Angular distribution in s-channel formation of the pentaquark $\Theta^+$ baryon

TL;DR

This study analyzes the angular distribution of the pentaquark $ heta^+$ in a specific reaction, providing evidence against certain spin-parity assignments and strengthening the case for its existence through angular dependence analysis.

Contribution

It introduces an analysis of the $ heta^+$ angular distribution, demonstrating interference effects and refining the statistical significance of the $ heta^+$ signal.

Findings

Interference between s-wave and higher-wave contributions explains the angular distribution.

Spin-parity 1/2$^-$ is statistically excluded for $ heta^+$.

Signal significance increases to 7.1 standard deviations with angular selection.

Abstract

Using the DIANA data on the charge-exchange reaction $K^+n \rightarrow pK^0$ on a bound neutron, in which the s-channel formation of the pentaquark baryon $\Theta^+(1538)$ has been observed, we analyze the dependence of the background-subtracted $\Theta^+ \rightarrow pK^0$ signal on the $K^0$ emission angle in the $pK^0$ rest frame. In order to describe the observed $\cos\Theta_K^\mathrm{cms}$ distribution, invoking the interference between the nonresonant s-wave and the $\Theta^+$-mediated higher-wave contributions to the amplitude of the charge-exchange reaction is required at a 2.8$\sigma$ level. The spin--parity assignment of 1/2$^-$ for the $\Theta^+$ baryon is ruled out at a statistical level of 2.9 standard deviations. A physically-meaningful selection in $\cos\Theta_K^\mathrm{cms}$ based on the observed angular dependence of the $\Theta^+ \rightarrow pK^0$ signal allows to boost the statistical significance of the signal up to 7.1 standard deviations. This is far in excess of previously reported signals and renders the $\Theta^+$ existence more credible.