4.1 sigma

Update:

Today at the conference Rencontres de Blois the CDF collaboration presented an update on the invariant mass of 2 jets produced in association with a W boson. Recall that 2 months ago CDF posted a paper based of 4.3 fb-1 of data claiming that this observable displays an unexpected bump near 150 GeV with a significance of 3.2 sigma. The bump could have been a fluke, an accounted for systematic effect or surprising new physics. Now the first option is no longer on the table: the same bump is also present in the more recent data with a large statistical significance. With 7.3fb-1 of the Tevatron data, after subtracting the known Standard Model backgrounds other than the WW and WZ production, the distribution of the jet pair invariant mass looks like this: The peak has become more pronounced! CDF quotes the significance of(the number 4.8 sigma I quoted earlier takes into account only statistical uncertainties; after including systematic uncertainties the significance drops to 4.1 sigma). In a collider experiment, such a huge departure from a Standard Model prediction is happening for the first time in the human history :-) I don't have to stress how exciting it is. However we're not celebrating the demise of the Standard Model yet, not before an independent confirmation DZero or from the LHC. In any case, this summer is going to be hot.For possible theoretical explanations of the bump, see my previous badly timed post . In the Blois slides CDF adds one important new piece of information. They say the bump cannot be due to the Standard Model top quark background, contrary to what was suggested in a couple of theory paper. Basically, there is no sign of enhanced b-jet content in the excess events, and in any case the top quark endpoint would show up below 150 GeV due to different jet energy scale corrections for b-jets.CDF has released more plots and the note describing the update.