Localized 4σ and 5σ dijet mass excesses in ALEPH LEP2 four-jet events

Abstract

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 ∼ 110 GeV; about half of the excess is concentrated in the region M 1 ∼ 80 GeV, M 2 ∼ 25 GeV, with a local significance between 4.7σ and 5.5σ, depending on assumptions about hadronization uncertainties. The other half of the events are in a broad excess near M 1 ∼ M 2 ∼ 55 GeV; these display a local significance of 4.1-4.5σ. 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.