Observation of an excess at 30 GeV in the opposite sign di-muon spectra of ${\mathrm{Z}}^0\to b\overline{b}+\mathrm{X}$ events recorded by the ALEPH experiment at LEP

Abstract

The re-analysis of the archived data recorded at the ${\mathrm{Z}}^0$ resonance by the ALEPH experiment at LEP during the years 1992-1995 shows an excess in the opposite sign di-muon mass spectra at 30.40 $\pm$ 0.46 GeV in events containing b quarks. The excess has a natural width of 1.78 $\pm$ 1.14 GeV. The di-muon excess has a local significance around $5\sigma$ ($Z_{\mathrm{asym}}$), depending on the background model used. The significances for background models based on a kernel density approximation stay close to $3\sigma$ ($Z_{\mathrm{freq},\mathrm{lee}}$), when including a look elsewhere effect. Another method to obtain a significance value results in at least $2.6\sigma$ ($Z_{\mathrm{Bi}}$). A compatible, but smaller excess is visible in the opposite di-electron mass spectrum as well. This paper uses the data collected by the ALEPH experiment in the years 1992-1995, which have been archived to allow their use for physics analyses after the closure of the collaboration.

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