Measurement of the I=1/2 $K \pi$ $\mathcal{S}$-wave amplitude from Dalitz plot analyses of $\eta_c \to K \bar K \pi$ in two-photon interactions

TL;DR

This paper measures the $K \, \pi$ S-wave amplitude in $\, \eta_c$ decays via Dalitz plot analysis, revealing a new resonance and providing model-independent phase and amplitude measurements up to 1.5 GeV/c².

Contribution

It presents the first model-independent measurement of the $I=1/2$ $K \, \pi$ S-wave amplitude and phase from $\, \eta_c$ decays, discovering a new $a_0(1950)$ resonance.

Findings

Observation of $\, \eta_c$ decays to $K \, \bar{K} \, \pi$ final states.

Model-independent measurement of the $K \, \pi$ S-wave amplitude and phase.

Identification of a new $a_0(1950)$ resonance with specific mass and width.

Abstract

We study the processes $\gamma \gamma \to K^0_S K^{\pm}\pi^{\mp}$ and $\gamma \gamma \to K^+ K^- \pi^0$ using a data sample of 519~$fb^{-1}$ recorded with the BaBar detector operating at the SLAC PEP-II asymmetric-energy $e^+ e^-$ collider at center-of-mass energies at and near the $\Upsilon(nS)$ ($n = 2,3,4$) resonances. We observe $\eta_c$ decays to both final states and perform Dalitz plot analyses using a model-independent partial wave analysis technique. This allows a model-independent measurement of the mass-dependence of the $I=1/2$ $K \pi$ $\mathcal{S}$-wave amplitude and phase. A comparison between the present measurement and those from previous experiments indicates similar behaviour for the phase up to a mass of 1.5 $GeV/c^2$. In contrast, the amplitudes show very marked differences. The data require the presence of a new $a_0(1950)$ resonance with parameters $m=1931 \pm 14 \pm 22 \ MeV/c^2$ and $\Gamma=271 \pm 22 \pm 29 \ MeV$.