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The production of a heavy quark is accompanied by gluon bremsstrahlung withangular and momentum spectra predicted by perturbative Quantum Chromo Dynamics(QCD). The radiation off heavy quarks is predicted to be suppressed for largemomentum particles, as a consequence of the angular ``dead cone effect''. Inthis paper, we studied this effect using data from Z boson decays to c- orb-quarks in $e^+e^-$ annihilation. The momentum spectra for charged particlesare reconstructed in the momentum fraction variable $x$ or $\xi=\ln(1/x)$ byremoving the decays of the heavy hadrons. We find an increasing suppression ofparticles with rising $x$ down to a fraction of $\lesssim 1/10$ for particleswith $x\gtrsim0.2$ in b-quark and $x\gtrsim0.4$ in c-quark jets in comparisonto light quark momentum spectra. The sensitivity to the dead cone effect in thepresent momentum analysis is larger than in the recently presented angularanalysis. The suppression for c- and b-quark fragmentation is in goodquantitative agreement with the expectations based on perturbative QCD withinthe Modified Leading Logarithmic Approximation (MLLA) in the central kinematicregion. The data also support a two parameter description in the MLLA of thesephenomena (``Limiting Spectrum''). The sensitivity of these measurements to theheavy quark mass is investigated.

The dead cone effect in heavy quark jets observed in momentum space and its QCD explanation