  The semileptonic decay asymmetry $\mathcal{A}_{\Delta m}$ is studied within
the open quantum systems approach to the physics of the neutral meson
$B^0$-$\overline{B^0}$ system: this extended treatment takes into account
possible non-standard, dissipative effects induced by the presence of an
external environment. A bound on these effects is provided through the analysis
of available experimental data from the Belle Collaboration.
