PI: Christopher Thomas
2024 Science Highlight for “Hadron Resonances from Lattice QCD”
(used Data Intensive Service at Cambridge)

Over the last couple of decades experiments have collected a wealth of data on hadrons, strongly bound clusters of quarks, and have made a number of unexpected observations that have driven theoretical efforts to understand their nature. A particularly interesting example is the recent observation of the doubly-charmed Tcc(3875) by the LHCb experiment at CERN. This contains two charm quarks and is charged, and so it is a manifestly exotic state that cannot be explained in the conventional picture of a quark-antiquark system.

Quantum Chromodynamics (QCD) is the theory of the strong interaction, but the quarks and gluons of QCD interact strongly and so making predictions from the fundamental equations is challenging. A first-principles, systematically-improvable approach is lattice QCD where the equations are solved numerically in a finite, discretised spacetime using large computational facilities such as DiRAC

In this work [arXiv:2405.15741], using lattice QCD with unphysically-heavy light quarks corresponding to a pion mass of 391 MeV, we computed coupled-channel D D* — D* D* scattering amplitudes with spin J=1 and parity P=+ involving pseudoscalar charm mesons (D) and vector charm mesons (D), as shown in the left panel of the figure. We found enhancements in some of the scattering amplitudes and traced these back to the two pole singularities shown in the right panel of the figure. The virtual bound state just below D D threshold corresponds to the Tcc and the resonance just below D* D* threshold corresponds to a Tcc’ which has not yet been observed in experiment, the first time this state had been seen in a first-principles calculations. The results support heavy quark spin symmetry ideas and imply that the Tcc’ should be observable in ongoing experiments.