1) Claudio Verzegnassi : PALM. A new program to calculate cross sections, asymmetries and ---------------------------------------------------------------- virtual effects of new physics at LEP2 -------------------------------------- PALM stands for PAviaLecceMontpellier, since it has been written by people from there. Technical questions should be send to the people in Pavia G.Montagna (guido.montagna@pv.infn.it) F.Piccinini (fulvio.piccinini@pv.infn.it) Note that these two are also the authors of TOPAZ0. Why PALM? The original motivation was to compute in a semi-analytical (!) simple and realistic way virtual effects of general models of new physics in the process e+e- -> ffbar at LEP2. Then it was soon realized that also the calculation of the MSM (minimal SM) predictions (at the one-loop level) was welcome. What are the important aspects of PALM? a) it is semi-analytical, (thus probably fast), and relatively simple b) it replaces systematically Gmu (the Fermi coupling) by "LEP1 observables", such as Z partial widths and asymmetries. By doing this, certain higher order corrections have not to be computed at high energy, since they are effectively accounted for in the measured quantities. The LEP1 observables are measured with a precision of approx. 1 in a thousand, so the induced error is much smaller than the expected experimental precision at LEP2. Then he gave more details on the calculations, such as the grouping of the one-loop corrections in four separately gauge-invariant blocks, and the relative importance of a proper treatment of boxes. ISR is included, also for boxes (which is not the case in other programs). He claims that the effect of ISR-FSR interference should me small (note by the editor: apparently we observe the opposite....) Next he gave a comparison between PALM and TOPAZ0, no numerical differences larger than 2 per mille could be found at the LEP2 energies, which is within the statistical and numerical precision. Finally he points at a possible effect ("relevant, but small, or small, but relevant") of boxes on the tau polarization.
2) Marie-Noelle Minard: Comments on the x-sections at 130-183 GeV and the integration range -------------------------------------------------------------------- Marie-Noelle has discouvered that ZFITTER gives considerably different predictions for the cos(theta) distribution of quarks at low angles. E.g., if computing the hadronic x-section with and without interference she observes a difference of 3.8%, which is even bigger for specific flavours such as uubar. Also in the dimuon case a difference of 12% is found. The asymmetries used in KORALZ do not include ISR/FSR interference, so there is a possible danger (note that PYTHIA is even much worse). On the other hand, our efficiency decreases rapidly down to approx. 40% above |cos theta|> 0.95. So the proposal is to evaluate the cross sections in the restricted acceptance range |cos theta| < 0.95, without extrapolating to full acceptance. Then the theoretical systematic error due to interference is only 1.5% instead of 3.8%. Now a discussion came up if we really should quote results for a reduced acceptance. This makes it difficult to compare to other experiments. It seems also too early (due to limited statistics and low efficiency at low angles) to use the data to check how big the effect really is. In the future we should measure the costheta* distribution for quarks. It was proposed to quote the results for reduced acceptance in the conference note, and for the upcoming paper one can do further investigations and eventually give both numbers, extrapolated and not extrapolated. This proposal was generally accepted.
2) Marie-Noelle Minard: First look at the 189 data --------------------------- Marie-Noelle showed first results for the hadronic selection on the 189 data. The typical distributions such as Mvis, thrust, sprime etc. (those given also in the paper) were shown. Generally good agreement between data and MC is observed. Also the cross sections agree with the expectations (so unfortunately no hint yet for new physics).
- Next meeting ---> to be announced
Guenther Dissertori
Thursday June 25, 16:59:38 MET DST 1998