The hadronic k{sub perpendicular}-spectrum inside a high-energy jet is determined including corrections of relative magnitude O({radical}({alpha}{sub s})) with respect to the Modified Leading Logarithmic Approximation (MLLA) in the limiting spectrum approximation (assuming an infrared cutoff Q{sub 0}={lambda}{sub QCD}) and beyond (Q{sub 0}{ne}{lambda}{sub QCD}). The results in the limiting spectrum approximation are found to be, after normalization, in impressive agreement with preliminary measurements by the CDF Collaboration, unlike what occurs at MLLA, pointing out small overall nonperturbative contributions. Within the same framework, 2-particle correlations inside a jet are also predicted at next-to-MLLA and compared to previous MLLA calculations.

Next-to-modified leading logarithmic approximation corrections to single inclusive k⊥ distributions and two-particle correlations in a jet

We propose four simple event-shape variables for semi-inclusive $$e^+e^- \rightarrow 4$$
 -jet events. The observables and cuts are designed to be especially sensitive to subleading aspects of the event structure, and allow to test the reliability of phenomenological QCD models in greater detail. Three of them, $$\theta _{14}$$
 , $$\theta ^*$$
 , and $$C_2^{(1/5)}$$
 , focus on soft emissions off 3-jet topologies with a small opening angle, for which coherence effects beyond the leading QCD dipole pattern are expected to be enhanced. A complementary variable, $$M_L^2/M_H^2$$
 , measures the ratio of the hemisphere masses in 4-jet events with a compressed scale hierarchy (Durham $$y_{23}$$
 –
 $$y_{34}$$
 ), for which subleading $$1\rightarrow 3$$
 splitting effects are expected to be enhanced. We consider several different parton-shower models, spanning both conventional and dipole/antenna ones, all tuned to the same $$e^+e^-$$
 reference data, and show that a measurement of the proposed observables would allow for additional significant discriminating power between the models.

/pdf/revisiting-radiation-patterns-in-e-e-collisions-msh1asfhfy.pdf

Revisiting radiation patterns in $$e^+e^-$$ collisions

The charged particle multiplicity in hadronic three-jet events from Z decays is investigated. The topology dependence of the event multiplicity is found to be well described by a modified leading logarithmic prediction. A parameter fit of the prediction to the data yields a measurement of the colour factor ratio C_A/C_F with the result C_A/C_F = 2.261 +/- 0.014 (stat.) +/- 0.036 (exp.) +/- 0.066 (theo.) in agreement with the SU(3) expectation of QCD. The quark-related contribution to the event multiplicity is subtracted from the three-jet event multiplicity resulting in a measurement of the multiplicity of two-gluon colour-singlet states over a wide energy range. The ratios r=N_{gg}(s)/N_{q qbar}(s) of the gluon and quark multiplicities and r^{(1)}=N_{gg}'(s)/N_{q qbar}'(s) of their derivatives are compared with perturbative calculations. While a good agreement between calculations and data is observed for r^{(1)}, larger deviations are found for r indicating that non-perturbative effects are more important for r than for r^{(1)}.

/pdf/charged-particle-multiplicity-in-three-jet-events-and-two-47cny02n82.pdf

Charged Particle Multiplicity in Three-Jet Events and Two-Gluon Systems

A study of QCD coherence is presented based on a sample of about 397000 $e^+e^-$ hadronic annihilation events collected at $\sqrt{s}=91$ GeV with the OPAL detector at LEP. The study is based on four recently proposed observables that are sensitive to coherence effects in the perturbative regime. The measurement of these observables is presented, along with a comparison with the predictions of different parton shower models. The models include both conventional parton shower models and dipole antenna models. Different ordering variables are used to investigate their influence on the predictions.

/pdf/measurement-of-observables-sensitive-to-coherence-effects-in-16nb8dul97.pdf

Measurement of observables sensitive to coherence effects in hadronic Z decays with the OPAL detector at LEP

A study of QCD coherence is presented based on a sample of about 397,000 $$e^{+}e^{-}$$
 hadronic annihilation events collected at $$\sqrt{s}=91$$
  GeV with the OPAL detector at LEP. The study is based on four recently proposed observables that are sensitive to coherence effects in the perturbative regime. The measurement of these observables is presented, along with a comparison with the predictions of different parton shower models. The models include both conventional parton shower models and dipole antenna models. Different ordering variables are used to investigate their influence on the predictions.

/pdf/measurement-of-observables-sensitive-to-coherence-effects-in-11j9cwu2mg.pdf

E. Mandl

Papers

Coherent soft particle production in Z decays into three jets