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Blessing the Z to bb signal today! February 23, 2007

Posted by dorigo in news, personal, physics, science.
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Today, if all goes as expected, the exactly 11 years of work I devoted to the search for Z->bb decays will finally produce their first fruit.

I discussed earlier the difficulty to extract the decay signal of a Z boson in two b-quarks from hadronic collisions. So, if the four LEP experiments detected millions of these decays, why bother extracting only a few thousands of them at the Tevatron ?

Indeed, the Z boson is one of the best known animals in the particle zoo. In fact,  just because it is so well known, it makes a perfect calibration line. Any experiment measuring the Z mass with whatever tool they have, have a chance to calibrate their tool to a part in hundred thousand – that is the precision with which we know the Z mass.

That is what I have worked for so long to demonstrate was doable: calibrate the jet energy response of our detector using the Z. Of course, I was not alone in this endeavour: but I was the first to work at it, and I alone saw a signal of Z->bb decays in Run I with CDF, which motivated others to study the feasibility of jet calibration in Run II.

Now, five years into Run II, the Z->bb working group includes nine physicists: two from the University of Chicago (Mel Shochet, Shawn Kwang),  three from the University of Padova (Julien Donini, TD, Mia Tosi – not shown today!), two from Rockefeller University (Ken-ichi Hatakeyama, Tomonobu Tomura), and two from the University of Pennsylvania (Christopher Neu, Daniel Whiteson). The pictures of our american colleagues are shown here in the order I cited them.

The measurement of the Z peak allows to determine the b-jet energy scale with a 2% accuracy. This translates in a 2 GeV systematic uncertainties in top quark measurements based on the very clean dilepton final state – which only includes b-jets in the signature. Compare with the best measurements so far obtained by CDF in that final state: 

  • the current matrix-element measurement of top mass from dilepton events with 955/pb of data (incidentally, almost twice as much data as that on which we base our Z-driven 2% scale measurement) obtains Mtop=167.3+-4.6(stat)+-3.8(syst) GeV, where 3.3 GeV is the jet energy scale systematics alone. Including our result this should decrease roughly by a factor of two.
  • a different measurement, using a kinematic method, finds Mtop=168.1+-5.6(stat)+-4.0(syst) GeV, again 3.3 GeV being the systematics due to the b-jet energy scale alone.

All other top mass measurements in CDF will also benefit greatly from the new scale measurement. However, more work will be required to understand the uncertainties due to extrapolating a result obtained with b-tagged jets reconstructed with a cone of 0.7 radians to generic jets reconstructed with a narrower 0.4 radians cone. Also, the energy spectrum of jets from top quark decay is not coincident (although pretty close) to that of jets from Z decay, and that will also be the subject of investigation.

Of course, whenever one reaches a goal, many other goals appear on the horizon – the most obvious of which is publishing the result. Nonetheless, now it is the time to cheer at the result. 

It is therefore with a lot of pride that I will sit in our conference room in my University tonight, connected by video with the CDF Theatre, hearing Julien’s blessing talk.

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