A hot new Mtop measurement from CDF reinforces the mystery… July 7, 2006
Posted by dorigo in news, physics, science.4 comments
A brand new top mass measurement by CDF in the dilepton final state is out.
It is based on 78 events, 50 of which from ttbar production, in 1.02 fb-1 of data selected by requiring two high-Pt leptons, missing transverse energy, and two hadronic jets. The mass measurement is performed by a matrix element technique, with parametrized transfer functions for the parton-jet energy conversion.
The systematic uncertainties amount to 3.9 GeV, and are totally dominated by the jet energy scale, 3.5 GeV by itself. The statistical error is equal to the total systematics, 3.9 GeV.
What makes headlines today is that the measurement is LOW! Mt = 164.5 GeV (with a total 5.4 GeV uncertainty if stat and syst are combined in quadrature, a dubious but standard procedure).
In Run I, people noticed that the more jets were produced in the ttbar decay, the higher the mass measurement turned out. The effect was strange, but not inconsistent with zero. Now, with a 164.5 GeV measurement in the dilepton channel, I am sure we’ll get a new wave of arguments by the usual dreamers.
If you want to join the club, one hint is that the current lepton+jets measurement is 1.5 sigma below the world average mass. However, the WA is also pushed down by previous dilepton measurements more than it is pushed up by any all-hadronic measurements, so one could build some case for a strange supersymmetric decay present in the dataset, which affects the dilepton topology…
I prefer to look at it another way. If there is an effect, the jet energy scale is off. It will be my duty to verify that with the sample of Z->bb decays I am currently analyzing with Julien… We are by now very close to producing a energy scale determination with a 2% uncertainty. That will surely help!
Another thing to note: the measurement discussed above will pull down the world average of Mt. What that means to me is that the most probable value of Higgs boson mass, from indirect fits to radiative corrections in electroweak measurements, goes down! And if the Higgs mass goes down, the Tevatron rejoices, for a light higgs is more at reach. Not just that. MSSM addicts will also rejoice, for the model is alive and well if MH<135 GeV…
We will see what happens. As for me, I already know what the top mass is. it is 246 GeV divided by square root of two…
Speeding July 7, 2006
Posted by dorigo in personal, politics, travel.1 comment so far
Today I ran a few errands in Venice. Among them, collecting a letter at the post office. It turned out to be a speeding ticket - the fourth since three years ago, when a new law was passed which assigns every driver an initial score of 20 points, and reduces them when they make infractions to the rules of the road.
I have no objections to the law, which did reduce the number of road accidents and resulting deaths. I, however, feel it is not fair to control electronically the speed of cars on the road… I prefer the american way, when you either get caught en plein air, with noise and colored lights, or you get away with it.
Anyway, of my original score of 20, I am now down to 12 points… I have to be careful, or I’ll soon have to use a bike to move around. I have a hidden resource however: my wife very rarely drives our car, and we can testify she was driving. Next time I commit an infraction, we’ll deduct the points from her score…
