LISA pathfinder appreciably constrains collapse models

Spontaneous collapse models are phenomological theories formulated to address major difficulties in macroscopic quantum mechanics. We place significant bounds on the parameters of the leading collapse models, the continuous spontaneous localization (CSL) model, and the Diosi-Penrose (DP) model, by using LISA Pathfinder's measurement, at a record accuracy, of the relative acceleration noise between two free-falling macroscopic test masses. In particular, we bound the CSL collapse rate to be at most $(2.96\ifmmode\pm\else\textpm\fi{}0.12)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\text{ }\text{ }{\mathrm{s}}^{\ensuremath{-}1}$. This competitive bound explores a new frequency regime, 0.7 to 20 mHz, and overlaps with the lower bound ${10}^{\ensuremath{-}8\ifmmode\pm\else\textpm\fi{}2}\text{ }\text{ }{\mathrm{s}}^{\ensuremath{-}1}$ proposed by Adler in order for the CSL collapse noise to be substantial enough to explain the phenomenology of quantum measurement. Moreover, we bound the regularization cutoff scale used in the DP model to prevent divergences to be at least $40.1\ifmmode\pm\else\textpm\fi{}0.5\text{ }\text{ }\mathrm{fm}$, which is larger than the size of any nucleus. Thus, we rule out the DP model if the cutoff is the size of a fundamental particle.