Measurement of the ZZ Cross Section via the ZZ →4l Channel

Authors: Tatiana Rodriguez, Elliot Lipeles
Blessed: April 28, 2011
Data: CDF Run II 6.1/fb


Measuring heavy diboson characteristics in pp collisions is a way of probing the structure of the electroweak sector, and provides milestones on the way to sensitivity to rare processes such as Higgs boson production. The ZZ is the last diboson channel without a precision cross-section measurement. We measure the ZZ cross-section in the four-lepton channel. We base our analysis heavily on the work done by a previous group at CDF. In addition to the increased statistics from a later iteration of the analysis, we also optimize our signal acceptance to reduce our cross-section error. The ZZ is a rare process and the highest sensitivity channel, in ZZ→llll, has a branching fraction of only 0.46%. In increasing our yield as much as possible while still controlling our background we attempt to bring the ZZ into the realm of precision Standard Model physics. Using 6.1 fb^-1 of data collected by the CDF experiment in pp collisions at 1.96 TeV we expected 10.4 signal events and 0.26 background events; we observed 14 ZZ → 4l candidate events (where l is a charged lepton). Which gives us a cross-section measurement of: σ(pp → ZZ)x BR(ZZ→4l) = 2.18 +0.63 -0.63 (stat) ± 0.30 (sys) This agrees with the Standard Model prediction NLO cross-section of 1.4 pb at just over one sigma.
Plots

Measuring heavy diboson characteristics in pp collisions is a way of probing the structure of the electroweak sector, and provides milestones on the way to sensitivity to rare processes such as Higgs boson production. The ZZ is the last diboson channel without a precision cross-section measurement.

We measure the ZZ cross-section in the four-lepton channel. We base our analysis heavily on the work done by a previous group at CDF. In addition to the increased statistics from a later iteration of the analysis, we also optimize our signal acceptance to reduce our cross-section error. The ZZ is a rare process and the highest sensitivity channel, in ZZ→llll, has a branching fraction of only 0.46%. In increasing our yield as much as possible while still controlling our background we attempt to bring the ZZ into the realm of precision Standard Model physics.

Using 6.1 fb^-1 of data collected by the CDF experiment in pp collisions at 1.96 TeV we expected 10.4 signal events and 0.26 background events; we observed 14 ZZ → 4l candidate events (where l is a charged lepton). Which gives us a cross-section measurement of:

σ(pp → ZZ)x BR(ZZ→4l) = 2.18 +0.63 -0.63 (stat) ± 0.30 (sys)

This agrees with the Standard Model prediction NLO cross-section of 1.4 pb at just over one sigma.

Plots

Highlights: Signal Region

eps file

eps file
We believe these plots indicate that we have recovered good events, since all fall within the expected Z mass peaks and the lepton types are all well-predicted except one, and that one is the cleanest and strongest type of them all.

Background Cross-check:

eps file
We believe this same sign lepton plot indicates that we have a good understanding of our background, it primarily coming from boson + jets and boson + photon mismeasurement in the detector.