First measurement of and
Using a sample of events collected by the BESIII detector, this study reports the first observation of the decays and with measured branching fractions, while finding no significant evidence for hidden-strange pentaquark candidates and and setting corresponding upper limits.
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Technical Summary: First Observation of and Decays and Search for Hidden-Strange Pentaquarks
Problem and Motivation
In the conventional quark model, hadrons are composed of two () or three ($qqq$) quarks. However, Quantum Chromodynamics (QCD) permits exotic hadrons with larger numbers of quarks. Following the 2015 LHCb observation of hidden-charm pentaquarks (), theoretical interest has shifted toward analogous hidden-strange pentaquark states. Specifically, interactions between and mesons are predicted to generate resonant structures similar to the states but with content instead of . Two such candidates, (from interaction) and (from interaction), have been proposed as hidden-strange partners to and , respectively. Prior to this work, direct observations of these resonance states in charmonium hadronic decays had not been reported.
Methodology
The analysis utilizes a data sample of events collected by the BESIII detector at the BEPCII collider. The study focuses on two decay chains:
- , with and .
- , with and .
Event selection requires charged tracks within specific polar angles and vertex constraints, alongside particle identification (PID) using $dE/dx$ and time-of-flight (TOF) measurements. Photon candidates are identified in the electromagnetic calorimeter (EMC). Reconstruction involves kinematic fits (1C for , 5C for the full decay chain) to constrain invariant masses and improve resolution.
To determine detection efficiencies and model the decay dynamics accurately, the authors employ a Partial-Wave Analysis (PWA). This approach accounts for intermediate resonances (e.g., and ) and non-resonant contributions, generating Monte Carlo (MC) samples that better match the data than simple phase-space (PHSP) models. Signal yields are extracted via unbinned maximum-likelihood fits to the relevant invariant mass spectra. Systematic uncertainties are evaluated by varying fit ranges, signal/background shapes, kinematic fit parameters, and detection efficiency models.
Key Contributions and Results
First Observation of Decays: The paper reports the first observation of the decays and
- The branching fraction for is measured as .
- The branching fraction for is measured as .
Search for Hidden-Strange Pentaquarks: The authors searched for the and candidates in the invariant mass spectra of and , respectively.
- No significant signals were observed for either candidate.
- Upper limits on the branching fractions at the 90% confidence level (C.L.) were established:
- .
- .
Significance
The paper claims that these results provide the first experimental evidence for the specific decay modes involving and in decays. Furthermore, by setting stringent upper limits on the production of and , the study provides critical experimental input and constraints for theoretical models attempting to describe hidden-strange pentaquark states. The absence of observed signals suggests that if these states exist, their production rates in decays are significantly lower than previously hypothesized or that their properties differ from the simple molecular interpretations analogous to the states.
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