A Quantum Diaries Survivor

Just a few days ago D0 made available on their web server a new conference note describing their analysis of 1.7 inverse femtobarns of proton-antiproton collisions, collected since 2002 at the Fermilab Tevatron collider.

The analysis is an improvement over the former result, which was based on 1.1/fb. The additional 0.6/fb of data blended in comes only from the dimuon final state, however. If one had to take this datum at face value, one would expect an only minor improvement in their results, since the channel accounts for only 1/81 of possible WW final states, as opposed to the 3/81 of the other topologies included in the 1.1/fb (1/81 from and 2/81 from , ): all in all, the increase would amount to 0.6/(1.1*4)=13%, with an expected improvement in the Higgs limit equal to $\sqrt 1.13$ or 6%.

That back-of-the-envelope estimate is wrong for two different reasons. One, muons are detected with much larger acceptance by the D0 detector, which makes their weight larger in the sum computed above. Two, the estimate neglects the continuous improvement of tools made possible by larger datasets and more time to think. The latter has always been the Tevatron experiments’ secret weapon, as is shown by the incredible precision they have been recently obtaining on the top quark mass (now known with 1% accuracy - a goal thought impossible to achieve with twice more data than that available now just six years ago), a result entirely due to a continuous refining of complex techniques.

But there is actually a bonus reason to be interested in this new result: because after obtaining a limit from the extended data, D0 includes the recently analyzed samples missing from the picture, obtaining their full 1.7/fb result for the WW final state!

So let us have a look at the works very briefly. The analysis consists in a selection of dilepton events with missing transverse energy - the signature of two leptonic W boson decays - which is optimized as a function of the Higgs mass. Great care is used to define the selection cuts, since their best value depends on : for instance, at low mass of the Higgs boson (below 160 GeV, which is the threshold for resonant WW production) special care has to be used to maintain high efficiency. Then a neural network classifier - called “multi-layer perceptron”, MLP- is used to discriminate the signal from the sum of concurring backgrounds - the most important ones being continuum WW production and Drell-Yan production of lepton pairs.

The output of the MLP is studied for data before the application of all selection cuts in order to verify the understanding of the mixture of processes. In the figures below you can see that D0 does indeed an excellent job. Left to right, top to bottom; dielectron final state; electron-muon final state; dimuon final state, Run IIa 1.1/fb; dimuon final state, Run IIb 0.6/fb (the latest data). In all four plots, the Higgs is the irrelevant empty histogram at the bottom.

At the end of the game, the distribution of neural network output for the final candidates is interpreted with the CLs method using information of each bin rather than the integral of the distribution: this obviously improves the accuracy and the sensitivity. Below you can find the NN distributions and the number of events expected by D0 for different Higgs boson masses (remember, they correspond to different selections!).

In the table, for three reference Higgs mass values the number of events expected by SM processes for Higgs production and backgrounds are shown in the second and third line. The data appear to undershoot backgrounds a little, highlighting the absence of a Higgs contribution (but I warn you: the probability that among those 10 events lie as many as two 160 GeV Higgs decays is still a fat 2% if the Higgs is there)

In the end, a 95% confidence level limit is obtained on the cross section as a function of the Higgs mass, and from it a “times SM ratio” limit plot is extracted. The one shown below combines results from dielectron and electron-muon final states, so it is a genuine 1.7/fb result. In it, the curve shows the upper limit on the ratio of Higgs production rate over SM predictions. When the black curve will finally cross the line at unity, we will start to have a real exclusion of Higgs mass values. We are getting close!

After seeing this plot, which reaches a x2.4 SM value at 160 GeV, I am starting to be very curious to see the combination with CDF results (which stood at x1.9 SM at 160 GeV already last August). I think we will have to wait for winter conferences to get that plot, but I smell a x1.1 limit at 160 GeV: not yet any mass exclusion for winter 2008 (for the latest combination, yielding x1.4 SM, see here).