arxiv: 2604.18061 · v1 · submitted 2026-04-20 · ✦ hep-ex

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Enhanced evidence of X(7200) and improved measurements of X(6900) parameters from a combined LHCb-ATLAS-CMS analysis

Yuan Wang , Ran Li , Bin Zhong , Yaqian Wang

Authors on Pith no claims yet

Pith reviewed 2026-05-10 03:37 UTC · model grok-4.3

classification ✦ hep-ex

keywords di-J/ψ mass spectrumX(6900) resonanceX(7200) resonancefully-charmed tetraquarksinterference modelingcombined LHC analysisresonance significancetetraquark spectroscopy

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The pith

Combining data from three LHC experiments strengthens evidence for the X(7200) and refines the X(6900) parameters in the di-J/ψ spectrum.

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

The authors combine published di-J/ψ mass spectra from three LHC experiments in a simultaneous fit. They find the X(6900) resonance with a significance exceeding 12 sigma and report more precise values for its mass and width. Across several models that include interference between resonances, they observe consistent signals for the X(7200) with significances up to 6.6 sigma in the favored scheme. This work shows that accounting for interference is necessary to interpret the spectrum and extract reliable parameters for these potential tetraquark states.

Core claim

A joint fit to the di-J/ψ invariant mass distributions from three LHC experiments observes the X(6900) at greater than 12 sigma significance with improved precision on its mass and width. The X(7200) appears consistently in multiple interference models, reaching 6.6 sigma significance when adopting a three-resonance scheme, thereby providing enhanced evidence for this additional state.

What carries the argument

The simultaneous fit to the combined di-J/ψ mass spectrum that incorporates different interference models between the X(6900) and X(7200) resonances.

Load-bearing premise

The structures observed separately in each experiment correspond to the same physical resonances and the chosen interference and background models capture all relevant contributions without significant missing terms.

What would settle it

An independent high-statistics measurement from one experiment that finds no excess at the fitted mass of the X(7200) or incompatible parameters for the X(6900) would challenge the result.

Figures

Figures reproduced from arXiv: 2604.18061 by Bin Zhong, Ran Li, Yaqian Wang, Yuan Wang.

**Figure 1.** Figure 1: Mass resolutions for ATLAS and CMS. A. Model I This model serves as the non-interfering baseline. The total lineshape is given by: fI (m) = "X j cj |BWj (m; Mj , Γj | 2 # × s 1 − 4M2 J/ψ m2 ⊗ G(0; σ(m)) + B(m), (1) where the index j runs over X1, X2, X(6900), and X(7200); cj are free parameters to normalize each signal represented by BW; and q 1 − 4M2 J/ψ m2 is the phase space factor [PITH_FULL_IMAGE:figu… view at source ↗

**Figure 2.** Figure 2: Fit results (Model I) for the di-J/ψ invariant mass distributions from LHCb (left), ATLAS (middle), and CMS (right). The blue solid curves represent the best fits, while the individual contributions from the resonant components (solid) and the background (dotted) are also shown. B. Model II Following the LHCb Collaboration’s interference approach, Model II couples the broad structure (described by an inter… view at source ↗

**Figure 3.** Figure 3: Fit results (Model II) for the di-J/ψ invariant mass distributions measured by LHCb (left), ATLAS (middle), and CMS (right). The blue solid curves represent the best fit, and the individual contributions from the resonant components and the background are also shown. C. Model III Model III investigates the scenario where the three lower-mass structures (X1, X2, and X(6900)) share a common production mechan… view at source ↗

**Figure 4.** Figure 4: Fit results (Model III) for the di-J/ψ invariant mass distributions from LHCb (left), CMS (middle), and ATLAS (right). The blue solid curves represent the best fit, and the individual contributions from the resonant components and the background are also shown. D. Model IV Model IV is motivated by the three-resonance interference model used by CMS [8]. In this scenario, X1 is interpreted not as a genuine r… view at source ↗

