# Determination of spin and parity of the $Z_c(3900)$

## TL;DR

This study determines the spin and parity of the $Z_c(3900)$ state as $J^P=1^+$ with high significance, measures its pole mass and width, and investigates related production processes using BESIII data.

## Contribution

The paper provides the first definitive assignment of the $Z_c(3900)$ quantum numbers and precise measurements of its pole parameters, advancing understanding of its nature.

## Key findings

- $J^P=1^+$ quantum numbers confirmed with >7σ significance
- Pole mass measured as approximately 3881 MeV/c^2
- Pole width measured as approximately 52 MeV

## Abstract

The spin and parity of the $Z_c(3900)^\pm$ state are determined to be $J^P=1^+$ with a statistical significance larger than $7\sigma$ over other quantum numbers in a partial wave analysis of the process $e^+e^-\to \pi^+\pi^-J/\psi$. We use a data sample of 1.92 fb$^{-1}$ accumulated at $\sqrt{s}=4.23$ and 4.26 GeV with the BESIII experiment. When parameterizing the $Z_c(3900)^\pm$ with a Flatte-like formula, we determine its pole mass $M_\textrm{pole}=(3881.2\pm4.2_\textrm{stat}\pm52.7_\textrm{syst})\textrm{MeV}/c^2$ and pole width $\Gamma_\textrm{pole}=(51.8\pm4.6_\textrm{stat}\pm36.0_\textrm{syst})\textrm{MeV}$. We also measure cross sections for the process $e^+e^-\to Z_c(3900)^+\pi^-+c.c.\to J/\psi\pi^+\pi^-$ and determine an upper limit at the 90\% confidence level for the process $e^+e^-\to Z_c(4020)^+\pi^-+c.c.\to J/\psi\pi^+\pi^-$.