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Evidence for single top by D0 December 12, 2006

Posted by dorigo in news, physics, science.

Ok, on this one, D0 appears to beat us.

I recently discussed the complex situation of single top production searches in CDF in https://dorigo.wordpress.com/2006/11/07/the-elusive-single-top/ ( and see https://dorigo.wordpress.com/2006/11/20/a-low-mass-top-in-single-top-events/ for an exotic interpretation of those results). To summarize here, despite a huge effort by CDF, no clear indication of the signal is present in the data so far analyzed – one analysis finds a 2.6-sigma excess over backgrounds, but another study based on the same data sees no signal at all; and the 2.something-sigma effect arose suspicion in some that there be something unexpected in the data.

Things in D0 are brighter: they gave a wine and cheese seminar at Fermilab four days ago, when Dugan O’Neil showed the results of three different analysis methods, all consistently showing a clear evidence for a Standard Model signal of single top production. You can find Dugan’s slides at http://www-d0.fnal.gov/Run2Physics/WWW/results/prelim/TOP/T39/wine_and_cheese.pdf.

The a-priori best measurement of the set provides a cross section of 4.9+-1.4 picobarns, when 2.9+-0.3 pb is the next-to leading order theory prediction. This cannot be dubbed an “observation” yet (which is a word reserved for 5-sigma effects in physics jargon), but it comes close to it, and it would be very strange if the 3.6-sigma excess of D0 did not grow to observation level in the next few months, as more data will be fed to the analysis.

What’s more, D0’s excess appears to cluster at the right top mass value, and not at low mass as some of CDF’s – an indication that things are going fine and that the standard model still rulez. Indeed, single top production is a purely electroweak process – at least in one of the two production channels – and surprises there would be twice as much puzzling. You can see some of the D0 signal in the plot on the left, where the reconstructed top quark mass is plot for the data and compared to backgrounds (in grey, green, and red) and signal (the blue stuff).

If you are curious about the details of the analyses by D0, I encourage you to have a look at the slides linked above. If you are lazy, I offer below a poor-man description of the whole thing…

Events triggered by the presence of a high-momentum electron or muon are collected in a 0.9 inverse-femtobarn dataset, significant missing transverse energy is required, and two or more jets. A neural-network B-tagger finds very effectively jets which are likely to have originated by b-quark hadronization, thus enriching the data with the single top production signal, which should nominally yield a W (yielding the lepton and missing Et) and two b-quark jets (plus an additional light-quark jet in some cases). The data is then studied by a decision tree, which uses many kinematical variables to discriminate the signal from all known background sources. A cut on the decision tree output enriches the surviving data of signal to the level that an excess is observed. The most discriminating kinematical variables (such as the one shown in the plot above) are then studied to verify that the excess sits where it is expected from single top production. From the excess a cross section is computed by taking into account the amount of data analyzed and the selection efficiencies.

Still curious ? Go to the talk!



1. gordonwatts - December 18, 2006

Thanks for the nice post!

BTW, speaking of misleading plots — this one is misleading. All the analysis methods use fairly complex variables — including the top mass — as inputs. So they will shape any output distribution. So if you didn’t see this sort of a shape for this plot you might be worried that you had really screwed up. The two analyses from CDF should do the same, btw.

This is an unfortunate effect of these moder multi-variate analyses. To get to the edge of your sensitivity you take into account every last bit of information you can… which often destroys the visual simplicity of a plot like this. 😦

2. dorigo - December 18, 2006

Yes Gordon, I know… I wrote about it myself. When you squeeze everything from the kinematics, nothing is left to show but a sheer count of survivors…


3. Single top: new results from CDF! « A Quantum Diaries Survivor - October 11, 2007

[…] physics, science. trackback About one year ago D0, the Tevatron experiment competitor of CDF, announced they had obtained for the first time evidence for the long-sought standard model process whereby a single top quark is created in […]

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