Dead cone effect in charm and bottom quark jets

TL;DR

This paper investigates the dead cone effect in charm and bottom quark jets using LEP data, demonstrating strong suppression of high-momentum hadrons and confirming pQCD predictions with detailed fragmentation function analysis.

Contribution

It provides the first detailed experimental verification of the dead cone effect in heavy quark jets and compares fragmentation functions with theoretical models and simulations.

Findings

Strong suppression of high-momentum hadrons in heavy quark jets

Excellent agreement between experimental data and pQCD predictions

Identification of an ultra-soft gluon excess at small momentum fractions

Abstract

The evolution of a heavy quark initiated jet is mainly ruled by gluon bremsstrahlung. As a consequence of the dead-cone effect, this radiation is suppressed in the forward direction at angles smaller than that proportional to the heavy quark mass $M_Q$, i.e. $\Theta_0=M_Q/E_Q$ at energy $E_Q$ of the primary quark. In this paper, we unveil this effect in charm and bottom quark jets using DELPHI and OPAL data from Z$^0$ boson decays in $e^+e^-$ annihilation at center of mass energy 91.2 GeV. The analysis of the reconstructed heavy quark fragmentation function in momentum space shows the strong suppression of hadrons at high momenta in such events compared to light quark fragmentation by a factor $\lesssim1/10$. The amount of this suppression is well reproduced by perturbative QCD (pQCD) within the Modified Leading Logarithmic Aproximation and the compact scheme of Local Parton Hadron Duality (MLLA-LPHD). As a new result, we obtain an almost perfect agreement between the light quark fragmentation functions expected at $W_0\propto M_Q$ from DELPHI and OPAL data with Pythia8 and shed light on the reasons for the existence of the ultra-soft gluon excess at small momentum fraction in comparison with pQCD predictions.