Observation of e⁺e⁻ → π⁰π⁰h_c and a Neutral Charmoniumlike Structure Z_c(4020)°



Using data collected with the BESIII detector operating at the Beijing Electron Positron Collider at center-of-mass energies of √s = 4.23, 4.26, and 4.36 GeV, we observe e⁺e⁻ → π⁰π⁰h_c for the first time. The Born cross sections are measured and found to be about half of those of e⁺e⁻ → π⁺π⁻h_c within less than 2σ In the π⁰h_c mass spectrum, a structure at 4.02 GeV/c² is found. It is most likely to be the neutral isospin partner of the Z_c(4020)^± observed in the process of e⁺e⁻ → π⁺π⁻h_c being found. A fit to the π°h_c invariant mass spectrum, with the width of the Z_c(4020)° fixed to that of its charged isospin partner and possible interferences with non- Z_c(4020)° amplitudes neglected, gives a mass of (4023.9 ± 2.2 ± 3.8) MeV/c² for the Z_c(4020)°, where the first error is statistical and the second systematic.



Mass spectrometry, Isobaric spin, Mass spectrometers, Zc(4020), Charmonium


This work is supported in part by National Key Basic Research Program of China under Contract No. 2015CB856700; Joint Funds of the National Natural Science Foundation of China under Contracts No. 11079008, No. 11179007, No. U1232201, No. U1332201; National Natural Science Foundation of China (NSFC) under Contracts No. 10935007, No. 11121092, No. 11125525, No. 11235011, No. 11079023, No. 11322544, No. 11335008; the Chinese Academy of Sciences (CAS) Large-Scale Scientific Facility Program; CAS under Contracts No. KJCX2-YW-N29, No. KJCX2-YW-N45; 100 Talents Program of CAS; German Research Foundation DFG under Contract No. Collaborative Research Center CRC-1044; Istituto Nazionale di Fisica Nucleare, Italy; Ministry of Development of Turkey under Contract No. DPT2006K-120470; Russian Foundation for Basic Research under Contract No. 14-07-91152; U.S. Department of Energy under Contracts No. DEFG02- 04ER41291, No. DE-FG02-05ER41374, No. DEFG02- 94ER40823, No. DESC0010118; U.S. National Science Foundation; University of Groningen (RuG) and the Helmholtzzentrum fuer Schwerionenforschung GmbH (GSI), Darmstadt; WCU Program of National Research Foundation of Korea under Contract No. R32-2008-000- 10155-0.


©2014 American Physical Society