Physicists from the Beijing Spectrometer (BESIII) Collaboration at the Beijing Electron Positron Collider in China have detected the first candidate for a charged hidden-charm tetraquark with strangeness.
Illustration of the Zcs tetraquark. Image credit: BESIII Collaboration.
Dubbed Zcs(3985), the newly-discovered multi-quark structure has a mass of 3.98 GeV/c2.
It decays to a charged strange-charmed meson plus a neutral charmed meson, i.e., Ds– D*0+Ds*-D0.
“Studies of exotic hadron states, i.e., those that contain more valence-quark constituents than the conventional three-quark baryons and two-quark mesons, provide unique access to details of the non-perturbative effects and color confinement mechanisms in quantum chromodynamics (QCD) theory,” the BESIII physicists said.
“The existence of hidden-charm tetraquarks with non-zero strangeness, Zcs, was predicted by many theoretical models, but until now, no definitive experimental observations have been reported.”
“The new discovery emerged from analyses of data sets taken at five center-of-mass energy points ranging from 4.628 to 4.698 GeV in 2020.”
Using a novel partial reconstruction method, the researchers observed the events corresponding to e+ e–→K+ (Ds– D*0+Ds*-D0) by a distinct peak at the D*0 mass in the K+ Ds*- recoil mass spectrum.
“After removing random-combination backgrounds, a clear enhancement was seen near the Ds– D*0 and Ds*-D0 mass thresholds that could not be attributed to any known conventional excited (strange)-charmed mesons,” they explained.
“Therefore, the hypothesis of the presence of an exotic state Zcs(3985) in the Ds– D*0 and Ds*-D0 mass spectrum was introduced and found to fit to the observed excess very well.”
The BESIII team observed 127 decays events with a global significance of 5.3 standard deviations.
“The Zcs(3985) observation represents a significant breakthrough that opens up a new dimension of the tetraquark meson spectrum,” the scientists said.
“Curiously, the Zcs(3985) production is more pronounced at center-of-mass energies around 4.68 GeV than at nearby higher or lower energies, which indicates that this energy may be of particular interest.”
The team’s paper was published in the journal Physical Review Letters.
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M. Ablikim et al. (BESIII Collaboration). 2021. Observation of a Near-Threshold Structure in the K+ Recoil-Mass Spectra in e+ e–→K+ (Ds– D*0+Ds*-D0). Phys. Rev. Lett 126 (10): 102001; doi: 10.1103/PhysRevLett.126.102001
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