Direct Measurement of the Z Boson Invisible Width


We extract a direct measurement of the Z boson invisible width from an analysis of events with large
missing transverse energy and one or more energetic jets. The "monojet" event analysis from which this
result is extracted was originally performed in the context of a generic search for new physics. However,
in the observed absence of a data excess over the Standard Model expectation in this channel, we interpret
the result in the context of a direct measurement of Z boson invisible width.
The generic search for new physics used to make this measurment is based on ~1.0fb-1 of data collected
with the inclusive missing Et trigger path (MET > 45 GeV) using kinematic cuts of Et > 80 GeV on the
leading jet and MET > 80 GeV. We utilize the data-driven background estimates of the SM processes
contributing to the monojet topology determined in the generic search analysis (excluding our estimate
of the Z to nu nu background) to estimate the total number of Z decays to unobservable particles contained
within our data sample. We extract the Z boson invisible width from the experimentally measured ratio of

sigma(Z + 1-jet) x Br(Z -> invisible) over sigma(Z + 1-jet) x Br(Z -> lepton lepton)

where here "1-jet" refers to the full set of toplogical requirements used in the selection of our monojet event
sample. Based on this ratio, we use the precise experimental value of the Z boson charged lepton partial
width obtained at LEP to determine the value of &Gamma(Z -> invisible). The procedure is outlined in more detail

Kevin Burkett, Eric James
Pierre-Hughes Beauchemin, Pierre Savard
University of Toronto

  • Contact people : Eric, Hugo, Kevin, Pierre

  • Generic Search for New Physics:
  • Calculation of Z Boson Invisible Width: