Measurements of psi'-> pbar K+ sigma0 and chi_cj->pbar K+ Lambda

TL;DR

This paper reports the first observation of certain baryonic decays of psi' and improved measurements of chi_cJ decays, revealing an unexpected near-threshold enhancement in the pbar Lambda invariant mass.

Contribution

It provides the first measurement of psi' to pbar K+ Sigma0 decay and improved measurements of chi_cJ to pbar K+ Lambda decays, including an unexpected near-threshold enhancement.

Findings

First observation of psi' to pbar K+ Sigma0 decay.

Precise measurements of chi_cJ to pbar K+ Lambda branching fractions.

Detection of an anomalous near-threshold enhancement in the pbar Lambda invariant mass.

Abstract

Using a sample of $1.06\times10^8$ $\psi^\prime$ mesons collected with the BESIII detector at the BEPCII $e^+e^-$ collider and $\chi_{cJ}$ mesons produced via radiative transitions from the $\psi^\prime$, we report the first observation for $\psi^\prime\to \bar{p}K^+\Sigma^0 +c.c.$ (charge-conjugate), as well as improved measurements for the $\chi_{cJ}$ hyperon decays $\chi_{cJ}\to\bar{p}K^+\Lambda+c.c.$. The branching fractions are measured to be $\mathcal{B}(\psi^\prime\to \bar{p}K^+\Sigma^0+c.c)= (1.67\pm0.13\pm0.12)\times10^{-5}$, $\mathcal{B}(\chi_{c0}\to\bar{p}K^+\Lambda+c.c.)= (13.2\pm0.3\pm1.0)\times10^{-4}$, $\mathcal{B}(\chi_{c1}\to\bar{p}K^+\Lambda+c.c.)= (4.5\pm0.2\pm0.4)\times10^{-4}$ and $\mathcal{B}(\chi_{c2}\to\bar{p}K^+\Lambda+c.c)= (8.4\pm0.3\pm0.6)\times10^{-4}$, where the first error is statistical, and the second is systematic. In the decay of $\chi_{c0} \to \bar{p}K^+\Lambda + c.c.$, an anomalous enhancement near threshold is observed in the invariant mass distribution of $\bar{p}\Lambda+c.c.$, which cannot be explained by phase space.