**Figure 5.** Figure 5: Fit results (Model IV) for the di-J/ψ invariant mass distributions from LHCb (left), ATLAS (middle), and CMS (right). The blue solid curves represent the best fit, and the individual contributions from the resonant components and the background are also shown. Table I. Fitted resonance parameters for the X(6900) and X(7200) structures under four different fit models. The values of the mass (M), width (Γ), … view at source ↗

read the original abstract

We report enhanced evidence for the $X(7200)$ state and significantly improved measurements of the $X(6900)$ resonance parameters through a combined analysis of the di-$J/\psi$ mass spectrum using published data from LHCb, ATLAS, and CMS. By performing simultaneous fits to all three experiments, we observe the $X(6900)$ with overwhelming significance ($>12\sigma$) and determine its mass and width with improved precision. For the $X(7200)$, we find consistent signals across multiple interference models, with significances ranging from $3.7\sigma$ to $6.6\sigma$; the best-fit model (adopting the CMS three-resonance scheme) yields $6.6\sigma$ significance, providing substantially strengthened evidence for this state. Our results underscore the essential role of interference effects in fully-charmed tetraquark spectroscopy and offer new constraints on their production mechanisms at the LHC.

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.

Desk Editor's Note private letter to a colleague

The combined fit tightens X(6900) parameters and raises X(7200) significance to 6.6σ in one interference scheme, but that peak value comes from post-hoc model selection without a trials correction.

read the letter

The main takeaway is a simultaneous fit to the published di-J/ψ spectra from LHCb, ATLAS, and CMS that assumes shared resonance parameters and tests several interference models. X(6900) emerges above 12σ with noticeably smaller uncertainties on mass and width than any single experiment reported. X(7200) appears in every scheme they try, with significances between 3.7σ and 6.6σ; the paper adopts the CMS three-resonance parametrization as the best fit and quotes the upper end of that range as enhanced evidence. That is the concrete new piece: a multi-experiment combination that produces joint numbers and shows the signal survives different background and interference choices. The work is useful because it recycles existing data without new running time and highlights how interference affects the extracted widths. The soft spot is the handling of model choice. The 6.6σ figure is tied to one specific scheme selected after the fits are done, and nothing in the abstract indicates a correction for the discrete set of models examined. If the other schemes are equally plausible a priori, the effective significance is lower. It would also be good to see an explicit check for tension between the three datasets once the common parameters are imposed; any experiment-specific distortions could be absorbed into the fit and inflate the apparent significance. Background parametrization and the full systematic table are not visible in the abstract, but the full text presumably supplies them. This is the sort of reanalysis that people in exotic-hadron spectroscopy will want to read and discuss. The numbers are concrete enough and the approach is transparent enough that it deserves referee time rather than a desk rejection. Referees can press on the model-selection issue and the cross-experiment consistency, which is exactly what the paper needs.

Referee Report

2 major / 2 minor

Summary. The manuscript performs a simultaneous fit to published di-J/ψ invariant-mass spectra from LHCb, ATLAS, and CMS. It reports the X(6900) with >12σ significance and improved mass/width precision, while claiming enhanced evidence for the X(7200) with significances ranging from 3.7σ to 6.6σ across interference models; the highest value (6.6σ) occurs when adopting the CMS three-resonance scheme.

Significance. A robust combined analysis of existing LHC data on fully-charmed tetraquarks would be useful for constraining production mechanisms and lineshapes. The approach of fitting all three datasets together is a constructive step, but its impact depends on whether the quoted significances properly account for model selection and whether the assumption of universal resonance parameters is justified by the data.

major comments (2)

[abstract and results section on fit models] The headline claim of enhanced evidence for the X(7200) rests on the 6.6σ result obtained only after adopting the CMS three-resonance interference scheme. Other models yield 3.7σ. Because the model is chosen after inspecting the data (or by reference to one experiment's scheme), the reported significance does not incorporate a trials factor for the discrete choice of interference parametrization or background form. This directly affects the central claim in the abstract and results.
[description of the simultaneous fit] The combined fit assumes common resonance parameters (mass, width, and interference phases) across LHCb/ATLAS/CMS without an explicit test for tension or experiment-specific lineshape distortions that could mimic or suppress the structure. No quantitative consistency check (e.g., pull distributions or separate vs. joint fit comparison) is described, which is load-bearing for the reliability of the quoted combined parameters.

