Constraining Light Colored Particles with Event Shapes

TL;DR

This paper uses advanced theoretical techniques and LEP event shape data to set strong, model-independent bounds on new light-colored particles, significantly improving previous limits.

Contribution

It introduces a novel method leveraging Soft-Collinear Effective Theory to constrain light colored particles using event shape data.

Findings

Colored adjoint fermions below 51.0 GeV are ruled out at 95% CL.

The method improves previous bounds by nearly an order of magnitude.

Event shapes are highly sensitive to new particle thresholds across multiple energy scales.

Abstract

Using recently developed techniques for computing event shapes with Soft-Collinear Effective Theory, LEP event shape data is used to derive strong model-independent bounds on new colored particles. In the effective field theory computation, colored particles contribute in loops not only to the running of alpha_s but also to the running of hard, jet and soft functions. Moreover, the differential distribution in the effective theory explicitly probes many energy scales, so event shapes have strong sensitivity to new particle thresholds. Using thrust data from ALEPH and OPAL, colored adjoint fermions (such as a gluino) below 51.0 GeV are ruled out to 95% confidence level. This is nearly an order-of-magnitude improvement over the previous model-independent bound of 6.3 GeV.