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$a_{0} (980)$ production, triangle singularity, and non- $ϕ$ background in the $J / ψ \to ϕ η π^{0}$ reaction

Hai-Peng Li¹, Wei-Hong Liang ^1,2,*, Chu-Wen Xiao ^1,2,3, and Eulogio Oset ^1,4,†

¹Department of Physics, Guangxi Normal University, Guilin 541004, China
²Guangxi Key Laboratory of Nuclear Physics and Technology, Guangxi Normal University, Guilin 541004, China
³School of Physics, Central South University, Changsha 410083, China
⁴Departamento de Física Teórica and IFIC, Centro Mixto Universidad de Valencia-CSIC, Institutos de Investigación de Paterna, Apartado 22085, 46071 Valencia, Spain

^*Contact author: liangwh@gxnu.edu.cn
^†Contact author: Oset@ific.uv.es

Phys. Rev. D 113, 054028 – Published 18 March, 2026

DOI: https://doi.org/10.1103/z1vf-q4dw

Abstract

We study the $J / ψ \to ϕ η π^{0}$ reaction measured recently by the BESIII Collaboration with high precision, paying attention to three important aspects: (1) the production of the $a_{0} (980)$ in the $π^{0} η$ mass distributions, with the typical narrow width observed in isospin violating processes; (2) the origin of two peaks in the $ϕ π^{0}$ mass distributions that were branded as “non- $ϕ$ ” contribution in the experimental analysis; (3) the existence of two triangle mechanisms developing a triangle singularity at the same energy where the non- $ϕ$ contribution peaks in the experiment. However, we also show that the strength of these peaks is very small relative to the observed ones, a feature that is tied to the experimental technique used to identify the $ϕ$ looking at $K^{+} K^{-}$ in a narrow window of invariant mass around the $ϕ$ mass. We suggest that these triangle singularities could be observed with other methods to identify the $ϕ$ .

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Phys. Rev. D 113, 054028– Published 18 March, 2026

Vol. 113, Iss. 5 — 1 March 2026

Received 19 December 2025
Accepted 2 March 2026

DOI: https://doi.org/10.1103/z1vf-q4dw

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

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$a_{0} (980)$ production, triangle singularity, and non- $ϕ$ background in the $J / ψ \to ϕ η π^{0}$ reaction

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a0(980) production, triangle singularity, and non-ϕ background in the J/ψ→ϕηπ0 reaction

Abstract

Physics Subject Headings (PhySH)

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References (62)

$a_{0} (980)$ production, triangle singularity, and non- $ϕ$ background in the $J / ψ \to ϕ η π^{0}$ reaction