Measuring Invisible Particle Masses Using a Single Short Decay Chain

TL;DR

This paper introduces a novel method to determine the masses of three invisible particles in a decay chain at hadron colliders by analyzing the distribution of events in a specific energy-invariant mass plane, overcoming previous limitations.

Contribution

The authors demonstrate that the event distribution in a log energy ratio versus invariant mass-squared plane contains enough information to extract all three invisible particle masses from a single decay chain.

Findings

All three invisible particle masses can be individually determined.

The method is robust against experimental smearing and combinatorial issues.

Reasonable accuracy in mass measurements is achievable in many cases.

Abstract

We consider the mass measurement at hadron colliders for a decay chain of two steps, which ends with a missing particle. Such a topology appears as a subprocess of signal events of many new physics models which contain a dark matter candidate. From the two visible particles coming from the decay chain, only one invariant mass combination can be formed and hence it is na\"ively expected that the masses of the three invisible particles in the decay chain cannot be determined from a single end point of the invariant mass distribution. We show that the event distribution in the $\log(E_{1T}/E_{2T})$ vs. invariant mass-squared plane, where $E_{1T}$, $E_{2T}$ are the transverse energies of the two visible particles, contains the information of all three invisible particle masses and allows them to be extracted individually. The experimental smearing and combinatorial issues pose challenges to the mass measurements. However, in many cases the three invisible particle masses in the decay chain can be determined with reasonable accuracies.