Observation of the electromagnetic radiative decays of the \boldmath{$\Lambda(1520)$} and \boldmath{$\Lambda(1670)$} to \boldmath{$\gamma\Sigma^0$}

TL;DR

This study reports the first observation of electromagnetic radiative decays of b1(1520) and b1(1670) to b3a0, measuring branching fractions and partial widths, and explores their resonance structures using a large J/a9/a3 event sample.

Contribution

It provides the first experimental measurements of these radiative decays and compares them with theoretical models, revealing discrepancies with existing predictions.

Findings

First observation of b1(1520) and b1(1670) radiative decays to b3a0 with high significance.

Measured branching fraction of b1(1520)b4b3a0 decay is 1 2.95 1 0.28 (stat.) 1 0.56 (syst.) d7 10^{-3}.

Observed a resonance at 1.67 GeV/c^2 in the b3a0 mass spectrum attributed to b1(1670).

Abstract

Using $(10087\pm 44)\times10^6$ $J/\psi$ events collected with the BESIII detector, we report the first observation of the electromagnetic radiative decays of the $\Lambda(1520)$ and $\Lambda(1670)$ to $\gamma\Sigma^0$, with a statistical significance of $16.6\sigma$ and $23.5\sigma$, respectively. The ratio of the branching fractions $\frac{\mathcal{B}(\Lambda(1520)\to\gamma\Lambda)}{\mathcal{B}(\Lambda(1520)\to\gamma\Sigma^0)}$ is determined to be $2.88\pm0.27(\text{stat.})\pm0.21(\text{syst.})$, which is in good agreement with flavor SU(3) symmetry. The branching fraction of $\Lambda(1520)\to\gamma\Sigma^0$ is measured to be $\mathcal{B}(\Lambda(1520)\to\gamma\Sigma^0)=(2.95\pm0.28(\text{stat.})\pm0.56(\text{syst.}))\times 10^{-3}$, corresponding to a partial width of $\Gamma(\Lambda(1520)\to\gamma\Sigma^0)=(47.2\pm4.5(\text{stat.})\pm9.0(\text{syst.}))$ keV, which is inconsistent with predictions from the relativized constituent quark model and the Algebraic model. Additionally, we observe a clear resonant structure in the $\gamma\Sigma^0$ mass spectrum around 1.67 GeV/$c^2$, attributed to the $\Lambda(1670)$. The product branching fraction $\mathcal{B}(J/\psi\to\bar\Lambda\Lambda(1670)+c.c.)\times\mathcal{B}(\Lambda(1670)\to\gamma\Sigma^0)$ is measured for the first time as $(5.39\pm0.29(\text{stat.})\pm 0.44(\text{syst.}))\times 10^{-6}$. However, no corresponding structure is seen in the $\gamma\Lambda$ mass spectrum, so an upper limit on the product branching fraction $\mathcal{B}(J/\psi\to\bar\Lambda\Lambda(1670)+c.c.)\times\mathcal{B}(\Lambda(1670)\to\gamma\Lambda)$ is determined to be $5.97\times10^{-7}$ at the 90\% confidence level.