Systematics of doubly heavy strange and nonstrange tetraquarks

Masses, magnetic moments, and color-spin structures of nonstrange and strange tetraquarks with two heavy quarks are systematically studied in a QCD string picture with a chromomagnetic interaction. Our mass computations, combined with weak and radiative decays, indicate that there are two doubly heavy tetraquarks, the bottom-charmed tetraquark <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msub><a:mi>T</a:mi><a:mrow><a:mi>c</a:mi><a:mi>b</a:mi></a:mrow></a:msub><a:mo stretchy="false">(</a:mo><a:mn>7173</a:mn><a:mo>,</a:mo><a:mn>0</a:mn><a:msup><a:mn>1</a:mn><a:mo>+</a:mo></a:msup><a:msup><a:mo stretchy="false">)</a:mo><a:mn>0</a:mn></a:msup></a:math> and the doubly bottom tetraquark <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:msub><e:mi>T</e:mi><e:mrow><e:mi>b</e:mi><e:mi>b</e:mi></e:mrow></e:msub><e:mo stretchy="false">(</e:mo><e:mn>10406</e:mn><e:mo>,</e:mo><e:mn>0</e:mn><e:msup><e:mn>1</e:mn><e:mo>+</e:mo></e:msup><e:msup><e:mo stretchy="false">)</e:mo><e:mo>−</e:mo></e:msup></e:math>, that are stable against strong decay, having lifetimes of 0.069 and 326 fs, respectively. Magnetic moments are also computed and are found to be sensitive to chromomagnetic mixing of the color-spin configurations of these tetraquarks, making them a valuable probe to experimentally detect the nontrivial color configuration <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:msub><i:mn>6</i:mn><i:mi>c</i:mi></i:msub><i:mo stretchy="false">⊗</i:mo><i:msub><i:mover accent="true"><i:mn>6</i:mn><i:mo stretchy="false">¯</i:mo></i:mover><i:mi>c</i:mi></i:msub></i:math>.