Study of the doubly charmed tetraquark Tþ cc

Quantum chromodynamics, the theory of the strong force, describes interactions of coloured quarks and gluons and the formation of hadronic matter. Conventional hadronic matter consists of baryons and mesons made of three quarks and quark-antiquark pairs, respectively. Particles with an alternative quark content are known as exotic states. Here a study is reported of an exotic narrow state in the D0D 0π+ mass spectrum just below the D*+D0 mass
threshold produced in proton-proton collisions collected with the LHCb detector at the Large Hadron Collider. The state is consistent with the ground isoscalar Tþ
cc tetraquark with a quark content of ccud and spin-parity quantum numbers JP = 1+. Study of the DD mass spectra disfavours interpretation of the resonance as the isovector state. The decay structure via intermediate off-shell D *+ mesons is consistent with the observed D0π+ mass distribution. To analyse the mass of the resonance and its coupling to the D* D system, a dedicated model
is developed under the assumption of an isoscalar axial-vector Tþ cc state decaying to the D* D channel. Using this model, resonance parameters including the pole position, scattering length, effective range and compositeness are determined to reveal important information about the nature of the Tþ cc state. In addition, an unexpected dependence of the production rate on track multiplicity is observed.