MT2-reconstructed invisible momenta as spin analizers, and an application to top polarization

TL;DR

This paper introduces a method to reconstruct invisible momenta in semi-invisible decays using MT2, enabling analysis of spin correlations and polarization in top quark pair production, with potential applications to other processes.

Contribution

The paper presents a novel approach to estimate invisible momenta using MT2, facilitating detailed spin and polarization studies in complex decay events.

Findings

Event-by-event momentum estimation using MT2 is feasible.

The method accurately reproduces spin-sensitive observables.

Applicable to various processes with semi-invisible decay topologies.

Abstract

Full event reconstruction is known to be challenging in cases with more than one undetected final-state particle, such as pair production of two states each decaying semi-invisibly. On the other hand, full event reconstruction would allow to access angular distributions sensitive to the spin fractions of the decaying particles, thereby dissecting their production mechanism. We explore this possibility in the case of Standard-Model t-tbar production followed by a leptonic decay of both W bosons, implying two undetected final-state neutrinos. We estimate the t and tbar momentum vectors event by event using information extracted from the kinematic variable MT2. The faithfulness of the estimated momenta to the true momenta is then tested in observables sensitive to top polarization and t-tbar spin correlations. Our method thereby provides a novel approach towards the evaluation of these observables, and towards testing t-tbar production beyond the level of the total cross section. While our discussion is confined to t-tbar production as a benchmark, the method is applicable to any process whose decay topology allows to construct MT2.