  Despite the unquestionable empirical success of quantum theory, witnessed by
the recent uprising of quantum technologies, the debate on how to reconcile the
theory with the macroscopic classical world is still open. Spontaneous collapse
models are one of the few testable solutions so far proposed. In particular,
the continuous spontaneous localization (CSL) model has become subject of an
intense experimental research. Experiments looking for the universal force
noise predicted by CSL in ultrasensitive mechanical resonators have recently
set the strongest unambiguous bounds on CSL; further improving these
experiments by direct reduction of mechanical noise is technically challenging.
Here, we implement a recently proposed alternative strategy, that aims at
enhancing the CSL noise by exploiting a multilayer test mass attached on a high
quality factor microcantilever. The test mass is specifically designed to
enhance the effect of CSL noise at the characteristic length $r_c=10^{-7}$ m.
The measurements are in good agreement with pure thermal motion for
temperatures down to 100 mK. From the absence of excess noise we infer a new
bound on the collapse rate at the characteristic length $r_c=10^{-7}$ m, which
improves over previous mechanical experiments by more than one order of
magnitude. Our results are explicitly challenging a well-motivated region of
the CSL parameter space proposed by Adler.
