Multiplicity difference between heavy- and light-quark jets revisited

Abstract

The perturbative QCD approach to multiparticle production predicts a characteristic suppression of particle multiplicity in a heavy quark jet as compared to a light quark jet. In the Modified Leading Logarithmic Approximation (MLLA) the multiplicity difference δ Qℓ between heavy and light quark jets is derived in terms of a few other experimentally measured quantities. The earlier prediction for b-quarks needs revision in the light of new experimental results and the improvement in the understanding of the experimental data. We now find δ bℓ = 4.4 ± 0.4. The updated MLLA results on δ bℓ and δ cℓ are compared with the present data from e + e − annihilation. Their expected energy independence is confirmed within the energy range between 29 and 200 GeV; the absolute values are now in better agreement with experiment than in the previous analysis, and the remaining difference can be attributed largely to next-to-MLLA contributions, an important subset of which are identified and evaluated.

Tests of quantum chromo dynamics at e+e− colliders

“…Assuming direct correspondence of parton and hadron distributions (LPHD as discussed above) allows to make predictions in MLLA QCD e.g. for the difference in charged particle multiplicities of light and heavy quark events, δ bl [45,235,261]. The MLLA QCD prediction is independent of cms energy and its latest value is δ bl = 4.4 ± 0.4 [261].…”

“…for the difference in charged particle multiplicities of light and heavy quark events, δ bl [45,235,261]. The MLLA QCD prediction is independent of cms energy and its latest value is δ bl = 4.4 ± 0.4 [261]. In contrast, the so-called naive prediction for δ bl considers only the reduction in phase space due to the heavy quark mass.…”

Exposing the dead cone effect with jet substructure techniques

“…The difference is negligible for narrow jets and only leads to a small distortion for the R ¼ 1.0 jet radius used here. 6 Bottom FSR is proportional to α s logðE Ã b =m b Þ, where E Ã b is the bottom energy in the top rest frame, while top FSR is proportional to α s logðE Ã t =m t Þ, where E Ã t is the top energy in the tt rest frame, so the former dominates at the moderate top boosts considered here.…”

“…Moreover, the total gluon FSR from the b quark is expected to be larger than from the top quark. 6 While imposing a Θ 2 S > 0 restriction could help isolate the phase space region away from the b quark, a more aggressive way to "clean up" the dead cone region is to force the b candidate to have a large value of Θ b .…”

“…The reason is simple: massive particles decay, and the same angular scale m=E appears both in the dead cone effect as well as in the characteristic opening angle between the decay products. In this way, the dead cone is effectively "filled," so, while the overall suppression of gluon radiation for heavy quarks can be inferred through inclusive [4][5][6][7][8] or semi-inclusive [9][10][11] observables, the universal angular radiation pattern around the massive particle is obscured. More direct probes of the dead cone for bottom and charm quarks have been put forward, for example in e þ e − collisions at LEP [12,13] and in ep collisions at HERA [14], and the dead cone effect is included in the shower deconstruction approach to top tagging [15].…”