Observation of Z0Z0 Diboson Production
 Figure 1 : Weak gauge boson double production reactions allowed in the electroweak unified theory. |
 Figure 2 : Weak boson double production reactions not allowed in the electroweak unified theory. |
Just over a year ago, the CDF experiment, which includes participation from the High Energy Physics Laboratory at Osaka City University, reported the first observation of the simultaneous production of the gauge bosons W± and Z0 of the weak interaction. This time, we introduce the observation of the simultaneous double production of Z0 bosons. In the current particle theory, the weak interaction is unified with the electromagnetic interaction, forming the electroweak unified theory by Weinberg and Salam. This theory is a highly refined system that aligns perfectly with all currently observed or measured experimental results. By using this, we can delve into the fundamental structure of the particle world by closely examining the consistency of phenomena newly predicted by this theory. One such intriguing phenomenon is the simultaneous double production of electroweak gauge bosons. Within the framework of the electroweak unified theory, reactions between electroweak gauge bosons are meticulously regulated as an inevitable consequence of theoretical calculations. For example, the reaction shown in Figure 1 is allowed, while the reaction in Figure 2 is not. Although the reaction in Figure 2 might seem consistent if we consider only the movement of charge, it cannot exist within the electroweak unified theory. Therefore, if the production cross-section of simultaneous double production of electroweak gauge bosons observed deviates from the theoretical predictions, it could indicate the existence of new, unknown physics. The production cross-section of Z0Z0 double production, in particular, is considered to be the smallest among the combinations of gauge bosons, making it more sensitive to even slight discrepancies. For instance, if an RS graviton mediating higher-dimensional gravity exists, it is predicted to decay into Z0Z0 at a certain rate.
 Figure 3 : Cross section of the weak gauge boson. The most right is the ZZ production observed this time. |
 Figure 4 : The cross-sectional view of the CDF detector and a candidate event of ZZ → μμμμ reaction. Tracks shown with the green and blue bold lines are those of μ leptons. |
In this CDF experiment, we measured the production cross-section of Z0 simultaneous double production by searching for the processes where the final state consists of four charged leptons or two charged leptons and two neutrinos:
qq → Z0Z0 → ℓ ℓ ℓ ℓ
qq → Z0Z0 → ℓ ℓ ν ν
The data used for the analysis is based on proton-antiproton collisions with a luminosity of 1.9 fb−1. By employing numerous computers to guarantee a high probability of selecting events that produce charged leptons or neutrinos, we observed three events that appeared to be Z0Z0 production where only 0.1 background events were expected from non-Z0Z0 sources. Statistically, this signifies a 4.4σ signal significance, strongly suggesting the observation of Z0 simultaneous double production. The probability of observing this number of events due to background alone is 1×10−1. From this, the production cross-section of Z0 simultaneous double production was calculated to be:
σ(Z0Z0; exp) = 1.4 ± 0.7 pb
Moreover, the predicted value from the electroweak unified theory is:
σ(Z0Z0; thoroy) = 1.4 ± 0.1 pb
Both values were found to be in good agreement within the margin of error. Figure 3 shows the comparison of this reaction cross-section with those of other reactions. Figure 4 displays the tracks in the detector for an event considered to be:
qq → Z0Z0 → μ μ μ μ
As a result, no signs of new physics were observed this time, but as the CDF experiment continues to accumulate more data, we may discover phenomena that cannot be explained by current theories in the near future.