Determination of the spin and parity of all-charm tetraquarks
Summary
The CMS Collaboration reports an analysis of structures observed in the J/ψ–J/ψ (di‑J/ψ) mass spectrum using proton–proton collision data at √s = 13 TeV. By studying the four‑muon final state and exploiting angular observables, CMS performs hypothesis tests of competing spin‑parity (J^P) assignments for fully‑charmed tetraquark candidates (cc¯cc¯). The study uses optimal discriminants, backgroundsubtraction and a detailed treatment of systematic uncertainties. The collaboration releases underlying data to HEPData and notes public availability of core software on GitHub.
Key Points
- CMS analysed four‑muon events from di‑J/ψ decays to probe the quantum numbers of fully‑charmed tetraquark candidates observed in the J/ψJ/ψ mass spectrum.
- Angular observables defined in the resonance and decay frames were used to distinguish alternative spin‑parity hypotheses.
- Optimal discriminants (D_ij) were constructed to separate the preferred model from alternatives; comparisons include signal‑only and signal+background cases with post‑fit uncertainty bands.
- Hypothesis tests across a set of J^P models show a clear preference for a spin‑2, positive‑parity interpretation for at least one observed structure, disfavouring several other J^P assignments (detailed results given in Extended Data Table 1).
- Results are supported by comprehensive systematic studies, public data release (HEPData DOI) and the CMS software ecosystem (cmssw on GitHub), enabling reuse and follow‑up analyses by the community.
Content summary
The paper focuses on determining the spin and parity of the structures seen in the di‑J/ψ invariant mass spectrum. Using the four‑muon final state, CMS reconstructs angular variables in the relevant centre‑of‑mass frames and compares distributions to a suite of signal models with different J^P assignments. The analysis builds optimised observables to maximise separation power between pairs of hypotheses and performs statistical hypothesis tests (including systematic uncertainties and polarisation effects where relevant).
Extended data figures show angular distributions, optimal‑observable shapes and comparisons between data and model predictions. The collaboration summarises pairwise hypothesis test results in an extended data table, and makes the data and analysis resources available for scrutiny and reuse. The article is Open Access under CC BY 4.0.
Context and relevance
Discovering and characterising fully‑heavy tetraquarks (cc¯cc¯) is a major theme in contemporary hadron spectroscopy: such states test QCD dynamics in the heavy‑quark sector and challenge models of multiquark binding (diquark–antidiquark, molecular, string dynamics and coupled channels). This CMS result provides experimental quantum‑number information (spin and parity), not just mass peaks, which is crucial to discriminate among theoretical descriptions and to guide model refinements.
The measurement ties into a wider experimental picture (LHCb, ATLAS, earlier CMS observations) and a dense theoretical literature on fully‑heavy tetraquark spectra and decay patterns. By establishing preferred J^P assignments, the work narrows viable interpretations and will influence follow‑up searches and model building ahead of higher‑luminosity runs and future colliders.
Author style
Punchy: this is not a footnote in exotic‑hadron studies — it’s a step change. CMS moves beyond observing bumps and provides quantum‑number evidence for at least one fully‑charmed structure. If you care about how exotic hadrons fit into QCD, the paper’s details matter.
Why should I read this?
Short answer: because it tells you whether the new ‘all‑charm’ bumps are just curiosities or bona fide new forms of matter. The paper doesn’t just list peaks — it teases out spin and parity using angular tricks and proper statistics. If you follow particle spectroscopy, model building or LHC results, this saves you the legwork: CMS has done the heavy lifting and the raw material is public for deeper dives.