Perturbative QCD studies of multijet structures in electron-positron annihilation
To investigate the effect in perturbative QCD of multigluon emissions on the transverse momentum distributions of multijet final states in electron-positron annihilation, we use a simplified model based on the approximation that gluons are emitted independently. As a guide to these multigluon emissions, we study the two-gluon contribution in some detail and calculate the Q(_T)-distribution for four-jet events in e(^+)e̅(^-)annihilation, using suitable jet-defining cuts, needed both theoretically, to regularize the soft- and collinear-gluon singularities, as well as experimentally, to group the final-state particles into distinct jets. To ascertain the accuracy of our approximate model, we compare our results with the exact ones, obtained by a Monte Carlo generation of events using the full matrix elements. We find that, for realistic values of the cuts, there is a significant kinematic region of agreement. This agreement and the validity of our model are further elaborated by taking its Abelian QCD limit, calculating distributions in other event shape variables and studying the jet broadening phenomenon. The applicability of our model is also delineated by finding it to be in remarkable structural and numerical agreement with the more exact algorithm of Altarelli et al. Finally, to investigate the effect of higher order and virtual graphs corrections to low order tree-level results, we use our model to calculate the O(a(^2)(_s)) Or-distribution for three-jet events in e(^+)e(^-) annihilation with virtual contributions included. We study the dependence of these corrections on the resolution parameters used to perform the (analytic) cancellation of infrared and collinear singularities between real and virtual graphs and discuss their physical consequences.