Person:

Schwartz, Matthew

Starting from a factorization theorem in Soft-Collinear Effective Theory, the thrust distribution in e(^+)e(^-) collisions is calculated including resummation of the next-to-next-to-next-to leading logarithms. This is a significant improvement over previous calculations which were only valid to next-to-leading logarithmic order. The fixed-order expansion of the resummed result approaches the exact fixed-order distribution towards the kinematic endpoint. This close agreement provides a verification of both the effective field theory expression and recently completed next-to-next-to-leading fixed order event shapes. The resummed distribution is then matched to fixed order, resulting in a distribution valid over a large range of thrust. A fit to ALEPH and OPAL data from LEP 1 and LEP 2 produces (\alpha_s(m_Z))= 0.1172 (\pm) 0.0010 (\pm) 0.0008 (\pm) 0.0012 (\pm) 0.0012, where the uncertainties are respectively statistical, systematic, hadronic, and perturbative. This is one of the world's most precise extractions of (\alpha_s) to date.

The heavy jet mass distribution in e(^+)e(^−) collisions is computed to next-to-next-to-next-to leading logarithmic (N(^3)LL) and next-to-next-to leading fixed order accuracy (NNLO). The singular terms predicted from the resummed distribution are confirmed by the fixed order distributions allowing a precise extraction of the unknown soft function coefficients. A number of quantitative and qualitative comparisons of heavy jet mass and the related thrust distribution are made. From fitting to ALEPH data, a value of αs is extracted, α(_s)(m(_Z) ) = 0.1220 ± 0.0031, which is larger than, but not in conflict with, the corresponding value for thrust. A weighted average of the two produces α(_s)(m(_Z) ) = 0.1193 ± 0.0027, consistent with the world average. A study of the non-perturbative corrections shows that the flat direction observed for thrust between α(_s) and a simple non-perturbative shape parameter is not lifted in combining with heavy jet mass. The Monte Carlo treatment of hadronization gives qualitatively different results for thrust and heavy jet mass, and we conclude that it cannot be trusted to add power corrections to the event shape distributions at this accuracy. Whether a more sophisticated effective field theory approach to power corrections can reconcile the thrust and heavy jet mass distributions remains an open question.