The pole structures of the $X(1840)/X(1835)$ and the $X(1880)$

TL;DR

This paper investigates the pole structures of certain exotic states near the proton-antiproton threshold using a chiral-symmetry-respecting interaction model, revealing their nature and coupling characteristics through analysis of experimental invariant mass distributions.

Contribution

The study constructs a novel $Nar{N}$ interaction model respecting chiral symmetry and extracts pole positions, providing new insights into the nature of threshold enhancements and their relation to specific Riemann sheets.

Findings

Threshold enhancement in $par{p}$ is from a pole on the third Riemann sheet.

Broader structure in $3( ext{pi}^+ ext{pi}^-)$ comes from a pole on the physical sheet.

Large compositeness suggests a $par{p}$ resonance exists.

Abstract

Whether the $N\bar{N}$ interaction could form a state or not is a long standing question, even before the observation of the $p\bar{p}$ threshold enhancement in 2003. The recent high statistic measurement in the $J/\psi \to \gamma 3(\pi^+\pi^-)$ channel would provide a good opportunity to probe the nature of the peak structures around the $p\bar{p}$ threshold in various processes. By constructing the $N\bar{N}$ interaction respecting chiral symmetry, we extract the pole positions by fitting the $p\bar{p}$ and $3(\pi^+\pi^-)$ invariant mass distributions of the $J/\psi \to \gamma p \bar p$ and $J/\psi \to \gamma 3(\pi^+\pi^-)$ processes. The threshold enhancement in the $p\bar{p}$ invariant mass distribution is from the pole on the third Riemann sheet, which more couples to the isospin triplet channel. The broader structure in the $3(\pi^+\pi^-)$ invariant mass comes from the pole on the physical Riemann sheet, which more couples to the isospin singlet channel. Furthermore, the large compositeness indicates that there should exit $p\bar{p}$ resonance based on the current experimental data. In addition, we also see a clear threshold enhancement in the $n\bar{n}$ channel, but not as significant as that in $p\bar{p}$ channel, which is useful and compared with further experimental measurement.