The nature of ${\chi }_{c1}\left(3872\right)$ and $T_{cc}^{+}\left(3875\right)$

Abstract

Two decades ago the ${\chi }_{c1}\left(3872\right)$ was discovered in the hadron spectrum with two heavy quarks. The discovery fueled a surge in experimental research, uncovering dozens of so called XYZ exotics states lying outside the conventional quark model, as well as theoretical investigations into new forms of matter, such as quark-gluon hybrids, tetraquarks, pentaquarks, with the potential of disclosing new information about the fundamental strong force. Among the XYZs, the ${\chi }_{c1}\left(3872\right)$ and $T_{cc}^{+}\left(3875\right)$ stand out for their striking characteristics and unlashed many discussions about their nature. Here, we address this question using the Born--Oppenheimer Effective Field Theory (BOEFT) and show how QCD settles the issue of their composition. Not only we describe well the main features of the ${\chi }_{c1}\left(3872\right)$ and $T_{cc}^{+}\left(3875\right)$ but obtain also predictions in the bottomonium sector. This opens the way to systematic applications of BOEFT to all XYZs.

12 pages, 3 figures, 2 tables: v2: Minor updates, updated and new references, new supplementary section on Schroedinger equations, multiplets and spin-splittings, v3: minor updates, version published in Physical Review Letters. Results and conclusions unchanged

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• Unravelling Pentaquarks with Born--Oppenheimer effective theory
• Exotic hadrons associated with $b$-quark
• Binding energy of the $T_{bb}$ tetraquark from lattice QCD with relativistic and nonrelativistic heavy-quark actions
• Radiative decays of hadronic molecules: From confusion to inspiration
• The Compact X and Z and their Invisible Molecular Partners
• Predicting charmed-strange molecular tetraquarks with $K^{(\ast )}$ and $T$-doublet charmed or anticharmed meson
• Resummation of threshold double logarithms in quarkonium fragmentation functions