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<p>We propose a model explaining the strong isospin violations observed in <i>X</i>(3872) decays. The <i>X</i> is assumed to be a compact isosinglet tetraquark and its charged partners to form an isotriplet, together with an additional neutral state mixing with the isosinglet. The isotriplet results from the loose binding of open charm mesons and, in isolation, has strongly suppressed production rates. However, its neutral component can still mix with the isosinglet and induce the large isospin violations observed. The compact-isosinglet/molecular-isotriplet pattern appears reversed when the <i>Z</i>(3900) resonances are considered. The <i>Z</i> particles observed correspond to a compact isotriplet and there is no evidence of a neutral isosinglet <i>Z</i>. We show that no isospin violation in the <i>Z</i> decays is expected.</p>

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The compact X and Z and their invisible molecular partners

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年5月18日

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  • A. Carducci,
  • B. Grinstein,
  • D. Germani,
  • A. D. Polosa

摘要

We propose a model explaining the strong isospin violations observed in X(3872) decays. The X is assumed to be a compact isosinglet tetraquark and its charged partners to form an isotriplet, together with an additional neutral state mixing with the isosinglet. The isotriplet results from the loose binding of open charm mesons and, in isolation, has strongly suppressed production rates. However, its neutral component can still mix with the isosinglet and induce the large isospin violations observed. The compact-isosinglet/molecular-isotriplet pattern appears reversed when the Z(3900) resonances are considered. The Z particles observed correspond to a compact isotriplet and there is no evidence of a neutral isosinglet Z. We show that no isospin violation in the Z decays is expected.