|Comparison of expected and observed distributions of both Z invariant masses. The region CDF looked at contained only seven events (inside the blue lines).|
While the production of a single W or Z boson in particle collisions at the Tevatron happened quite often and has been a reference for precise electroweak measurements, a pair of such bosons was produced less frequently. Scientists predict that, in collisions between protons and antiprotons, a ZZ boson pair is produced only once for every 20,000 W bosons produced. This measurement is an important test of the Standard Model and has been calculated by theorists at Fermilab.
Z bosons decay predominantly into a pair of quarks in proton-antiproton collisions. However, this signature is hard to recognize among the plethora of jets produced. It is easier to detect Z bosons when they decay into a pair of charged leptons, even though the frequency of such decays is small.
In this analysis, the ZZ signature is detected when one Z decays into a charged-lepton pair and the second Z decays into either another charged-lepton pair or a pair of neutrinos.
The first case results in a very clean signature, which allows for the observation of a process with very few events. After all the cuts, background accounts for a mere 0.6 percent of the data from the detector. Our paper shows many comparison plots between data and the Monte Carlo simulation, one of which is given in the top figure.
The second case is harder because of the two neutrinos, which are not picked up by the detector, and is therefore identified with the help of a sophisticated algorithm. After cuts, in this analysis the sample of this kind of decay contained 288 events, of which 230 ± 16 were background events. By combining information from seven different variables, the sophisticated algorithm has provided us with a better separation of signal from background. The lower plot shows the data and the expected distribution of the signal and six different backgrounds. The result for the cross section is very consistent with our analysis for the lepton pair sample.
Scientists obtained the final result by combining the two states, exploiting the whole data set collected by CDF. The result is a legacy measurement. The measured ZZ cross section of 1.04 +0.32/-0.25 picobarns is in good agreement with the Standard Model value of 1.4 ± 0.1 picobarns. The result is also in good agreement with the DZero measurement of 1.32 +0.32/-0.28 picobarns.
—edited by Andy Beretvas
|A sophisticated algorithm is used to separate the ZZ signal from the background when one of the Z‘s decays into a pair of neutrinos.|
|CDF physicists Matteo Bauce and Donatella Lucchesi, both from the Unviersity of Padova and INFN, contributed to this data analysis.|