  Statistical mechanics can predict thermal equilibrium states for most
classical systems, but for an isolated quantum system there is no general
understanding on how equilibrium states dynamically emerge from the microscopic
Hamiltonian. For instance, quantum systems that are near-integrable usually
fail to thermalize in an experimentally realistic time scale and, instead,
relax to quasi-stationary prethermal states that can be described by
statistical mechanics when approximately conserved quantities are appropriately
included in a generalized Gibbs ensemble (GGE). Here we experimentally study
the relaxation dynamics of a chain of up to 22 spins evolving under a
long-range transverse field Ising Hamiltonian following a sudden quench. For
sufficiently long-ranged interactions the system relaxes to a new type of
prethermal state that retains a strong memory of the initial conditions. In
this case, the prethermal state cannot be described by a GGE, but rather arises
from an emergent double-well potential felt by the spin excitations. This
result shows that prethermalization occurs in a significantly broader context
than previously thought, and reveals new challenges for a generic understanding
of the thermalization of quantum systems, particularly in the presence of
long-range interactions.
