Investigating the internal structure of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>X</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mn>6900</mml:mn> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:math> in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mn>2</mml:mn> <mml:mi>J</mml:mi> <mml:mo>/</mml:mo> <mml:mi>ψ</mml:mi> </mml:mrow> </mml:math> decay channel

Assuming X ( 6900 ) is a tetraquark state, the decay width of X ( 6900 ) → 2 J / ψ is calculated in a covariant quark model, with the diquark-antidiquark [ c c ] [ c ¯ c ¯ ] picture. Two possible structures, vector-vector and axial-vector–axial-vector coupling, are investigated. The result indicates that the axial-vector–axial-vector coupling is consistent with the experiments. Additionally, as another application of the covariant quark model, the decay width of X ( 6900 ) → 2 η c is predicted to be 66–88 keV.