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A direct measurement of the invisible width of the Z from single photon counting

Research output: Contribution to journalJournal article


  • ALEPH Collaboration
<mark>Journal publication date</mark>1993
<mark>Journal</mark>Physics Letters B
Issue number3-4
Number of pages15
Pages (from-to)520-534
<mark>Original language</mark>English


The ALEPH detector at LEP is used to study single photon events in e+e- collisions at the Z resonance. In a total data sample of 15.7 pb−1 taken in 1990 and 1991 scanning the resonance, 400 events were recorded where each has a single deposition of more than 1.5 GeV measured in the polar angular region | cosθγ | < 0.74 of the electromagnetic calorimeter. It is shown that this number of events cab be accounted for by known processes. After subtraction of background events, the invisible width of the Z is determined to be 45 ± 34(stat.) ± 34(syst.) MeV. Using Z. resonance parameters derived by ALEPH, the corresponding number oflight neutrino generations is found to be 2.68 ± 0.20(stat.) ± 0.20(syst.).