- Systematic studies of reprocessed 183 GeV data ---------------------------------------------- Marie-Noelle has given a status report on the qqbar selection, based on Lint=59.35 pb-1. First she has compared the old with the new reprocessing. Three parameters have been looked at : Ntracks, Mvis, sqrt(s'/s). Scatter plots for old vs new were shown. For the sqrt(s'/s) variable some scatter around the diagonal is seen. The effect of the new reprocessing is that for sqrt(s'/s)>0.85 1.5% of the events move out from the region, whereas 1.4% of new events come in. She will study the relation of the fluctuation with fluctuations in the thrust axis. The inclusive hadronic x-section is 107.19 +- 1.37 +- 1.11 pb , expected : 104.1 pb For sqrt(s'/s)>0.85 : 26.75 +- 0.59 +- 0.33 pb, expected : 24.15 pb ========> +3.8 sigma!!!! Then she divided the full data sample in subsamples of about 7 pb-1. She observes a big time-variation of the ratio Nhad*10**3/Nbha, varying from 0.7 to 1.7. A discussion arises about possible problems with the luminosity measurement. Note : When writing these minutes, it become clear that there was a bug in this distribution, there is not such a drastic time dependence. Conclusions : More investigations needed.
- Study of dimuons at 183 GeV ------------------------------ Elsa has looked at dimuons for energies from 181 to 184 GeV. x-sections, efficiencies and backgrounds can be found on the copies of the transparencies. The forward-backward asymmetry comes out to be Afb(mu) = 0.558 +- 0.061 (Zfitter:0.589) Then she looked at the double radiative part of the sample, ie., 60 < Minv < 110 GeV and sqrt(s'/s)>0.9 : When taking events from this region, she finds an excess of events in the s'/s-distribution for sqrt(s'/s)>0.9: 1% of events expected, 2.6% observed. When scanning the events, she finds eg. events with 2 muons of 45 GeV and parallel photons of 2 GeV. Already at 172 GeV some excess was observed (2% expected, 11% observed). Those seem to be ISR, since a high-energetic photon was found. At 161 GeV no excess was observed. Conclusion : The excess is not understood, will be studied.
- Rb measurement at 183 GeV ------------------------- Rb is interesting in order to look for new physics (eg. leptoquarks), not so much for Z-studies due to the enhanced photon-exchange. He takes the hadronic selection by M.N.Minard, applies then an event tag, and uses Z data for calibration. The working point is at a purity around 95%. The he shows again that one has to scale a cut in QIPBTAG in order to get the same efficiencies at the Z and the higher energies (see previous meeting). He uses still the old reprocessing, results will have to be given with the new one. Limits: ------- We are sensitive to couplings of the form lambda*e*b*LQ (or R-parity violating terms). He performs a least-squares fit and obtains and exclusion region in the leptoquark parameter plane. Conclusion: New reprocessing needed, further he has to monitor the negative impact parameter distribution during the run. Ian asks about using a Neural Net. There one might face again the problem of selection criteria which do not scale correctly with the centre-of-mass energy. Anyway, Mario does not expect much improvement in efficiency.
- Single leptoquark production (Demokritos group) ----------------------- First Erietta described the ERATO Monte Carlo which generates single leptoquark production (e+e- --> e+qS). She discussed the relevant diagrams, the regularization of singularities, they assume a generic Yukawa coupling, which is mass and generation dependent. They have generated 1000 events for each leptoquark mass in the region 70 -> 183 GeV, in steps of 10 GeV. Then the optimized the selection cuts for the mass range 130-180 GeV. The signal: The LQ decays inside the apparatus, whereas the e+ disappears in the beam pipe. (-> keep events with 1 jet and an e-, apply lepton isolation cuts). Backgrounds are qq, Zee, ZZ. No events are found in the data (-> limits are given). Then they change the cuts in order to optimize the analysis for leptoquark masses > 45 GeV (such as OPAL has done). To do : To be more efficient in the region MLQ>45 GeV, cuts and parameters have to be changed. Systematics and ll backgrounds have to be studied, further one could also look for a vector LQ. Alain asks if this could be presented to the winter conferences? Answer: Yes. So, what is needed now is an Aleph Note.
- Comments on the candidate event in the Higgs searches ------------------------------------------ Sau Lan Wu has given a summary of the studies on the candidate event (Run 44577, event 9277) in the Hll channel, which has triggered an analysis for a 97 GeV particle decaying into bbar in four-jet events. This analysis is described in detail in the ALEPH Note ALEPH 97-113, PHYSICS 97-109, Dec 18, 1997.
- Comments on bbqq events at 183 GeV ---------------------------------- Andre first summarized the CART selection, a new selection scheme which can be somehow seen as a linear neural net. What he finds is an excess of bbarqqbar events in the high efficiency region. 8 events from this excess are in common with the Wisconsin events. No excess of events can be found before applying kinematic cuts. A Z*H scenario seems to be unlikely since not enough events are found in the side bands and the recoil mass is far from the kinematic limit. He showed that the CART selection would always find the wrong combination of jets (q-bbar, qbar-b) in Higgs decays with 4 jets, m(bbbar)>80 GeV. It is difficult to conclude something except that more work on the b-tagging is needed.
- Discussion ----------
- Next meeting ---> first week of February
Guenther Dissertori
Wed January 28, 11:20:38 MET DST 1998