A Quantum Diaries Survivor

Just a few days ago D0 released their new results from searches of the   decay process. That result has now been combined with all their other searches, in particular those for a lighter Higgs boson, which mostly decays to a b-quark pair if as the following graph explains.

In the graph, the line marked “bb” shows the fraction of times a H boson disintegrates into a pair of b-quarks, which subsequently - given the large energy each is endowed with, equal to - produces two hadronic jets in the D0 detector. The line marked “WW” shows instead the dominance of the WW final state if the Higgs mass is larger, an effect due to the fact that the elusive particle decays to the heaviest bodies available: as soon as two W bosons are energetically possible (and even a little before that, when one W is produced with less energy than its rest mass) they become the dominant decay. Note also the three regions with background color yellow, pale blue, and bluer blue: yellow marks the region where LEP II has excluded a Higgs boson, while pale blue marks the region where the bb final state is dominant, the so-called “light-higgs” region. 

To find a decay one needs some extra handle, because two b-jets are a quite common occurrence at the Tevatron - one event in a hundred thousand, as opposed to one in a hundred billion. The extra handle is provided by requiring that the H comes together with a W or Z boson, in a process called associated production, or Higgs-strahlung from vector bosons - the latter adapted from the german word bremsstrahlung, which means braking radiation, the one produced by a charged particle forced to slow down. Electrons (as well as any other charged body)produce real photons by bremsstrahlung when they are curved or decelerated, while W and Z bosons “emit” real H bosons when they have enough energy to do so.

D0’s combination is a pleasure to look at, because for once I do not need to wrestle too much into gory details, and can concentrate on describing the results. And you, dear reader, are presented with a series of very nice and clear plots, where everything is tidy and whose message is clear.

The D0 plots are made all with the same style: a neural network classifier is used to discriminate signal from background, and you get to see the data (black points) in a histogram of the NN output, compared to the expected sum of backgrounds (all together in a single distribution, marked with a red line).

The agreement of data and background speaks by itself, but in addition you get to see how the Higgs signal would show up in the distribution. To see it better, it is multiplied by a factor of x15. Thank god, a common factor across the board (for all WH/ZH searches, while the higher S/N of searches forces the removal of the enhance factor), so that one can easily compare visually the statistical power of different searches. Kudos to D0 for their excellence in data display!

So here are the individual search plots. They are based on data samples ranging from 0.6 to 1.7 inverse femtobarns.

Above, the search results. The expected signal is in blue, multiplied by a factor of 15.

Above, the   search results. The expected signal is in blue, multiplied by a factor of 15.

Above, the search results. The expected signal is in blue, multiplied by a factor of 15.

Above, the search results (l stands for a charged lepton, either electron or muon). The expected signal is in blue, multiplied by a factor of 15. This plot is made with events containing two b-tagged jets, for maximum signal purity.

Above, the search results (l stands for a charged lepton, either electron or muon). The expected signal is in blue, multiplied by a factor of 15. This plot is made with events containing only one b-tagged jet, for a larger signal efficiency.

Above, the search results. The expected signal is in blue, multiplied by a factor of 15. This analysis searches for the spectacular signal of three W bosons, which is difficult to see because of the price to pay in branching fraction of W bosons to leptons. Backgrounds, however, are smaller than in most other searches. Here, instead than a NN output, a discriminant is plot on the x axis. Its meaning, however, is exactly the same: a value close to 1 is more likely for signal-like events.

Above, the search result from the latest chunk of 0.6/fb data analyzed by D0. Here, the signal (still in blue) is not multiplied by 15, but the y axis is logarithmic.

Finally, all searches are combined to extract a global 95% confidence level limit. You can see it in the plot below. The limit, as usual with Higgs searches at the Tevatron, is on the number of times the Higgs production could exceed the predicted Standard Model rate without having been seen by D0.

The black curve is the limit found by D0, while the hatched red line is the limit expected given the sensitivity of the searches and the data available. You can see that a big progress has been made in the searches at low Higgs mass, where the limit is sitting at only a few times the SM. This leaves room to hopes that the Tevatron may say something meaningful also in the low mass region before LHC becomes the only game in town.

For more information, please visit the D0 site of this analysis.