minor comments (2)

[abstract] The abstract states that signals are 'consistent across multiple interference models' but does not enumerate how many models were tested or the precise selection criterion used to identify the 'best-fit' model.
[systematics discussion] Systematic uncertainties arising from the choice of background parametrization and from possible differences in experimental acceptance or resolution are mentioned only briefly; a dedicated table or section quantifying their impact on the X(7200) significance would improve clarity.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the detailed and constructive report. The comments highlight important aspects of model dependence and fit consistency that we address below. We have revised the manuscript to strengthen the presentation of our results.

read point-by-point responses

Referee: The headline claim of enhanced evidence for the X(7200) rests on the 6.6σ result obtained only after adopting the CMS three-resonance interference scheme. Other models yield 3.7σ. Because the model is chosen after inspecting the data (or by reference to one experiment's scheme), the reported significance does not incorporate a trials factor for the discrete choice of interference parametrization or background form. This directly affects the central claim in the abstract and results.

Authors: We thank the referee for raising this important point on model selection and trials factors. Our analysis considers several interference schemes motivated by the individual LHCb, ATLAS, and CMS publications, and we report the full range of significances (3.7σ to 6.6σ) in both the abstract and results section. The 6.6σ corresponds to the scheme with three resonances as used by CMS, which provides the best description of the combined data. While we do not apply an additional trials factor for the discrete model choices, as they are not arbitrary but based on prior experimental findings, we acknowledge the referee's concern. In the revised manuscript, we have expanded the discussion to more clearly state that the significances are model-dependent and present the range prominently, avoiding any implication that 6.6σ is the sole measure without qualification. We believe this addresses the concern without altering the core findings. revision: partial
Referee: The combined fit assumes common resonance parameters (mass, width, and interference phases) across LHCb/ATLAS/CMS without an explicit test for tension or experiment-specific lineshape distortions that could mimic or suppress the structure. No quantitative consistency check (e.g., pull distributions or separate vs. joint fit comparison) is described, which is load-bearing for the reliability of the quoted combined parameters.

Authors: We agree that explicit consistency checks are valuable for validating the assumption of universal resonance parameters. Although the simultaneous fit achieves a good chi-squared per degree of freedom and the parameters are consistent with individual experiment results within uncertainties, we did not provide detailed quantitative comparisons in the original manuscript. In the revision, we have added a new subsection in the results section describing the comparison between separate fits to each experiment's data and the joint fit. We also include pull distributions for the combined fit to demonstrate the absence of significant tensions or experiment-specific distortions. These additions confirm that the shared parameters are justified by the data. revision: yes

Circularity Check

0 steps flagged

No circularity: combined fit to independent published datasets

full rationale

The paper reports a simultaneous fit to the di-J/ψ mass spectra published by LHCb, ATLAS, and CMS. All quoted significances (>12σ for X(6900), 3.7–6.6σ for X(7200) under different models) and parameter values are direct outputs of this fit to external data. No equation, ansatz, or result reduces by construction to a quantity previously fitted by these authors; references to the CMS three-resonance scheme point to an independent experiment rather than a self-citation chain. The derivation is therefore self-contained against the input datasets and does not exhibit any of the enumerated circularity patterns.

Axiom & Free-Parameter Ledger

3 free parameters · 1 axioms · 0 invented entities

The central claim rests on standard resonance-fitting assumptions plus several free parameters that are adjusted to the combined spectrum.

free parameters (3)

X(6900) mass and width
Fitted parameters whose values are extracted from the simultaneous likelihood.
interference phases and amplitudes
Model-dependent parameters varied in each interference scheme.
background shape coefficients
Polynomial or other coefficients adjusted to the mass spectrum.

axioms (1)

domain assumption The observed mass spectrum is the sum of resonance amplitudes, interference terms, and a smooth background that can be parametrized independently of the signals.
Invoked when constructing the likelihood function for the fits.

pith-pipeline@v0.9.0 · 5473 in / 1273 out tokens · 28709 ms · 2026-05-10T03:37:17.478992+00:00 · methodology

discussion (0)

Reference graph

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