Study of the η π 0 spectrum and search for a J PC = 1 − + exotic meson

A partial wave analysis ~PWA! of the hp 0 system ~where h!gg) produced in the charge exchange reaction p 2 p!hp 0 n at an incident momentum of 18 GeV/c is presented as a function of the hp 0 invariant mass mhp 0 and momentum transfer squared t p2!hp , from the incident p 2 to the outgoing hp 0 system. S, P, and D waves were included in the PWA. The a0(980) and a2(1320) states are clearly observed in the overall hp 0 effective mass distribution as well as in the amplitudes associated with S wave and D waves, respectively, after partial wave decomposition. The observed distributions in moments ~averages of spherical harmonics! were compared to the results from the PWA and the two are consistent. The distribution in t p2!hp for individual D waves associated with natural and unnatural parity exchange in the t channel are consistent with Regge phenomenology. Of particular interest in this study is the P wave since this leads to an exotic J PC 51 21 for the hp system. A P wave is present in the data; however, attempts to describe the mass dependence of the amplitude and phase motion with respect to the D wave as a Breit-Wigner resonance are problematic. This has implications regarding the existence of a reported exotic J PC 51 21 meson decaying into hp 0 with a mass near 1.4 GeV/c 2 .