This paper studies how charmonium — a bound state of a charm quark and an anti‑charm quark — behaves at high temperature. The authors use lattice QCD, a first‑principles numerical method, to probe temperatures between about 153 MeV and 305 MeV. Their main finding is that the charmonium states that lie below the open‑charm threshold still appear to exist in this temperature range. However, these states develop a noticeable thermal width that grows with temperature, and larger excited states broaden more than the smaller ground state.

To reach these conclusions the researchers computed Euclidean‑time correlation functions on ensembles with 2+1 dynamical light and strange quark flavors. They used the highly improved staggered quark (HISQ) action for the sea quarks and a relativistic Wilson clover action for the valence charm quarks. The work was done on lattices with spatial size 64^3 and two lattice spacings, about a = 0.0493 fm and a = 0.0404 fm, and temperatures were changed by varying the temporal extent of the lattice. The bare charm mass was tuned nonperturbatively by matching the lattice J/ψ mass to its experimental value.

A technical but important choice was to use extended meson operators built with Gaussian smearing. These operators separate the quark and antiquark in space and therefore have better overlap with true bound states than pointlike operators. That improved overlap made the correlators more sensitive to bound states and to changes caused by the thermal medium. The authors also applied one level of HYP smearing to the gauge links to reduce short‑distance noise.

Why this matters: charmonium is a key probe of the quark–gluon plasma created in heavy‑ion collisions. Whether and how charmonium states dissolve in the medium is connected to deconfinement and to an effective in‑medium potential that can acquire an imaginary part. Observing a growing thermal width supports the picture that dynamical in‑medium effects (often encoded as an imaginary part of the potential) play a role in reducing the lifetime of quarkonium states, even when the states remain identifiable.