Measuring masses in semi-invisible final states at electron-positron colliders

Abstract

Mass measurement of a particle whose decay products including invisible particles is a challenging task at colliders. For a new physics model involving a dark matter candidate $N$ and a ${Z}_{2}$ symmetry that stabilizes it, a typical new process at ${e}^{+}{e}^{\ensuremath{-}}$ colliders is the pair production ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}Y\overline{Y}$ followed by decay processes $Y\ensuremath{\rightarrow}aN$ and $\overline{Y}\ensuremath{\rightarrow}b\overline{N}$, where $a$ and $b$ are visible but $N$ is invisible. In this work, we propose a new method to measure the physical masses in this topology by making use of the kinematic equations given by momentum-energy conservation and on-shell conditions. For each event, the solvability of these equations determines a limited region on the trial ${m}_{Y}\text{\ensuremath{-}}{m}_{N}$ plane. The edge of this region can be used to define two variables, ${m}_{Y}^{\text{edge}}$ and ${m}_{N}^{\text{edge}}$, whose distributions are utilized to derive the measurement values of ${m}_{Y}$ and ${m}_{N}$. The measurement deviations and uncertainties are also estimated after including detector effects and background contamination.