Tuesday 16th February 1999 Ian Tomalin MINUTES OF THE 27TH BEW MEETING ================================= Agenda ======= Terry Medcalf : Excited lepton search at 189 GeV Mario Antonelli : Single top quark production at 189 GeV Marie-Noelle Minard : Update to 189 GeV difermion results Corinne Goy : R_b and Afb(bb) at 189 GeV Ian Tomalin : New Physics Limits from difermion results Errietta Simopoulou : Direct leptoquark search at 189 GeV Terry Medcalf : Excited lepton search at 189 GeV ================================================= Searches for excited leptons at 189 GeV were presented. The analyses are designed to select radiative decays l*-->l gamma from single or pair production of excited leptons. i.e. The final state has charged leptons plus one or two (isolated) photons. Searches for excited neutrinos have also been presented, using events with one or two photons. Mass limits very close to beam energy for pair production. Limits on couplings provided up to 189 GeV for single production. For single excited neutrino production, radiative nu nubar gamma events rejected by cutting on |M(miss) - M(Z0)|. The data seems to have too many events near M(Z0). Not yet understood. Mario Antonelli : Single top quark production at 189 GeV ========================================================= Search for Z -> top-anticharm production in 189 GeV data. The existing limit is BR(Z -> t cbar) < 33% from CDF. Reconstruct t -> b W, where W decays hadronically or leptonically. The selection cuts for these two channels follow those of 183 GeV analysis (ALEPH-98-038), but with a few improvements. Combined selection efficiency is 17%. See 18 events, when expect only 10 (6.5 qq, 2.8 WW, 0.6 ZZ, 0.1 Zee). The Poisson prob. to see >= 18 events is onlyt 1.4%. The origin of this excess is not understood. It is present only for the W -> q qbar channel. Systematics assessed from fraction of events rejected by each cut on its own after preselection. => Background uncertain by +-20%. This increases confidence level from 1.4 -> 6%. Further study needed to understand origin of excess. May be signal ... Marie-Noelle Minard : Update to 189 GeV difermion results ========================================================== Further studies of Bhabha channel which was 5 sigma away from SM predictions during ALEPH week. Discoveries - 1) SM prediction was obtained using too few MC events. With more BHWIDE events, discprenancy drops to 3 sigma. 2) At small polar angles, polar angle measured by TPC disagrees with that measured in ECAL by 1-2 mrads. As Bhabha cross section is very forward peaked, this gives significant systematic error. 3) At small polar angles, tracks in data have many more TPC hits than tracks in MC. => As small polar angles not understood, 189 GeV results will be obtained without using tracks in region 0.9 < |cos(theta)| < 0.95 . Strange polar angular distribution of hadronic events, which was presented during ALEPH week, also investigated, and traced to incorrect definition of signal/background at MC truth level. Corinne Goy : R_b and Afb(bb) at 189 GeV ========================================= Corinne updated the measurement of R_b at 189 GeV. Using Z0 calibration data, value of R_b at Z0 peak disagrees with SM prediction. Attempt to determine cause of this using single/double tags. Assign systematic to 189 GeV result by assuming that discrepancy in R_b at Z0 peak is due to background or due to efficiency. => R_b = 0.151 +/- 0.011 +/- 0.004 Afb(bb) has been assessed using a simple jet charge method (based on momentum component parallel to jet axis), as in 183 GeV paper. Result: Afb(bb) = 0.347 +/- 0.171 +/- 0.017, where systematic taken from effect of corrections used in LEP1 jet charge analyses. Afb(bb) has also been determined by adding to jet charge, the lepton charge etc. using neural network. Gives slightly better performance, but systematics not yet studied. Corinne also showed that measuring asymmetries using is better than using a simple counting method based on the fraction of time the forward hemisphere contains the most positive jet. Ian Tomalin : New Physics Limits from 189 GeV difermion results ================================================================ Contact term limits from Frederic Teubert shows 35% improvement over 183 GeV for dileptons. For hadrons, no improvement is seen, probably because jet charge asymmetries not yet used. Will do so before Thursday meeting. For Bhabha channel, theoretical error on SM prediction now much larger than experimental errors. Limits on R-parity violatiing sneutrinos derived by Ian Tomalin from dilepton measurements, show improved limit on Yukawa coupling by factor of two at large (300 GeV) masses. Limits on leptoquarks obtained by Ian using hadronic events improve by about 35% over 183 GeV results. Limits on Z' from Marc Swynghedauw show improvements for LRS and SSM models, but not for E6. The poor E6 limits this year arise as a result of our measured tau cross section at 189 GeV being 2.5 sigma low. Note also that our E6 limits at 183 GeV were better than expected. Errietta Simopoulou : Direct leptoquark search at 189 GeV ========================================================== Presented direct leptoquark search in 189 GeV data, using the process e+e- -> e+ q LQ. The new analysis improves over old 183 GeV one using suggestions made at previous BEW meeting. Notably (i) Different selection cuts at each LQ mass, (ii) Limits now set on same LQ models as in BEW paper, with same notation. (iii) Careful treatement of cases where 1, 2 or 3 hadronic jets found to avoid bias in reconstructed LQ mass. Limits are set in the range 170-180 GeV for most LQ types. In several cases, these limits are better than those obtained indirectly from the hadronic cross sections (Ian Tomalin talk).