  The COMPASS Collaboration measured the transverse momentum dependent
multiplicities of charged hadrons produced in Deep Inelastic Scattering (DIS)
off unpolarized protons. Complementing previous COMPASS measurements obtained
with an isoscalar target, the data have been collected in 2016 and 2017 with
160 GeV/$c$ muon beams and a liquid hydrogen target. The multiplicities are
studied as a function of the square of the hadron transverse momentum with
respect to the virtual photon direction $P_{hT}^2$ in bins of the Bjorken
variable $x$, of the photon virtuality $Q^2$ and of the fraction $z$ of the
photon energy carried by the hadron. Preliminary results of this analysis,
performed on a fraction of the available data sample, are shown here for the
first time.
