The Precision Determination of Invisible-Particle Masses at the LHC

TL;DR

This paper introduces new variables, m_2C and m_2C,UB, for precise mass determination of invisible particles at the LHC, utilizing event distributions and the kink in Max m_T2, achieving high accuracy with realistic data.

Contribution

The paper presents novel constrained mass variables that improve the accuracy of invisible particle mass measurements at hadron colliders, especially using information away from the endpoint.

Findings

Achieved 4.1 GeV precision in mass measurement with 100 fb^-1 data.

Demonstrated the utility of m_2C and m_2C,UB variables in a supersymmetry case study.

Showed that the method outperforms or matches existing techniques in accuracy.

Abstract

We develop techniques to determine the mass scale of invisible particles pair-produced at hadron colliders. We employ the constrained mass variable m_2C, which provides an event-by-event lower-bound to the mass scale given a mass difference. We complement this variable with a new variable m_2C,UB which provides an additional upper bound to the mass scale, and demonstrate its utility with a realistic case study of a supersymmetry model. These variables together effectively quantify the `kink' in the function Max m_T2 which has been proposed as a mass-determination technique for collider-produced dark matter. An important advantage of the m_2C method is that it does not rely simply on the position at the endpoint, but it uses the additional information contained in events which lie far from the endpoint. We found the mass by comparing the HERWIG generated m_2C distribution to ideal distributions for different masses. We find that for the case studied, with 100 fb^-1 of integrated luminosity (about 400 signal events), the invisible particle's mass can be measured to a precision of 4.1 GeV. We conclude that this technique's precision and accuracy is as good as, if not better than, the best known techniques for invisible-particle mass-determination at hadron colliders.