Electromagnetic form factors and structure of the $T_{bb}$ tetraquark

We present the first lattice QCD calculation of electromagnetic form factors of a tetraquark, focusing on the $T_{bb}=bb\bar{u}\bar{d}$ with quantum numbers $I(J^P)=0(1^{+})$. The electromagnetic current probes the charge monopole, magnetic dipole and the electric quadrupole distributions within the tetraquark. From it, we find evidence that its structure consists of a compact heavy diquark $[bb]$ in spin one, color-antitriplet configuration, and a light antidiquark $[\bar{u}\bar{d}]$ in spin zero, color-triplet configuration. The computations were performed on a single CLS ensemble with $N_f=2+1$ dynamical quarks at a lattice spacing $a\approx 0.064$ fm and with a pion mass $m_{\pi }\approx 290$ MeV.