Observation of $ψ(3686)\to γη(1405)$ via $η(1405)\to f_0(980)π^0$

TL;DR

This BESIII study reports the first observation of the radiative decay ψ(3686)→ γ η(1405) with η(1405)→ f0(980)π^0, yielding a branching fraction of $ (3.77±0.43±0.30)×10^{-7}$. It also observes isospin-violating ψ(3686)→ γ f1(1285)→ γ f0(980)π^0 decays with a branching fraction of $ (7.36±2.25±2.36)×10^{-8}$ and finds a ratio $ rac{ ext{B}(ψ(3686)→ γ η(1405))}{ ext{B}(J/ψ→ γ η(1405))} = (2.51±0.44) ext{%}$, indicating a violation of the 12% rule. The results show that η(1405)→ f0(980)π^0 cannot be explained by f0–a0 mixing alone and likely involve additional dynamics such as triangle singularities. Upper limits are set on η_c decay channels, and the findings have implications for understanding isospin-violating processes and radiative charmonium decays.

Abstract

The decay $ψ(3686)\toγπ^+π^-π^0$ is studied using a sample of $(2712.4\pm14.3)\times10^6$ $ψ(3686)$ events collected with the BESIII detector. The decay $η(1405)\toπ^+π^-π^0$ is observed for the first time in $ψ(3686)$ decays via the intermediate state $f_0(980)$ and the product branching fraction $\mathcal{B}(ψ(3686)\toγη(1405))\times\mathcal{B}(η(1405)\to f_0(980)π^0)\times \mathcal{B}(f_0(980)\toπ^+π^-)$ is determined to be $(3.77\pm0.43\pm0.29)\times10^{-7}$, where the first uncertainty is statistical and the second is systematic. The isospin-violating decay of $ψ(3686)\toγf_1(1285)\toγf_0(980)π^0\toγπ^+π^-π^0$ has been observed with signal significance of $2.9σ$. And the branching fraction $\mathcal{B}(ψ(3686)\toγf_1(1285)\toγf_0(980)π^0\toγπ^+π^-π^0)$ is determined to be $ (7.36\pm2.25\pm2.26)\times 10^{-8}$. Since no $η_c$ signal is evident in either the $π^+π^-π^0$ or $f_0(980)π^0$ mass spectrum, upper limits are set to be $\mathcal{B}(ψ(3686)\toγη_c)\times\mathcal{B}(η_c\toπ^+π^-π^0)<3.09\times10^{-7}$ and $\mathcal{B}(ψ(3686)\toγη_c)\times\mathcal{B}(η_c\to f_0(980)π^0)\times\mathcal{B}(f_0(980)\toπ^+π^-)<7.97\times10^{-8}$ at 90\% confidence level, respectively.

Figures (3)

• Figure 1: (a) The $M(\pi^+\pi^-)$ distribution. The red arrows and pink arrows show the $f_0(980)$ signal region and the sideband regions, respectively. (b) The $M(\pi^+\pi^-\pi^0)$ distributions after selecting events in the $f_0(980)$ mass window. The black dots with error bars are data, the red solid line is the total fit result, the blue dashed line is the $\eta(1405)$ signal, the brown long-dashed line is the $f_{0}(980)$ sideband, and the pink long-dashed line is the $f_{1}(1285)$ peak.
• Figure 2: Fits to the $M(\pi^+\pi^-\pi^0)$ distributions in the $\eta_c$ mass region (a) without and (b) with the requirement that $M(\pi^+\pi^-)$ is within the $f_0(980)$ mass window. The black dots with error bars are data, the red solid line is the total fits result, the blue dashed line is the $\eta_c$ signal shape, the pink dashed line is the $J/\psi$ background, and the green dashed line is the combinatorial background.
• Figure 3: Normalized likelihood distribution before (blue dots) and after (red dots) considering the multiplication uncertainties for (a) $\eta_c\to\pi^+\pi^-\pi^0$ and (b) $\eta_c\to f_0(980)\pi^0$.