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The new number January 7, 2007

Posted by dorigo in news, physics, science.

Ok, in the last post I had originally intended to put the new top mass measurement along with an explanation of the analysis, but I ended up describing more generally the problem of detecting all-hadronic decays of top quark pairs… So I need a new post to write about the more technical information. This one.

The new result in the all-hadronic decay mode by CDF, based on 943 pb-1 of Run II data, is:

 Mt=171.1+-3.7(stat+JES)+-2.3(syst) GeV

where the first uncertainty is the combination of statistical uncertainty and jet energy scale systematics, and the second combines all remaining systematic uncertainties.

The picture on the left shows the result for the subset of selected events which contain two jets b-tagged by the vertex identification algorithm: the yellow distribution is the estimated mass shape of the top events in the sample.

The result is obtained through a technique which is becoming a standard in CDF. The technique consists in performing a kinematic fit with an additional parameter beyond the top mass: the jet energy scale, which receives a strong constraint from the W->jj resonance. The jet energy scale is a number close to 1.0 which tells us if the energy correction algorithm we apply on Monte Carlo simulated jets behaves the same way or not on real jets: if we over-correct the jets in the data, we will then have a positive bias in whatever measurements we obtain from a comparison of Monte Carlo distributions to the data. Conversely, if we over-correct the Monte Carlo, we will tend to underestimate the measurement. I discussed in detail the issue of the determination of the jet energy scale in three posts a few months ago, see:




Anyway, let me discuss for a second the new result and its implications.

Overall, the total error bar is 4.3 GeV wide, and this measurement will therefore contribute appreciably to the shrinking of the Tevatron average (now sitting at Mt=171.4+-2.1 GeV).

It is to be noted, however, that the data sample on which this result is based is the same of previous measurements in the all-hadronic final state, the latest of which (based on 1020 pb-1) was given last summer at Mt=174.0+-2.2(stat)+-4.8(syst) GeV and entered in the Tevatron average quoted above, and shown in the plot on the left. So it is kind of hard to eyeball what will this measurement do to the world average, especially given the correlation of the jet energy scale systematic uncertainty with the corresponding systematics of the most precise results based on the single lepton final states. I would guess that the average will go down by a few tenths of a GeV, and the uncertainty will also shrink by maybe a tenth of a GeV.

So, what is the bottom line? That the Higgs boson is getting leaner by the day!!! Yesterday I quoted the new W mass result, which, being higher than the previous world average, drives down the Higgs mass estimate. Today, the top quark is getting lighter, also driving the Higgs down… I am quite curious to see these new results fed into the global electroweak fits!



1. Bee - January 8, 2007


2. Bee - January 8, 2007

Ah. Ahem. Sorry for the empty comment. I’ve tried yesterday to comment something but it didn’t work. Forgot what I wanted to say though.

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