PI: Christopher Thomas
The LHCb experiment at CERN has discovered an unprecedented number of new hadrons (bound states of quarks and gluons), around 70 to date. Many of these are seen as short-lived resonances decaying through the strong interaction to lighter, stable hadrons. Numerous of them lie at unexpected energies compared to what is predicted in simple models of hadrons such as the quark model.
It is essential to have first-principles predictions for these resonances from Quantum Chromodynamics (QCD), the fundamental theory of the strong interaction. Up to now this has been challenging, but through algorithmic advances and facilities such as DiRAC, it has become possible to numerically compute the properties of these hadrons through an approach known as lattice QCD where equations are solved numerically in a finite, discretised spacetime.
LHCb produces copious amounts of charm quarks. When a charm quark and a light antiquark combine, they produce a “D-meson”. In a recent study [JHEP 07 (2025) 060, arXiv:2502.04232] using the DiRAC facility in Cambridge, we determined the properties of the first few excitations of these D-mesons where the particles have non-trivial amounts of internal “spin” angular momentum. These can decay to different types of lighter hadron in a variety of ways and so required us to carefully study “coupled-channel” scattering. The resulting amplitudes feature rich structures with broad and narrow peaks and dips – examples are shown in the first figure. Underlying these features are pole singularities corresponding to hadron resonances. We identify three such resonance poles in the spin-1 channel for the first time (JP=1+ where J is the spin and P is the parity) – these are shown in red in the second figure, where m is the mass and Γ is the width, inversely proportional to the lifetime. We also find a related hadron in the spin-2 channel (JP=2+) which is shown in blue. These results help elucidate some puzzles involving D-mesons, including conjectured exotic two-pole structures and a symmetry relating the spin-0 and spin-1 channels.
