Momentum-dependence of $\rho-\omega$ mixing in the pion vector form factor and its effect on $(g-2)_\mu$

Abstract

The inclusion of the $\rho-\omega$ mixing effect is essential for a precise description of the pion electromagnetic form factor in the $e^+e^- \rightarrow \pi^+\pi^-$ process, which quantifies the two-pion contribution to the anomalous magnetic moment of the muon $a_\mu$. In this paper, we analyse the momentum dependence of the $\rho-\omega$ mixing by considering loop contributions at the next-to-leading order in the $1/N_C$ expansion within the framework of resonance chiral theory. We revisit the work [Y. H. Chen, D. L. Yao, and H. Q. Zheng, Commun. Theor. Phys. 69 (2018) 1], considering the contribution arising from the kaon mass splitting in the kaon loops and the latest experimental data. We perform two kinds of fits (with momentum-independent or momentum-dependent $\rho-\omega$ mixing amplitude) to describe the $e^+e^-\rightarrow \pi^+\pi^-$ and $\tau\rightarrow \nu_{\tau}2\pi$ data within the energy region of 600$\sim$900 MeV and the decay width of $\omega \rightarrow \pi^+\pi^-$, and compare their results. Our findings indicate that both the momentum-independent and momentum-dependent $\rho-\omega$ mixing schemes provide appropriate descriptions of the data. However, the momentum-dependent scheme exhibits greater self-consistency, considering the reasonable imaginary part of the mixing matrix element $\Pi_{\rho\omega}$ obtained. Regarding the contribution to the anomalous magnetic moment of the muon, $a_\mu^{\pi\pi}|_{[0.6,0.9]\text{GeV}}$, the results obtained from the fits considering the momentum-dependent $\rho-\omega$ mixing amplitude agree well with those obtained without incorporating the momentum dependence of the $\rho-\omega$ mixing, within the margin of errors. Furthermore, based on the fitted values of the relevant parameters, we observe that the decay width of $\omega \rightarrow \pi^+\pi^-$ is predominantly influenced by the $\rho-\omega$ mixing effect.