Experimental Tests of QCD and Fragmentation Models in Electron-Positron Annihilation

Citation

Stone, Howard D. (1988) Experimental Tests of QCD and Fragmentation Models in Electron-Positron Annihilation. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/2ezn-qk36. https://resolver.caltech.edu/CaltechTHESIS:03072013-134658154

Abstract

This thesis presents a detailed study of the hadronic data obtained with the MARK J detector from e⁺e⁻ annihilation. The predictions of several new combinations of Monte Carlo parton generators and fragmentation schemes have been tuned and compared directly to the data.

A study, performed in the limit of perfect detector resolution and efficiency, has shown that the string effect, which some groups claimed to have observed, cannot be attributed to soft gluon coherence effects occurring at the soft perturbative stage of the event evolution.

It is found that all the fragmentation schemes employing the string formalism are unable to correctly describe the form of the Thrust and Energy-Energy Correlation distributions obtained with the high energy data above 40 GeV center-of-mass energy. The problems encountered have their origins in the modelling of the fragmentation process rather than in the perturbative QCD aspects of the models. All the models investigated, however, are capable of describing a broad range of distributions at the lower center-of-mass energies.

No evidence for the presence of the string effect has been found in the MARK J data.

The new fragmentation models have been used to extract values for the strong coupling constant from the data. The systematic uncertainties assigned to previous α₈ measurements have been re-assessed in the light of this study, and it has been found that previous error assignments require no adjustment.

A recent 2nd order calculation for the three-jet partial cross section has been investigated and converted into a form where it can be directly compared to previous calculations. The calculations are found to be in excellent agreement, and it is concluded that the systematic errors that should be assigned to α₈ measurements due to theoretical uncertainties are negligible.

Motivated by the observation of an excess number of low thrust inclusive muon events at the highest PETRA energies, the MARK J hadronic data above 46.3 GeV has been compared to a lower energy data sample. The presence of new phenomena in the highest energy hadronic data has been ruled out at the level of available statistics.

A brief study of the characteristics to be expected at LEP energies for events originating in the production of top quarks has been made, and a method for isolating a top-enriched sample has been investigated.

Thesis Files