First Evidence for an Unambiguous Triangle Singularity from ψ(2S) → pp̅η

Abstract

Triangle singularities, predicted by Landau in 1959, are purely kinematic enhancements arising from hadronic rescattering loops. Despite their proposed role in various anomalous decay processes and exotic hadron candidates, a direct experimental confirmation has remained elusive for more than six decades. We analyze the pη/p̅η invariant mass spectrum in ψ(2S) → pp̅η measured by the BESIII Collaboration. The data exhibit a clear cusp-like structure around 1.564~GeV in the N(1535) region, in precise agreement with the kinematic position predicted for the triangle singularity. Including the triangle singularity loop in the fit substantially improves the description of the data, with χ²/d.o.f. decreasing from 1.22 to 0.90, corresponding to a significance of ∼ 3.8σ for the triangle singularity contribution. The precise alignment of the observed excess with the predicted kinematic position provides the first compelling evidence for the triangle singularity effect.

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BibTeX

@misc{huang2026first,
  title = {{First Evidence for an Unambiguous Triangle Singularity from $ψ(2S) \to p\barpη$}},
  author = {Qi Huang and Yi-Jia Zeng and Xiao-Rui Lyu and Rong-Gang Ping and Jia-Jun Wu},
  howpublished = {arXiv},
  year = {2026},
  month = aug,
  doi = {10.48550/arxiv.2608.29006},
  eprint = {2608.29006},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2608.29006},
  note = {CC BY 4.0},
}

RIS

TY  - UNPB
TI  - First Evidence for an Unambiguous Triangle Singularity from ψ(2S) → pp̅η
AU  - Qi Huang
AU  - Yi-Jia Zeng
AU  - Xiao-Rui Lyu
AU  - Rong-Gang Ping
AU  - Jia-Jun Wu
PY  - 2026
DA  - 2026/08/29
PB  - arXiv
DO  - 10.48550/arxiv.2608.29006
UR  - https://arxiv.org/abs/2608.29006
AB  - Triangle singularities, predicted by Landau in 1959, are purely kinematic enhancements arising from hadronic rescattering loops. Despite their proposed role in various anomalous decay processes and exotic hadron candidates, a direct experimental confirmation has remained elusive for more than six decades. We analyze the pη/p̅η invariant mass spectrum in ψ(2S) → pp̅η measured by the BESIII Collaboration. The data exhibit a clear cusp-like structure around 1.564~GeV in the N(1535) region, in precise agreement with the kinematic position predicted for the triangle singularity. Including the triangle singularity loop in the fit substantially improves the description of the data, with χ^2/d.o.f. decreasing from 1.22 to 0.90, corresponding to a significance of ∼ 3.8σ for the triangle singularity contribution. The precise alignment of the observed excess with the predicted kinematic position provides the first compelling evidence for the triangle singularity effect.
ER  -

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