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<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mover accent="true"><mml:mi>c</mml:mi><mml:mo>¯</mml:mo></mml:mover><mml:mi>c</mml:mi></mml:math>purity of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>ψ</mml:mi><mml:mo stretchy="false">(</mml:mo><mml:mn>3770</mml:mn><mml:mo stretchy="false">)</mml:mo></mml:math>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>ψ</mml:mi><mml:mo>′</mml:…
It is suggested that the resonance $\ensuremath{\psi}(3770)$ may contain a sizeable ($O(10%)$ in terms of the probability weight factor) four-quark component with the up- and down-quarks and antiquarks in addition to the $c\overline{c}$ pair, which component in itself has a substantial part with isospin $I=1$. Furthermore such a four-quark part …