The CDF Collaboration blessed yesterday afternoon the results of a search for massive Gravitons decaying into pairs of Z bosons. And it is a startling new result!
Usually after a blessing (which is the result of a collaboration-wide presentation when the analysis is given a final scrutiny) the results are not immediately made public: this non-written rule has the purpose of allowing the analysis authors to be the first to present the results at a conference or other public event. But the rule written in the CDF bylaws, on the other hand, say that after a blessing the result is public, so for this time I will stick to the written one, and fair play be darned this time... The chances to announce what might be the first evidence of gravitons is too appealing!
So what is this all about ? It turns out that events featuring pairs of well-identified Z bosons, decaying into electron-positron pairs or muon-antimuon pairs, are exceedingly rare in the standard model, especially if their total mass is larger than a couple of hundred GeV. Because of that, and because of the clean signal these four charged leptons provide, one does not expect to see any such event even in the very large dataset collected by CDF at the Tevatron collider.
Instead, the recent analysis by CDF has spotted three very clean ZZ events. And they all have a total mass compatible with the same value, in the 850-GeV ballpark! Previous analyses overlooked two of these events because of too tight lepton identification criteria. Below you can see event displays of the three candidates, taken directly from the web site of the CDF analysis (link below).
Above, event 1. Note the four clean electron signals. The masses of the two pairs are shown in the top right: both are well consistent with the Z boson mass, 91 GeV.
Event 2 features two electrons and two muons. One electron is in the forward direction and does not have a track pointing to the electromagnetic deposit (shown on the right in the top panel with a pink square). The masses of the reconstructed Z bosons is again in good agreement with the Z boson mass. Note that this event had been previously overlooked because of the forward electron.
And event 3 has two electrons and two jets. Normally hadronic jets bring in large backgrounds, but in this case the event is very clean and definitely a third candidate for the graviton signal!
I am unable to quote a significance for the result, because CDF is still in the process of evaluating it in a rigorous fashion. But since total backgrounds are well below a tenth of an event, these three events are almost certain to constitute the discovery of a new, extremely massive object. A graviton might be the explanation, but other particles might produce the same signature: a Higgs boson, for instance! For the Higgs boson, the cross section appears too large, but this, too, requires more careful investigations. CDF could not sit and wait for these more detailed calculations, however, so I think I am not breaking any rule if I say that the blessing of these three events represents a clear indication that this is a startling new discovery!
It appears ironic that this occurs one day after the Large Hadron Collider comes in operation, at an energy three point five times larger than that of the Tevatron. But this is also probably the reason why CDF decided to come out with the new result exactly today!
I am currently on vacation so this post is much shorter than I would like it to be. I plan to add a more careful explanation of the whole search here tomorrow, but for the time being you can find some additional information in the web site of the CDF analysis.
Post - scriptum: in case it is not clear, read the date of this post: this is an April Fool.
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