A Quantum Diaries Survivor

Today my colleague Wei-ming Yao is showing the new combined limits on Higgs boson production by CDF at the Aspen winter conference. The result has been blessed just a week ago, and it is the result of the combination of several different analyses searching the Higgs boson in all the final states the Tevatron is sensitive to.

The data analyzed varies from 1.1 to 1.9 inverse femtobarns, and is thus only about a half of what has so far been collected by CDF. That is because these analyses are really difficult to perform, and the required accuracy demands a periodic re-tuning of all the devilish details - which takes a huge effort from the physicists involved in the analyses.

The limit is an update of the combination of results CDF published last summer, which I discussed back then in another post. The progress from the former numbers is small but important: in particular, a significant decrease of the cross section limit can be observed at low Higgs masses - the region where associated production of WH or ZH pairs contributes the most. Associated production of a vector boson and the Higgs is a tough signal to search for, because one needs to refine two of the most complex tools at a hadron collider: b-quark tagging -which allows to select the decay- and jet energy resolution, which affects the reconstruction of a mass bump.

The plot below summarizes the current state of affairs: as by now every reader of this blog should be familiar with, the x axis in this plot is the unknown value of the Higgs mass. On the y axis is the ratio between observed limit on higgs production cross section and standard model predictions for the same process: a limit at y=2.0 means that the Higgs has been determined to not be produced at rates exceeding twice those predicted by Standard Model.

You can see several lines in the plot. Each color corresponds to a different search: red stands for the associated production search, while blue stands for the analysis of  decays when the Z goes to neutrinos; in purple, instead, are the results of a search for the   final state. Finally, the green lines detail the results of the search for direct H production and decay to a pair of W bosons.

For each search, the two lines have a different meaning: the hatched line is the limit CDF expected to set, given the analysis details and the amount of analyzed data; the full line shows instead the limit actually obtained. The differences show where CDF has been “lucky” (if one may claim to be lucky when observing fewer events than background processes should give) or unlucky.

What matters most, however, is the black line, which is the combined limit, obtained from a complex statistical procedure on which I have no desire to delve right now. The yellow band shows the variability of the expected combined limit.

What should we take home from this plot ? Well, several things. First, we see that the addition of 70% more data has made the  search the most sensitive at very low masses. This brought down the limit from last summer’s x10 at Mh=120 GeV to today’s x5.5. Help came also from the WH search, which has added about 15% more data since last summer.

All in all, the CDF limit stands at x1.8 times the Standard Model for higgs boson mass of 160 GeV. I think we will have to wait for summer 2008 to see the first chunk of Higgs masses being excluded right about there by a combination of CDF and D0 results.