|Dijet invariant mass distribution with fit results overlaid for events passing all selection criteria (top) and the same background-subtracted distribution with the fitted diboson contribution overlaid (bottom).|
In an older Fermilab Today article, CDF reported the observation of an unusual excess of events in which a W boson appeared together with two jets (which are among the most commonly produced particles at the Tevatron). The jets appeared to match well with the possibility of being produced through a totally unexpected new particle. While the CDF collaboration was cautious in its interpretation of the data, the scientific community was shocked by the story the data seemed to be telling. A few months later, collaborators at the DZero experiment, on the opposite side of the collider, repeated exactly the same study using their own data, finding no such excess. This was an apparent impasse that required further action in order to shed light on a clearly compelling case.
The CDF collaboration recently reported another study of collisions giving rise to a W or Z boson and two quarks (jets). Scientists purposefully selected the data to be completely independent of the data that gave rise to the excess in the earlier study. The original data set required a lepton with a high transverse momentum; the current analysis vetoes any event with a high-transverse-momentum lepton. The idea was to cross-check earlier results and, at the same time, to probe the scenario in which the hypothetical new particle would truly exist and would also appear together with a Z boson, as suggested by several scientists. In this new analysis, scientists applied additional corrections to reconstructed jets in simulated events. These corrections more accurately model particle showers that are initiated by both quarks and gluons.
The original selection resulted in more than 2 million events. The principal background is multi-jet events, which are produced by the strong interaction. After using up-to-date analysis tools, the number of multi-jets was reduced to 6,280 ± 1190 (see above figure). The experiment finds 2,900 ± 183 diboson events (WW, WZ, ZZ). This number of diboson events translates into a measured cross section of 13.8 +3.0/-2.7 picobarns. This number is in agreement with the Standard Model value of 16.8 ± 1.0.
The most important result of this analysis is that no anomalies (no second peaks) are observed in the dijet mass spectrum. This story summarizes beautifully many of the salient features of the scientific journey: the appearance of an experimental anomaly in a well-established framework, leading to great excitement; the process of independently checking the validity of the result; and finally the improved understanding of nature that inherently follows either its confirmation or disproval. Here, what appeared to be a potential game changer to particle physics ended up producing a sounder understanding of important physics processes.
—edited by Andy Beretvas