Isospin strikes back

IF 4.8 2区物理与天体物理 Q2 PHYSICS, PARTICLES & FIELDS The European Physical Journal C Pub Date : 2025-03-07 DOI:10.1140/epjc/s10052-025-13976-7

Francesco Rosini, Simone Pacetti

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Abstract

Assuming isospin conservation, the decay of a $c\bar{c}$ vector meson into the $\Lambda \bar{\Sigma }^0+\mathrm{c.c.}$ final state is purely electromagnetic. At the leading order, the $c\bar{c}$ vector meson first converts into a virtual photon that, then produces the $\Lambda \bar{\Sigma }^0+\mathrm{c.c.}$ final state. Moreover, such a mechanism, i.e., the virtual photon coupling to $\Lambda \bar{\Sigma }^0+\mathrm{c.c.}$, is the sole intermediate process through which, in Born approximation, the reaction $e^+e^-\rightarrow \Lambda \bar{\Sigma }^0+\mathrm{c.c.}$ does proceed. It follows that any significant difference between the amplitudes of the processes $c\bar{c}\rightarrow \Lambda \bar{\Sigma }^0+\mathrm{c.c.}$ and $e^+e^-\rightarrow \Lambda \bar{\Sigma }^0+\mathrm{c.c.}$ at the $c\bar{c}$ mass must be ascribed to an isospin-violating contribution in the $c\bar{c}$ decay. In Ferroli et al. (Eur Phys J C 80: 903, 2020) we studied the decay of the $\psi (2S)$ vector meson into $\Lambda \bar{\Sigma }^0+\mathrm{c.c.}$ and, on the light of the large branching fraction

$$\begin{aligned} \textrm{BR}_{18}(\psi (2S)\rightarrow \Lambda \bar{\Sigma }^0+\mathrm{c.c.})=(1.23\pm 0.24)\times 10^{-5}, \end{aligned}$$

published in the 2018 edition of the Review of Particle Physics (Tanabashi et al. in Phys Rev D 98: 030001, 2018), we claimed either the presence of a significant isospin-violating contribution or, with a lesser emphasis, a “not complete reliability of the only available datum”. In any case, we propose a new measurement. Apparently, our second and considered less serious hypothesis was the right one, indeed the branching fraction published in the 2024 edition of the Review of Particle Physics (Navas et al. in Phys Rev D 110: 030001, 2024) is

$$\begin{aligned} \textrm{BR}(\psi (2S)\rightarrow \Lambda \bar{\Sigma }^0+\mathrm{c.c.})=(1.6\pm 0.7)\times 10^{-6}, \end{aligned}$$

more than seven times lower with the error that increased from $\sim 20\%$ to $\sim 45\%$.

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同位旋反击

假设同位旋守恒，$c\bar{c}$矢量介子到$\Lambda \bar{\Sigma }^0+\mathrm{c.c.}$最终态的衰变是纯电磁的。在第一阶，$c\bar{c}$矢量介子首先转换成虚光子，然后产生$\Lambda \bar{\Sigma }^0+\mathrm{c.c.}$最终状态。此外，这种机制，即与$\Lambda \bar{\Sigma }^0+\mathrm{c.c.}$的虚光子耦合，是唯一的中间过程，在玻恩近似中，反应$e^+e^-\rightarrow \Lambda \bar{\Sigma }^0+\mathrm{c.c.}$确实是通过它进行的。由此可见，在$c\bar{c}$质量处$c\bar{c}\rightarrow \Lambda \bar{\Sigma }^0+\mathrm{c.c.}$和$e^+e^-\rightarrow \Lambda \bar{\Sigma }^0+\mathrm{c.c.}$过程振幅之间的任何显著差异，都必须归因于$c\bar{c}$衰变中违反同位旋的贡献。在Ferroli et al. （Eur Phys J C 80: 903,2020）中，我们研究了$\psi (2S)$矢量介子衰变为$\Lambda \bar{\Sigma }^0+\mathrm{c.c.}$，并且根据2018年版《粒子物理评论》（Tanabashi et al. In Phys Rev D 98: 030001, 2018）上发表的大分支部分$$\begin{aligned} \textrm{BR}_{18}(\psi (2S)\rightarrow \Lambda \bar{\Sigma }^0+\mathrm{c.c.})=(1.23\pm 0.24)\times 10^{-5}, \end{aligned}$$，我们声称存在重大的同位旋违反贡献，或者不那么强调，“唯一可用的数据不完全可靠”。无论如何，我们提出一种新的测量方法。显然，我们第二个被认为不那么严肃的假设是正确的，实际上，发表在2024年版的《粒子物理评论》（Navas et al. in Phys Rev D 110: 030001, 2024）上的分支分数$$\begin{aligned} \textrm{BR}(\psi (2S)\rightarrow \Lambda \bar{\Sigma }^0+\mathrm{c.c.})=(1.6\pm 0.7)\times 10^{-6}, \end{aligned}$$低了7倍多，误差从$\sim 20\%$增加到$\sim 45\%$。

本文章由计算机程序翻译，如有差异，请以英文原文为准。

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来源期刊

The European Physical Journal C 物理-物理：粒子与场物理

CiteScore

8.10

自引率

15.90%

发文量

1008

审稿时长

2-4 weeks

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