Localized 4-Sigma and 5-Sigma Dijet Mass Excesses in ALEPH LEP2 Four-Jet Events

Jennifer Kile

(CFTP, Instituto Superior Tecnico)

Sala 2.8.3, IST, Dept. de FĂ­sica
Thursday, June 29th, 2017 at 04:30 PM


We investigate an excess observed in hadronic events in the archived LEP2 ALEPH data. This excess was observed unexpectedly at preselection level during data-MC comparisons of four-jet events. 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 M1+M2~110 GeV; about half of the excess is concentrated in the region M1~80 GeV, M2~25 GeV, with a local significance between 4.8 and 5.6 sigma, depending on assumptions about hadronization uncertainties. The other half of the events are in a broad excess near M1~M2~55 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.

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