This morning I attended the first session of the Physics in Collisions conference in Kobe, which dealt with Electroweak Physics. The four talks I could listen to were all of very good quality, and I am not ashamed to say that I did learn a thing or two, despite this is a field of investigations on which I have focused for over a decade. Also, I decided that conferences featuring few, long talks are definitely better than ones which try to cram dozens of small contributions in tight schedules: at least, the session conveners are not required to play the watchdog and struggle to keep people within their allotted time, and post-talk questions and comments are actually encouraged rather than suppressed.

The conference venue is the Centennial Hall in Kobe University, a building on the slopes of Mount Rokko which sports a wonderful view on the bay area:
The interior of the hall is also quite remarkable. Here is the ceiling:

But let me try to summarize the talks. The first one was given by my colleague Bernd Stelzer, who discussed Top quark measurements. Bernd reviewed in detail the most precise measurements of top quark production cross section, mass, and properties produced by the CDF and Dzero experiments at the Tevatron, and then gave some highlights of the potentialities of the LHC in top physics. In my notebook I have scribbled the following annotations:

  • The top pair production cross section is now measured very precisely by CDF. In its most precise analysis CDF performs a normalization of the luminosity using the cross section of Z production (which, being a purely electroweak process, has a very precisely predicted cross section from theory): this technique eliminates the 6% uncertainty due to the amount of integrated luminosity used to collect the data. The combination of CDF measurements has thus now a 6.5 percent total error, which is much better than the theory error. This, added to the fact that a combination with Dzero results will reduce further the uncertainty, means that there is potential use of more precise theoretical predictions: top pair production is due mostly to quark-antiquark annihilation at the Tevatron and it is in a perturbative quantum-chromodynamical (pQCD) regime, where we do not expect surprises; but the accessibility of precision checks of pQCD calculations in this process is pleasant news.

  • Measurements of the spin-spin correlation of top quarks shows a discrepancy between Standard Model predictions and CDF measurements (at 1-sigma level) and Dzero measurement (at the 2-sigma level). Something to keep watching closely. I will have a post on this issue soon, if I manage to get on top of my schedule.

After Bernd, Pierre Petroff talked about W and Z production and properties. He gave a very nice talk. Here are some notes:

  • The W production asymmetry is now measured at the Tevatron with an accuracy which allows to determine the parton distribution functions of quarks in the proton more precisely than what global fits do. Of course, this only occurs in the very narrow range of the parameter space relevant to W production; but it is nice to see that we can squeeze more information on the structure of the proton from these measurements. One could say we continue to learn about hadronic matter by using electroweak probes, eighty years after the first studies of beta decay.

  • The limiting factor in the accuracy with which we can infer the unknown mass of the Higgs boson is now the uncertainty on the W mass, since the top mass is known with 0.75% uncertainty from combination of CDF and Dzero results. We currently have a total error of 25 MeV on the W mass in the world average value, and in order to squeeze as much information on the Higgs mass from the W mass as the one coming from the top quark, we would need to shrink the uncertainty on the former down to 8 MeV or so. This is quite a challenge for the Tevatron, and it is even more so for the LHC.

After a well-deserved coffee break, I got to hear about Multi-boson production by Paolo Mastrandrea. It was quite pleasing to see the quality and wealth of recent results from the Tevatron on this topic. Among them, Paolo showed the first observation of ZZ production by CDF, which is the effort of a student in Padova, Matteo Bauce.

Matteo followed my course on Subnuclear Physics last fall in Padova, and I am both happy and envious by seeing he has achieved an important new measurement in CDF during his undergraduate thesis studies: this is indeed quite a feat! I am certainly happy for him and for Padova, as well as for seeing this measurement from CDF; but I am also envious because I had originally hoped he would rather choose a thesis on the CMS experiment, and thus come to work with me. I would have been able to offer him a much less exciting study, based on putting together an analysis we cannot yet perform, having no real data to run it on. Of course his choice was good: the data that CDF has acquired and is still collecting can and will provide a reason for years of more studies, so students can still profitfully invest their time there, rather than flocking to LHC.

The final talk in the morning was given by Denis Bernard, who discussed the difficult topic of Hadronic contributions to the measurement of the muon anomaly, the so-called "g-2" measurement made by Brookhaven. I already discussed elsewhere this measurement and its implications (also see this nice two-part article); I will just draw from the conclusions of Bernard's talk that despite a new extremely precise and beautiful measurement by BaBar, the discrepancy between theory and experiment on this particular quantity remains at the 3-sigma level.

Bernard had not finished spelling that he evaluated it at 3.3 standard deviations, when a listener from the audience mentioned that tomorrow a paper will appear on the Arxiv, where that very number is now estimated at 3.1 standard deviations. The g-2 remains a quantity to watch closely, especially since new physics is expected to contribute to its value appreciably. This could indeed be the place where Supersymmetry first makes its appearance -or, if you are a sceptic like me, the place where Susy enthusiasts will receive another burning disillusion.