Results of 1 fb-1 search for H->WW February 3, 2007Posted by dorigo in news, physics, science.
CDF released this week their result on the search for SM Higgs bosons decays to W boson pairs, based on a dataset of 1 inverse femtobarn of data.
The H->WW decay is the “golden channel”, where that elusive particle can show up most clearly if its mass is close to twice the W mass, that is around 160 GeV.
Of course, sensitivity is not null for lower or higher values of Higgs mass, but it rapidly degrades both ways due to the smaller branching ratio (at masses below the 2Mw threshold, where one of the W bosons has to be “off mass-shell” to allow the decay) or to the rapidly falling expected rate (at higher masses).
The CDF analysis concentrates on the final state containing two high-Pt charged leptons (electrons or muons). Taus are so far excluded from this particular search.
The W bosons can in fact decay to pairs of leptons of the same species (electron plus electron-neutrino, muon plus muon-neutrino, or tau plus tau-neutrino), or to quark pairs (up and down, or charm and strange). Due to the fact that quarks come in three colors, they get three times as much pie, and the overall probability to see a W in the electron or muon final state is two ninths. However, electrons and muons are experimentally much, much cleaner than the other decay bodies.
CDF concentrated into trying to understand very well the sample of events containing lepton pairs, and they did so by first studying the Z boson decays, which can yield pairs of electrons or pairs of muons. Since electrons and muons can be seen in different parts of the detector, each of which has its own subtleties to understand, a good thing to do is to measure the Z cross section for each combination of lepton pairs.
The plot above shows that CDF measures a consistent Z boson cross section across the board: each point in the graph is an independent measurement, performed with leptons detected in a particular subdetector. All measurements are consistent with the predicted Z cross section to each lepton pair (251 pb).
Having determined that leptons are well understood, one needs to understand backgrounds. This is done by comparing many kinematical distributions with the simulation of background processes, in kinematical regions where no Higgs signal should be present.
And then, finally one opens the box and looks at the data where the Higgs signal could indeed show up.
One feature of the Higgs boson is that it possesses zero spin. (Spin is a quantum-mechanical concept, so one should avoid thinking of a spinning sphere). Having no spin, and decaying to two vector bosons, which each carry one unit of spin, means that the two W bosons will have their spin axes opposite to each other, because spin is a conserved quantity in the decay. And this is a crucial fact, because the W in turn decay to a lepton-neutrino pair, and they do so by emitting the charged lepton in a direction strongly dependent on the direction of their spin axis.
Making a long story short, one expects that the two charged leptons from Higgs decay will be emitted more likely in the same direction, while regular W pair production (by Standard Model processes which do not involve Higgs production) will not be subjected to that constraint. So one looks at the azimuthal angle between the detected leptons, and studies whether there is an excess at low angles.
The distribution on the left shows precisely that angle. One sees that the background is made of several different colors. Pink is SM WW production, the largest background, while the dark blue is the expected Higgs signal. Too small to be seen, for sure. But as we collect more data, an excess at low angles might start being an indication that Higgs bosons are present in the mixture.
For now, the plot above only shows that we see about as many events as we would expect to see from Standard Model processes. So we set a limit in the production of the Higgs boson. The limit can be computed for different values of the Higgs mass, by carefully changing selection cuts to make the best possible compromise between signal loss and background rejection.
The final result is shown above. On the x axis is the Higgs mass, and on the y axis is a multiplicative factor describing how far we are from the predicted Higgs cross section (blue line sitting at y=1). Basically, the black curve tells you that CDF so far can exclude that, for a 160 GeV Higgs, its production rate is more than 10 times what theory predicts. For masses below or above 160 GeV, the limit becomes less stringent.
The graph also shows that CDF has been slightly unlucky this time: the data was expected to allow excluding all points of the plane above the dashed black line, but we did worse than that, because we observed more events than predicted.
So what is the bottom line ? Not much. CDF has produced just another limit for Higgs production. The limit cannot exclude any values of the Higgs mass yet. Once combined with all other limits, derived from other data samples, as well with those obtained by the other experiment playing the same game (D0), the black line will get much closer to the blue one…
And with enough data we might be able to say something concrete about the existence of the Higgs, before LHC starts collecting their data.