Measuring Masses in Semi-Invisible Final States at Electron-Positron Colliders

TL;DR

This paper introduces a novel kinematic method to measure the masses of particles in collider events with semi-invisible final states, improving mass determination accuracy in new physics searches.

Contribution

It proposes a new approach using the solvability of kinematic equations to extract particle masses from collider data with invisible particles.

Findings

The method effectively determines mass edges from event data.

Simulation results show accurate mass measurements with uncertainties.

The approach accounts for detector effects and background contamination.

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 $e^+e^-$ colliders is the pair production $e^+e^- \to Y\bar{Y}$ followed by decay processes $Y\to aN$ and $\bar{Y}\to b\bar{N}$, 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 $m_Y$-$m_N$ plane. The edge of this region can be used to define two variables, $m_Y^\mathrm{edge}$ and $m_N^\mathrm{edge}$, 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.