Gosh, I was damn busy all day, and by now all major blogs have already run the story. Anyway, better late than never... CDF claims a 3 sigma excess in the dijet invariant mass distribution in the lepton + neutrino + 2 jets events. This analysis was originally devised to search for the diboson WW and WZ final states, where a W boson decays leptonically and the other electroweak boson decays into 2 jets. The latter should show up as a broad peak in the dijet invariant mass spectrum, on top of a much larger background from the generic W+jets production. Indeed, both D0 and CDF could see the peak below 100 GeV, which allowed them to pinpoint the diboson production at the 5 sigma level and measure the diboson production cross section. However, CDF had also a small blip around 150 GeV which was not expected.
All in all, the excess advertised in today's CDF paper is not exactly new, and it has been widely discussed among theorists. Moreover, the analysis published today has been publicly available for some time in the form a PhD thesis. What changed with respect to the earlier CDF diboson search is that the cuts have been slightly revamped to make the bump more pronounced. The excess over the standard model prediction is estimated to be slightly above 3 sigma.
It is well known that sigmas come in varieties: there or more significant 3 sigmas, less significant 3 sigmas, and astrophysical 3 sigmas. To my taste the latest CDF claim belongs to the 2nd category. We are dealing with a small hump on top of a huge background, and a small unaccounted for systematic error could easily show up as a false positive. Furthermore, D0 does not see anything; they actually have a small deficit near 150 GeV (although with much less data and different cuts). Finally, the recent experience with papers submitted simultaneously to arXiv and to popular news outlets is not quite encouraging ;-)
In spite of these caveats the effect is definitely interesting and we cannot exclude it is real. What could it be? It is not a Higgs; anything Higgsish with 150 GeV mass would prefer decaying to a pair of W bosons or b-quarks rather than to 2 light jets. The simplest explanation, proposed in this April Fools' paper, involves a 150 GeV Z' boson. A light Zprime with a significant coupling to leptons is excluded by LEP and the Tevatron, but if the coupling to leptons is small then the limits are surprisingly weak. In particular, 150 GeV Z' with electroweak size couplings to quarks is perfectly allowed, and would have the right cross section to produce a bump observed by CDF. One should note that Z' with the mass of that order could generate a large forward backward asymmetry of the top production, as observed in another CDF analysis. But one should also note that generating the asymmetry requires a large flavor violating coupling u t Z' which in principle is not related to the coupling to the light quarks that is probed by today's CDF paper. In a month or so, when I post an update, there will surely by dozens of new models explaining the bump, and some of them may link to the top asymmetry in a more direct way.
For more comments see Flip, Tommaso, Sean, and Michael, Peter, Lubos, and again Tommaso.