We study the statistical mechanics of supercooled liquids when the
system evolves at a temperature TT
with a field \epsilonϵ
linearly coupled to its overlap with a reference configuration of the
same liquid sampled at a temperature T_0T0.
We use mean-field theory to fully characterize the influence of the
reference temperature T_0T0,
and we mainly study the case of a fixed,
low-T_0T0
value in computer simulations. We numerically investigate the extended
phase diagram in the (\epsilon,T)(ϵ,T)
plane of model glass-forming liquids in spatial dimensions
d=2d=2
and d=3d=3,
relying on umbrella sampling and reweighting techniques. For both
2d2d
and 3d3d
cases, a similar phenomenology with nontrivial thermodynamic
fluctuations of the overlap is observed at low temperatures, but a
detailed finite-size analysis reveals qualitatively distinct behaviors.
We establish the existence of a first-order transition line for nonzero
\epsilonϵ
ending in a critical point in the universality class of the random-field
Ising model (RFIM) in d=3d=3.
In d=2d=2
instead, no phase transition is found in large enough systems at least
down to temperatures below the extrapolated calorimetric glass
transition temperature T_gTg.
Our results confirm that glass-forming liquid samples of limited size
display the thermodynamic fluctuations expected for finite systems
undergoing a random first-order transition. They also support the
relevance of the physics of the RFIM for supercooled liquids, which may
then explain the qualitative difference between
2d2d
and 3d3d
glass-formers.