# Testing Invisible Momentum Ansatze in Missing Energy Events at the LHC

## TL;DR

This paper compares various methods for estimating invisible particle momenta in SUSY-like events at the LHC, demonstrating that M_2 variables generally outperform other approaches in reconstructing particle masses.

## Contribution

It provides a comparative analysis of multiple invisible momentum ansatze, highlighting the superior performance of M_2 variables and the benefit of endpoint preselection.

## Key findings

- M_2 variables produce sharper invariant mass peaks.
- Preselecting events near kinematic endpoints improves performance.
- M_2 methods outperform MAOS in mass reconstruction accuracy.

## Abstract

We consider SUSY-like events with two decay chains, each terminating in an invisible particle, whose true energy and momentum are not measured in the detector. Nevertheless, a useful educated guess about the invisible momenta can still be obtained by optimizing a suitable invariant mass function. We review and contrast several proposals in the literature for such ansatze: four versions of the M_T2-assisted on-shell reconstruction (MAOS), as well as several variants of the on-shell constrained M_2 variables. We compare the performance of these methods with regards to the mass determination of a new particle resonance along the decay chain from the peak of the reconstructed invariant mass distribution. For concreteness, we consider the event topology of dilepton ttbar events and study each of the three possible subsystems, in both a ttbar and a SUSY example. We find that the M_2 variables generally provide sharper peaks and therefore better ansatze for the invisible momenta. We show that the performance can be further improved by preselecting events near the kinematic endpoint of the corresponding variable from which the momentum ansatz originates.