  Preliminary results obtained by the COMPASS experiment for
longitudinal-spin-dependent azimuthal asymmetries in single-hadron
muoproduction off protons are presented for the first time. The analysis was
carried out on the full data sample collected by COMPASS with longitudinally
polarized proton ($NH_3$) target in 2007 and 2011 using 160~\gvc and 200~\gvc
muon beams, correspondingly. Within QCD parton model approach extracted
asymmetries are giving access to the specific convolutions of certain
transverse-momentum-dependent \textit{twist-2} and \textit{higher twist} parton
distribution functions and fragmentation functions. In order to access
different features of the involved distributions, the asymmetries are extracted
as functions of different kinematic variables for both positive and negative
single-hadron productions. The significant amount of collected data and wide
kinematic range give unique importance to the results. Obtained asymmetries are
compared with previous measurements performed by HERMES experiment and with
available model predictions.
