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

TL;DR

This study observes the dead cone effect in heavy quark jets through momentum spectra analysis in Z boson decays, confirming QCD predictions and enhancing understanding of heavy quark fragmentation.

Contribution

It provides the first detailed momentum space measurement of the dead cone effect in heavy quark jets, validating perturbative QCD predictions with experimental data.

Findings

Suppression of high-momentum particles in b- and c-quark jets compared to light quarks.

Quantitative agreement with MLLA predictions within the central kinematic region.

Enhanced sensitivity of momentum analysis over angular analysis for dead cone detection.

Abstract

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