Quantum numbers of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>X</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mn>1880</mml:mn> <mml:mo stretchy="false">)</mml:mo> </mml:math>

We study the properties of the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>X</a:mi> <a:mo stretchy="false">(</a:mo> <a:mn>1880</a:mn> <a:mo stretchy="false">)</a:mo> </a:math> , the structure around the <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mover accent="true"> <e:mi>p</e:mi> <e:mo stretchy="false">¯</e:mo> </e:mover> <e:mi>p</e:mi> </e:math> threshold that appears in the <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:mrow> <i:mn>3</i:mn> <i:mo stretchy="false">(</i:mo> <i:msup> <i:mrow> <i:mi>π</i:mi> </i:mrow> <i:mrow> <i:mo>+</i:mo> </i:mrow> </i:msup> <i:msup> <i:mrow> <i:mi>π</i:mi> </i:mrow> <i:mrow> <i:mo>−</i:mo> </i:mrow> </i:msup> <i:mo stretchy="false">)</i:mo> </i:mrow> </i:math> invariant mass spectrum in the decay process of <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"> <m:mrow> <m:mi>J</m:mi> <m:mo>/</m:mo> <m:mi>ψ</m:mi> <m:mo stretchy="false">→</m:mo> <m:mi>γ</m:mi> <m:mn>3</m:mn> <m:mo stretchy="false">(</m:mo> <m:msup> <m:mrow> <m:mi>π</m:mi> </m:mrow> <m:mrow> <m:mo>+</m:mo> </m:mrow> </m:msup> <m:msup> <m:mrow> <m:mi>π</m:mi> </m:mrow> <m:mrow> <m:mo>−</m:mo> </m:mrow> </m:msup> <m:mo stretchy="false">)</m:mo> </m:mrow> </m:math> . Nucleon-antinucleon rescattering is taken into account in our analysis, and the decay amplitude of <r:math xmlns:r="http://www.w3.org/1998/Math/MathML" display="inline"> <r:mrow> <r:mi>J</r:mi> <r:mo>/</r:mo> <r:mi>ψ</r:mi> <r:mo stretchy="false">→</r:mo> <r:mi>γ</r:mi> <r:mn>3</r:mn> <r:mo stretchy="false">(</r:mo> <r:msup> <r:mrow> <r:mi>π</r:mi> </r:mrow> <r:mrow> <r:mo>+</r:mo> </r:mrow> </r:msup> <r:msup> <r:mrow> <r:mi>π</r:mi> </r:mrow> <r:mrow> <r:mo>−</r:mo> </r:mrow> </r:msup> <r:mo stretchy="false">)</r:mo> </r:mrow> </r:math> can be obtained by the distorted wave Born approximation. With these amplitudes, we analyze the contributions to the <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"> <w:mi>X</w:mi> <w:mo stretchy="false">(</w:mo> <w:mn>1880</w:mn> <w:mo stretchy="false">)</w:mo> </w:math> from different partial waves. Our analysis suggests that the <ab:math xmlns:ab="http://www.w3.org/1998/Math/MathML" display="inline"> <ab:mi>X</ab:mi> <ab:mo stretchy="false">(</ab:mo> <ab:mn>1880</ab:mn> <ab:mo stretchy="false">)</ab:mo> </ab:math> should be isoscalar <eb:math xmlns:eb="http://www.w3.org/1998/Math/MathML" display="inline"> <eb:msup> <eb:mn>0</eb:mn> <eb:mrow> <eb:mo>−</eb:mo> <eb:mo>+</eb:mo> </eb:mrow> </eb:msup> </eb:math> , and it is generated by the threshold behavior.