- Status of BEW work ------------------ Guenther first reminded what is the main purpose of the BEW group, namely to put limits on physics beyond the SM via looking at the total and/or differential cross sections for e+e- -> ffbar (fermion pairs). The new physics can either be something very general such as contact interactions, or something more specific such as lepto-quarks. Then he pointed out that the group is working on three fronts: A) X-section measurements B) Study of systematics C) Limits on new physics where each has a short term and a long term program. A) X-section measurements : --------------------------- (done by M.N. Minard and E. Merle) SHORT TERM : The short term program is practically done, as we have measurements at 133 GeV : Published 161,172 GeV : ALEPH Note, paper and contribution for Jerusalem in preparation, see also talk by M.N.Minard at the Thursday meeting of June 12. LONG TERM : We will measure the x-sections at 183 GeV up to the final LEP energies of 200 - epsilon. The measurements should then finally be combined for all experiments. B) Study of systematics : ------------------------- SHORT TERM : Here the same holds as outlined under point A). We have estimates of the systematic errors at 133,161 and 172 GeV. LONG TERM : In the future we will have to reduce the systematic error as much as possible. For example, the systematic error on the hadronic x-section is of the order of 4%. Up to now this was not a problem because of the large statistical error. However, in the future this error will go down, especially after combination of all ADLO results. An example were there is room for improvement: The understanding of ISR ------------------------ A problem has been discovered for the implementation of ISR in Pythia (see a previous talk by M.N.Minard). There are not enough highly energetic ISR photons generated at large angles. This is due to a too small upper scale in the ISR shower implementation. A way out is to take process ISUB=19, where the matrix element for e+e- gamma -> Z is calculated, and which is best for high-pt photons. However, for low-pt it cannot be used. There the standard process (ISUB=1) is o.k. So finally a "cut-and-paste" approach should be taken (this is actually done by Marie-Noelle). Another way out is to increase the cut-off scale in the ISR shower. This is implemented in a new version of Pythia and will appear in Kingal as PYTH05 (B.Bloch is working on it). Finally ISR can also be studied by looking at different MCs, such as KORALZ (KORL08). Guenther has generated samples with ISR and ISR+FSR at 172 GeV. They are tested at the moment and will be put on the official MC production page soon. B) Limits on new physics : -------------------------- SHORT TERM : The short term goal is to give the most general limits on contact terms for Jersualem. F.Teubert is working on the hadronic channel, I. Tomalin on the leptonic channel. The things one has to take care of here are the correct understanding of the x-sections which are going into the fits. For example, is the improved Born approximation still a good approximation at the high energies? Or: What are exactly the diagrams which are already in ZFITTER, and which diagrams have to be computed separately and to be taken as background? Here we need input/help from theorists. To illustrate that there is room for improvement: the theoretical uncertainty of the x-section at the LEP2 energies is still of the order of 1-2 %. It is hoped that results for Jerusalem can be presented at the next Thursday meeting. LONG TERM : Improve the understanding of the theoretical input of the fits, and start to study other physics such as lepto-quark etc.
- Discussion ---------- It has been pointed out that people from Greece and a student of H.Videau want to contribute to the work of the BEW group.
- Next meeting ---> to be announced yet
Guenther Dissertori
Thu June 26 , 17:30:38 MET DST 1997