- Hadronic x-section at 183 GeV ----------------------------- Marie-Noelle has analyzed 55.88 pb-1 at 183 GeV and 6.178 pb-1 at 130 GeV (not yet finalized). In the invariant mass distribution a slight deficit of the events in the Z return peak is observed in MC w.r.t. data. The energy distribution of radiated photons exhibits a bump at 75 GeV, which is being investigated and probably comes from W decays with an electron. The angular distribution of the photons as obtained with KORALZ is in good agreement with the data. Dieter points out that the new MC has a problem with photons in the ECAL endcap. This should be followed up. In an alternative analysis she cuts at thrust>0.88 in order to reduce the backgrounds. Indeed, a background reduction of a factor of 5 is obtained, but also the efficiency drops. The resulting x-section is consistent with the one from the standard analysis. Systematics: ------------ The leakage of double-radiative events into the high-s' region is studied with the Mvis-distribution. The WW background is checked using a neural-net output. The error on the efficiency from the energy-scale is estimated by a 1.5% variation on type 4 Eflow objects and a 4% variation on type 5 objects. A high contribution from gamma-gamma is observed in the high-s' region. Marie-Noelle points out that it comes from the double-tagged events, and is not reduced because she does not cut on the visible energy. From a comparison of different ways to handle the tagged gamma-gamma component with HERWIG she estimates a 1.4 % systematic error. Results at 183 GeV: ------------------- Below one finds preliminary numbers obtained with 55.88 pb-1 : sqrt(s'/s) > 0.1 sqrt(s'/s) > 0.85 ------------------------------------------------------------------- Eff 82.5% 96.2% --- +- 0.6%(MCstat) +- 0.6% (s' definition) +- 0.7%(E scale) +- 0.7% (E scale) Backgrounds: ------------ gamma-gamma 1.32 +- 0.2 % 1.4 +- 0.3 % WW 13 +- 0.3 % 23.9 +- 0.6 % (WW x-sect=15.71 pb) ZZ 1.6 +- 0.3 % 0.2 +- 0.3 % Zee 1.5 +- 0.3 % 0.2 +- 0.1 % (Note that the uncertainties on the backgrounds are correlated) x-sections: ----------- 106.13 +- 1.69 +- 1.11 pb 26.57 +- 0.70 +- 0.32 pb Expected: 104.1 24.15 Note: The high energy x-section is about three sigma too high !! ----- For the 130/136 GeV data she will combine the 1995 and 1997 data. She is working on it at the moment.
- Leptonic x-sections at 183 GeV ------------------------------ Elsa is looking at the dimuon, ditau and dielectron channels at 183 GeV. Preliminary results were presented. Detailed tables of results can be found on the copies of the transparencies and are not given here. There is a small discrepancy in the total intergated luminosity she obtained from scanbook w.r.t. to Marie-Noelle's one. Elsa finds 55.80 pb-1. To be checked. Muon Channel: ------------- The FB-asymmetry at sqrt(s'/s)>0.85 is calculated using the counting method and corrected for selection biases using the MC. The corrections are of the order of 1%, much smaller than the statistical error. The question comes up if this analysis is affected by the TPC problems. Will be checked. It is stated that the database bug is not affecting the results. Tau Channel: ------------ She points out that for this channel the asymmetry is flipped in sign. Will be checked. Some excess in the last bin of the FB-asymmetry is observed, but also the background shows a rather strange behaviour, since it rises and then suddenly drops to zero in the last bin. Also this will be studied in more detail. The x-section turns out to be rather low. Bhabha : ------- Here the x-section is in agreement with the SM expectations. Elsa is also working on the 130/136 GeV data. The selection is ready for the dimuons, further checks remain to be done, however. It is stated that there was no big improvement regarding the collaboration between Marc Swynghedauw and Elsa.
- Contact Term Fits with the 183 GeV data --------------------------------------- A first look at the contact terms shows that in general the results improve by 30-50% w.r.t. to the limits sent to Jerusalem regarding the leptons. In this case the likelihood funtions are much better behaved as previously (due to the increased statistics), only in the tau case some asymmetric errors remain. In the hadronic case the situation is quite different. There the limits do not improve. This is due to the 3-sigma excess of our measured x-section. The results are compatible with the SM at the 95% CL, but not at the 68% level. The likelihood shows again the two-peak structure. It is decided that these results will not be shown at the LEPC.
- Comments on the s' definition ----------------------------- Frederic has studied the OPAL definition for s': They define sqrt(s') to be the centre-of-mass energy after ISR. It is computed from the invariant mass of the fermion pair in the final state, actually from the jet directions and sqrt(s). They have to correct for FSR and ISR-FSR interference effects using Zfitter. The interference effect is of the order of 1%. They obtain a corrected x-section by subtracting the interference contribution from the measured x-section, assuming that the efficiencies are the same with and without interference. The problem is : The interference effects are only known to order(alpha). There are indications that these order(alpha) predictions are not reliable. At the Z peak the effects are small because the finite lifetime of the Z "separates physically" the initial and final state wavefunctions. However, if one cuts on the photon energy, and e.g. allows only very soft gammas to be left, the photon wavelength increases and interference is restored. Indeed, this has been proposed by Jadach et al. as a means to measure the Z width, and DELPHI has published a paper on that. The main point there is that they had to apply a 60% correction relative to the order(alpha) prediction, in the opposite direction! Frederic himself has performed studies of interference effects in his thesis and the radiative dimuon paper. He found that the interference effects are less important in the data than in the order(alpha) calc. The conclusion is : Order(alpha) seems not to be appropriate enough to estimate interference effects. So what should we do? We need to be consistent between the definition and the variable we cut on. He proposes to define sqrt(s') = invariant mass of the final state fermions It has the advantage to be well defined theoretically and experimentally. It has the disadvantage that is does not strictly separate hard from soft processes (FSR). He proposes to apply the OPAL procedure to correct the efficiency from the MC, but to compare data and MC to estimate the systematic uncertainty.
- Discussion (on the s' definition) --------------------------------- Ian: ---- He proposes to define s' as the invariant mass of the final state fermions (from the angles), but to apply no subtraction (such as OPAL does). Instead we should study the efficiency in bins of angles and invariant mass. His proposal is generally accepted.
- Next meeting ---> end of November, or beginning of December
Guenther Dissertori
Fri November 21 , 18:50:38 MET DST 1997