Very recently the LHCb collaboration reported their observation of the first two fully open-flavor tetraquark states, the X 0(2900) of JP = 0+ and the X 1(2900) of JP = 1−. We study their possible interpretations using the method of QCD sum rules, paying special attention to an interesting feature of this experiment that the higher resonance X 1(2900) has a width significantly larger than the lower one X 0(2900). Our results suggest that the X 0(2900) can be interpreted as the s-wave D *– K *+ molecule state of JP = 0+, and the X 1(2900) can be interpreted as the p-wave c ¯ s ¯ u d compact tetraquark state of JP = 1−. Mass predictions of their bottom partners are also given.
We apply the method of QCD sum rules to study the $$s s {\bar{s}} {\bar{s}}$$sss¯s¯ tetraquark states of $$J^{PC} = 0^{-+}$$JPC=0-+. We construct all the relevant $$s s {\bar{s}} {\bar{s}}$$sss¯s¯ tetraquark currents, and find that there are only two independent ones. We use them to further construct two weakly-correlated mixed currents. One of them leads to reliable QCD sum rule results and the mass is extracted to be $$2.51^{+0.15}_{-0.12}$$2.51-0.12+0.15 GeV, suggesting that the X(2370) or the X(2500) can be explained as the $$ss{\bar{s}}{\bar{s}}$$sss¯s¯ tetraquark state of $$J^{PC} = 0^{-+}$$JPC=0-+. To verify this interpretation, we propose to further study the $$\pi \pi /K {\bar{K}}$$ππ/KK¯ invariant mass spectra of the $$J/\psi \rightarrow \gamma \pi \pi \eta ^\prime /\gamma K {\bar{K}} \eta ^\prime $$J/ψ→γππη′/γKK¯η′ decays in BESIII to examine whether there exists the $$f_0(980)$$f0(980) resonance.
We study the hybrid mesons with the exotic quantum number I G J PC = 0+1−+ and investigate their decays into the η η′, a 1(1260) π, f 1(1285) η, f 1(1420) η, K * ( 892 ) K ¯ , K 1 ( 1270 ) K ¯ , and K 1 ( 1400 ) K ¯ channels. It is found that the QCD axial anomaly enhances the decay width of the η η′ channel although this mode is strongly suppressed by the small p-wave phase space. Our results support the interpretation of the η 1(1855) recently observed by BESIII as the s ¯ s g hybrid meson of I G J PC = 0+1−+. The QCD axial anomaly ensures the η η′ decay mode to be a characteristic signal of the hybrid nature of the η 1(1855).
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