A semi-inclusive measurement of charged hadron multiplicities in deep inelastic muon scattering off an isoscalar target was performed using data collected by the COMPASS Collaboration at CERN. The following kinematic domain is covered by the data: photon virtuality Q2 > 1 ðGeV=cÞ2, invariant mass of the hadronic system W > 5 GeV=c2, Bjorken scaling variable in the range 0.003 <x< 0.4, fraction of the virtual photon energy carried by the hadron in the range 0.2 <z< 0.8, and square of the hadron transverse momentum with respect to the virtual photon direction in the range 0.02 ðGeV=cÞ2 < P2hT < 3 ðGeV=cÞ2. The multiplicities are presented as a function of P2hT in three-dimensional bins of x, Q2, z and compared to previous semi-inclusive measurements. We explore the small-P2hT region, i.e. P2hT < 1 ðGeV=cÞ2, where hadron transverse momenta are expected to arise from nonperturbative effects, and also the domain of larger P2hT, where contributions from higher-order perturbative QCD are expected to dominate. The multiplicities are fitted using a single-exponential function at small P2hT to study the dependence of the average transverse momentum hP2hTi on x, Q2 and z. The power-law behavior of the multiplicities at large P2hT is investigated using various functional forms. The fits describe the data reasonably well over the full measured range.
