_tokens":0,"reasoning_tokens":447}},"tokens_in":1044,"tokens_out":532,"cost_source":"openrouter_usage","duration_ms":6932,"image_pages":[],"temperature":1.0,"reasoning_tokens":447,"transport_provider":"openrouter","cache_read_input_tokens":0,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-09-15T02:05:21.263200+00:00","model_set":{"reader":"deepseek/deepseek-v4-flash"},"falsifier":"Remeasure the modulation in the narrower interval $p_T^{ee}<0.1$ GeV/$c$, where the hadronic fraction drops to $0.14\\pm0.02$: the polarization–geometry picture requires $A_2^{\\rm phot}$ to stay clearly negative there rather than drifting toward the near-zero hadronic value. A data-driven cross-check would replace the Tsallis extrapolation with a hadronic $p_T$ template taken from an event class or collision system where photoproduction is negligible, and require the same negative $A_2^{\\rm phot}$ to survive.","supporting_citations":[{"cited_title":"Reaction plane alignment with linearly polarized photon in heavy-ion collisions","cited_arxiv_id":"2302.10458","evidence_quote":"Proposes the reaction-plane-aligned polarization observable and supplies the EPA–VMD calculation that the measured A2 is compared against."},{"cited_title":"Observation of excess J/$\\psi$ yield at very low transverse momenta in Au+Au collisions at $\\sqrt{s_{\\rm{NN}}} =$ 200 GeV and U+U collisions at $\\sqrt{s_{\\rm{NN}}} =$ 193 GeV","cited_arxiv_id":"1904.11658","evidence_quote":"Establishes that coherent photoproduction dominates J/psi production at very low pT in hadronic collisions and supplies the Monte Carlo signal shape used in the invariant-mass fits."},{"cited_title":"Photoproduction of low-$p_{\\rm T}$ J/$\\psi$ from peripheral to central Pb$-$Pb collisions at 5.02 TeV","cited_arxiv_id":"2204.10684","evidence_quote":"Documents photoproduction of low-pT J/psi from peripheral to central Pb–Pb collisions, the cross-system evidence that the low-pT excess persists with hadronic overlap."},{"cited_title":"Methods for analyzing anisotropic flow in relativistic nuclear collisions","cited_arxiv_id":"nucl-ex/9805001","evidence_quote":"Supplies the pT-weighted Q-vector method used to reconstruct the event plane and to evaluate its resolution R2."},{"cited_title":"Coherent J/$\\psi$ photoproduction in hadronic heavy-ion collisions","cited_arxiv_id":"1705.01460","evidence_quote":"Provides the coherent J/psi photoproduction calculation in hadronic heavy-ion collisions that motivates treating the pT<0.2 GeV/c sample as photoproduction-dominated."},{"cited_title":"Evidence of spin-interference effects in exclusive J/ψ→e +e− photoproduction in ultraperipheral heavy-ion collisions","cited_arxiv_id":null,"evidence_quote":"Prior STAR evidence for polarization-induced angular modulations in exclusive J/psi photoproduction, which this measurement extends by using the event plane instead of the pair momentum as reference."},{"cited_title":"Examine the species and beam-energy dependence of particle spectra using Tsallis Statistics","cited_arxiv_id":"0912.0993","evidence_quote":"Provides the Tsallis functional form used to fit the hadronic J/psi spectrum and to extrapolate the hadronic background below 0.2 GeV/c."}],"review_version":1}