Precision determination of invisible-particle masses at the CERN LHC

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 $\mathtt{m}\mathtt{a}\mathtt{x}$ ${m}_{T2}$ 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 end point, but it uses the additional information contained in events which lie far from the end point. We found the mass by comparing the $\mathtt{H}\mathtt{E}\mathtt{R}\mathtt{W}\mathtt{I}\mathtt{G}$ generated ${m}_{2C}$ distribution to ideal distributions for different masses. We find that for the case studied, with $100\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ 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.