Odd and Even Partial Waves of $\eta\pi^-$ and $\eta'\pi^-$ in $\pi^-p\to\eta^{(\prime)}\pi^-p$ at $191\,\textrm{GeV}/c$

TL;DR

This study analyzes the partial waves of $ ext{ηπ}^-$ and $ ext{η'}π^-$ produced in high-energy pion-proton collisions, revealing similarities in even-L waves and notable differences in odd-L waves, indicating different underlying quantum states.

Contribution

First detailed partial-wave analysis comparing ηπ− and η′π− systems at high energy, highlighting differences in odd-L waves and resonance behaviors.

Findings

Similarities in L=2,4,6 wave intensities and phases for ηπ− and η′π−

Enhanced odd-L wave intensities in η′π− indicating non-qar q states

Resonance peaks at 1.4 and 1.7 GeV/c^2 for different waves

Abstract

Exclusive production of $\eta\pi^-$ and $\eta'\pi^-$ has been studied with a $191\,\textrm{GeV}/c$ $\pi^-$ beam impinging on a hydrogen target at COMPASS (CERN). Partial-wave analyses reveal different odd/even angular momentum ($L$) characteristics in the inspected invariant mass range up to $3\,\textrm{GeV}/c^2$. A striking similarity between the two systems is observed for the $L=2,4,6$ intensities (scaled by kinematical factors) and the relative phases. The known resonances $a_2(1320)$ and $a_4(2040)$ are in line with this similarity. In contrast, a strong enhancement of $\eta'\pi^-$ over $\eta\pi^-$ is found for the $L=1,3,5$ waves, which carry non-$q\bar q$ quantum numbers. The $L=1$ intensity peaks at $1.7\,\textrm{GeV}/c^2$ in $\eta'\pi^-$ and at $1.4\,\textrm{GeV}/c^2$ in $\eta\pi^-$, the corresponding phase motions with respect to $L=2$ are different.