  Understanding how closed quantum systems dynamically approach thermal
equilibrium presents a major unresolved problem in statistical physics.
Generically, non-integrable quantum systems are expected to thermalize as they
comply with the Eigenstate Thermalization Hypothesis. However, in the presence
of strong disorder, the dynamics can possibly slow down to a degree that
systems fail to thermalize on experimentally accessible timescales, as in spin
glasses or many-body localized systems. In general, particularly in long-range
interacting quantum systems, the specific nature of the disorder necessary for
the emergence of a prethermal, metastable state--distinctly separating the
timescales of initial relaxation and subsequent slow thermalization--remains an
open question. We study an ensemble of Heisenberg spins with a tunable
distribution of random coupling strengths realized by a Rydberg quantum
simulator. We observe a drastic change in the late-time magnetization when
increasing disorder strength. The data is well described by models based on
pairs of strongly interacting spins, which are treated as thermal for weak
disorder and isolated for strong disorder. Our results indicate a crossover
into a pair-localized prethermal regime in a closed quantum system of thousands
of spins in the critical case where the exponent of the power law interaction
matches the spatial dimension.
