# Localized $4\sigma$ and $5\sigma$ Dijet Mass Excesses in ALEPH LEP2 Four-Jet Events

## TL;DR

This paper reports a robust excess of four-jet events with specific mass combinations in ALEPH LEP2 data, with significances up to 5.5 sigma, and explores its stability under various analysis modifications.

## Contribution

The study identifies a significant, previously unreported excess in four-jet events at LEP2, demonstrating its robustness and ruling out systematic uncertainties as explanations.

## Key findings

- Excess observed at M1+M2~110 GeV with up to 5.5 sigma significance.
- Half of the excess concentrated near M1~80 GeV, M2~25 GeV.
- No similar excess found in LEP1 data.

## Abstract

We investigate an excess observed in hadronic events in the archived LEP2 ALEPH data. This excess was observed at preselection level during data-MC comparisons of four-jet events when no search was being performed. The events are clustered into four jets and paired such that the mass difference between the two dijet systems is minimized. The excess occurs in the region $M_1+M_2\sim 110\mbox{ GeV}$; about half of the excess is concentrated in the region $M_1\sim 80\mbox{ GeV}$, $M_2\sim 25\mbox{ GeV}$, with a local significance between $4.7\sigma$ and $5.5\sigma$, depending on assumptions about hadronization uncertainties. The other half of the events are in a broad excess near $M_1\sim M_2\sim 55\mbox{ GeV}$; these display a local significance of $4.1-4.5\sigma$. We investigate the effects of changing the SM QCD Monte Carlo sample, the jet-clustering algorithm, and the jet rescaling method. We find that the excess is remarkably robust under these changes, and we find no source of systematic uncertainty that can explain the excess. No analogue of the excess is seen at LEP1.