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My talk on new results from CDF May 20, 2008

Posted by dorigo in cosmology, news, personal, physics, science.
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This morning I gave my seminar at PPC08, and I was able to record it on my camera. So, rather than giving a transcript, I could in principle get away easily by pasting here a simple link to my presentation in .mpg format. However, I will be able to do that only as I go back home next week, since transferring 500mbytes via wireless is not something I want to entertain myself with. I am thus going to put here a few of the slides, commenting them as I did during my talk, and I will update the post next week to include the link to the video file. For now, you can get a pdf file with all the slides here.

Note: This post is dedicated to Louise Riofrio, who kindly mentions my talk in her wonderful blog today…

I started with the usual mention of the experimental apparata: the first is the Tevatron collider (see slide below), which has delivered 4 inverse femtobarns of 2-TeV proton-antiproton interactions to the CDF and D0 detectors so far. The inverse femtobarn is a unit of measure of integrated luminosity L, which tells you how many events N are produced for a process with a given cross section \sigma: since the total proton-antiproton cross section is about 60 millibarns, 4 inverse femtobarns correspond to N=\sigma L = 0.06 b \times 4 \times 10^{15} b^{-1} = 2.4 \times 10^{14}, or a total of 240 trillion proton-antiproton collisions. I mentioned that the Tevatron expects to double the delivered luminosity if it will be allowed to run through year 2010, and I promised to show what that means for the precision of some critical measurements and searches.

Next I discussed the CDF detector (see slide below). I pointed out that its original design dates back to the year 1980, and that it was constructed to discover the top quark – something it achieved in 1995, but it has produced an enormous amount of excellent measurements in addition. CDF is a magnetic spectrometer where a inner tracking system made of 7 layers of silicon microstrip sensors is embedded in a large drift chamber, and the two are contained within a 1.4 Tesla solenoid. Outside the magnet are electromagnetic and hadronic calorimeters, surrounded by muon chambers.

I discussed only a few results on top physics. The top quark is a remarkable particle: the heaviest of all known elementary bodies, it decays before having time to hadronize since its natural width is an order of magnitude larger than the scale of quantum chromodynamical interactions. So we can study its properties free from the hassle of non-calculable soft QCD effects. The large mass of the top begs the question: why is it so large ? Why is its yukawa coupling very close to unity ? Is some form of new physics hiding in the phenomenology of top production and decay ?

Production of top quark pairs at the Tevatron occurs mainly by quark-antiquark annihilation, the diagram shown on the left in the slide below. Since each top almost always decays to a W boson and a b-quark, one can classify the final states according to the decay products of the two W bosons. In the upper right graph one can see how the possible final states break down in terms of relative rates: the most probable -and most background-ridden- is the all-hadronic final state, where both W bosons decay to quark pairs, and you get six hadronic jets from the top pairs. I also discussed single top production mechanisms, shown in the diagrams at the bottom: these have a comparable production rate, but they are much harder to extract from the data due to larger backgrounds.

I then showed a summary of top pair cross section measurements, mentioning that there are by now dozens of different determinations. The average has a uncertainty of 12%, and its agreement with NNLO predictions shows that the technology of perturbative QCD calculations is in good shape.

After discussing one measurement of top cross section in detail, I went on with mass determinations. The technology of these measurements has improved greatly in the past few years, and by now the top mass is measured by CDF with a 1.1% uncertainty. The average with D0 has been carried out on results obtained with 2/fb of data, and it produces M_t = 172.6 \pm 1.4  GeV, which is a 0.8% uncertainty. The top quark mass is important for the building of models of new physics beyond the standard model, and of course it provides a stringent constraint to electroweak fits of standard model observables. It is foreseen that the full Run II dataset in 2010 will allow the Tevatron to reach a 1 GeV precision on the top mass, or even slightly better than that.

I next showed the combined measurement of single top production cross section. A very complicated and advanced technique using evolved neural networks and genetic algorithms allows to optimize the measurement, and a 3.7-sigma evidence for the signal is obtained by CDF. This is a unlucky chance, since the expected sensitivity exceeded 5-sigma. But we have already collected enough data to grant the canonical “observation-level” significance in the near future, as the data is analyzed. I also mentioned that the future measurements will allow to determine the V_{tb} matrix element with a precision of 7%.

As far as new results on B physics are concerned, I only showed a couple. One is about the discovery of the exhilarating \Xi _b baryons, which are seen through the exclusive decay chain to J/psi mesons and \Xi baryons, with the latter in turn producing two pions and a proton with two separate vertices. The other is the new CDF limit on the branching fraction of B_s mesons to muon pairs, a process heavily suppressed in the standard model, which is enhanced in SUSY models. I discussed the latter result elsewhere recently.

I then presented two searches for supersymmetry recently performed in CDF: one for chargino-neutralino production in events with three leptons and missing transverse energy, and another for squarks and gluino signatures in jets and missing transverse energy. I also discussed these results recently in this blog, in a recent series of posts about dark matter searches at colliders.

I showed results on W mass and width measurements, on which CDF has the best measurements in the world. In particular, the W mass measurement has been produced with only 200 inverse picobarns of data, which is a twentieth of the data we have collected this far: CDF may reach below 20 MeV on the accuracy of W mass determination.

Diboson production has been observed in all its manifestations by CDF: the latest ones were WZ and ZZ production, which may give rise to spectacularly clean events (see the event display shown in the slides below: a perfect WZ candidate). The measurements of cross section are in excellent agreement with SM predictions.

Finally, I discussed the current limits on Higgs boson production. I think I have discussed this particular topic frequently enough in this blog to allow myself to skip a description of my slides here. I concluded my talk mentioning the string of successes of CDF in the recent past, and the prospects for precision SM measurements and reach of Higgs searches. I pointed out that CDF is the longest lasting physics experiment ever (yeah yeah, if we exclude the pitch drop experiment)…

There were several questions by the audience, most of them centered on Higgs boson limits and searches. I was of course happy to answer them, in particular to show that the results have kept improving more than the increase of luminosity they relied upon. In conclusion, it is always a great pleasure to present CDF results… A remarkable experiment indeed!

Comments

1. Kea - May 21, 2008

Wow, Tommaso, I just don’t know how you find the time at a conference to be such an eloquent blogger! Thanks.

2. dorigo - May 21, 2008

Hah Kea, and you ain’t seen nothin’ yet… I have about 15 pages of notes from the talks I heard about today. But I want to pick the most interesting stuff out of that material…. Now I’m really jetlagged though so I will do that tomorrow.

Cheers,
T.


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