Spin Determination via Third Generation Cascade Decays

TL;DR

This paper proposes a novel method to determine the spin of new particles at the LHC by analyzing charge asymmetries in third-generation quark decays, improving model discrimination and reducing uncertainties.

Contribution

It introduces a charge-based asymmetry approach using third-generation quarks, enhancing spin measurement accuracy and model differentiation over previous methods.

Findings

Charge asymmetry for bottom quarks is similar to previous methods but more robust.

Top quark decay products can substitute for the top in spin analysis.

The method improves discrimination between Supersymmetry and extra-dimensional models.

Abstract

Once new particles are discovered at the LHC and their masses are measured, it will be of crucial importance to determine their spin, in order to identify the underlying new physics model. We investigate the method first suggested by Barr and later extended by others to distinguish between Supersymmetry and alternative models, e.g. Universal Extra Dimensions, in a certain cascade decay. This method uses invariant mass distributions of the outgoing Standard Model particles to measure the spin of intermediate particles, by exploiting the quark/anti-quark asymmetry of the LHC as a pp collider, which is limited for first generation quarks. In this work, we suggest instead to measure the charge of the outgoing quark, in case it is a third generation quark. The resulting asymmetry for a bottom quark is similar to the previous method, while it is independent of hadronic uncertainties. Furthermore, for a top quark, the asymmetry allows better distinction between the models, as demonstrated by a quantitative analysis of model discrimination. We also show that the top's decay products can be used instead of the top itself, when the reconstruction of the top momentum is difficult to accomplish, and still provide information about the spin.