Mixing angle of $K_1$ axial vector mesons

Abstract

Analyses of various experimental measurements all indicate that the mixing angle $\theta_{K_1}$ of $K_1(1270)$ and $K_1(1400)$ is in the vicinity of $33^\circ$ or $57^\circ$. However, whether $\theta_{K_1}$ is greater or less than $45^\circ$ is still quite controversial. For example, there were two very recent studies of the strong decays of $K_1$ mesons. One group claimed that $\theta_{K_1}\approx 60^\circ$, while the other group obtained $\theta_{K_1}=(33.6\pm4.3)^\circ$. Since the determination of the mixing angles $\alpha_{^3\!P_1}$ and $\alpha_{^1\!P_1}$ with the former (latter) being the mixing angle of $f_1(1285)$ ($h_1(1170)$) and $f_1(1420)$ ($h_1(1380)$) in the flavor basis through mass relations depend on $\theta_{K_1}$, we show that $\theta_{K_1}\approx 57^\circ$ is ruled out as it leads to a too large deviation from ideal mixing in the $^1\!P_1$ sector, inconsistent with the lattice calculation of $\alpha_{^1\!P_1}$ and the observation of strong decays of $h_1(1170)$ and $h_1(1380)$. We find that for $\theta_{K_1}\approx (28-30)^\circ$, the corresponding $\alpha_{^3\!P_1}$ and $\alpha_{^1\!P_1}$ agree well with all lattice and phenomenological analyses. This again reinforces the statement that $\theta_{K_1}\sim 33^\circ$ is much more favored than $57^\circ$.