Investigation of Power Corrections to Event Shape Variables measured in Deep-Inelastic Scattering

The H1 experiment has measured the distributions and the mean values of quantities related to the shape of events in deep-inelastic scattering over a large range of the momentum transfer Q, varied between 7 and 100 GeV. The shape measures are sensitive to the flow of momenta in the event and thus serve as indicators of the jet character of the interaction. Jets of particles are assumed to be formed around the quarks that participate in the hard scattering. These shape parameters have been known to be calculable in an infrared safe manner, on the basis of quark and gluon emissions. Such attempts, while qualitatively successful, have suffered from the presence of additional power like hadronisation corrections in the data. Recently, the power law corrections have been calculated in QCD. They are characterized by a non-perturbative parameter alpha_p, which should be universal for all such shape variables.
The H1 measurements address these questions by extracting simultaneously the coupling parameter alpha_s of the strong interaction and the non-perturbative parameter alpha_p (figure). Typical values of 0.5 are found for the non-perturbative parameter, the spread is typically 0.1. The corresponding values of alpha_s vary between different shape measures by more than the established experimental precision.
These findings are in agreement with similar observations of final states in e+e- scattering, where the first data with large variation in Q start to appear.

Last Update Dec 30, 1999, E. Elsen