Calculation of Power Corrections to Hadronic Event Shapes of Tagged b Events

We compute power corrections to mean values of hadronic event shapes - the thrust and the C parameter - of tagged b quark events in electron positron annihilation, using the dispersive approach. We find that the leading power corrections are of the same type of 1/Q corrections as for event shapes in the massless case, with the same non-perturbative coefficient times a perturbatively calculable mass-dependent coefficient. The effect of the mass correction in the power correction is to reduce the latter by 10-30% for tagged b events, for centre-of-mass energies ranging from the Z peak down to 20 GeV.