Missing pieces the key to measuring weak boson pairs

Neutrinos are inferred by a momentum imbalance after adding up all visible detector activity. This conservative estimator helps reject background events where that momentum imbalance is measured poorly.

The simultaneous production of two weak force carriers is a major background to the Higgs boson search. It also gives physicists an opportunity to study the weak sector of the Standard Model and probe for signs of new physics. A new DZero analysis focuses on the rarest of these processes, when a Z boson is produced at the same time as either a W boson or another Z boson. Each study starts by examining events with a pair of charged leptons that are consistent with a Z boson decay. But part, if not all, of the second boson’s decay products in this analysis are invisible.

The W boson and the Z boson can produce neutrinos when they decay, and neutrinos are not seen directly by detectors built for collider experiments. Instead, their existence must be inferred by the imbalance of momentum their absence leaves behind. If the sum of the transverse momentum for all observed particles is not zero, then the missing piece must reflect the total transverse momentum carried away by neutrinos. However, the uncertainty of this measurement is influenced by the finite resolution of the momentum of every particle observed in the detector.

The DZero analyzers combat this compounded resolution problem differently in WZ or ZZ production. When a W boson produces the neutrino, in addition to a third charged lepton, the estimate of the missing transverse momentum is constrained by performing a fit on the Z boson decay products. For events in which a second Z boson decays into two neutrinos, the analyzers calculate the minimum feasible missing transverse momentum when considering the resolution of all the particles observed in the event. The cut on this missing transverse momentum estimator was optimized before looking at the data in the signal region and was chosen to minimize the total uncertainty on the cross section measurement. The final results from DZero represent the most precise cross section measurement in both channels at a hadron collider to date.

—Mike Cooke

These physicists made major contributions to this analysis.

The data quality group reviewed all of the data recorded by the DZero detector before it was used in any physics analysis. Analyzers rely on the data quality group’s certification to ensure that every subdetector they require was delivering reliable information for all of the events they use.