- Marie-Noelle Minard : Hadronic events at LEP2 ---------------------------------------------- 1) PYTHIA produced to few events with large angle ISR, and hence got Z return cross-section wrong. This has now been fixed by changing a flag in the PYTHIA cards. 2) Proposal to define cross-section for hadronic events such that events with a Z propogator mass exceeding 0.9*s are called signal. (Instead of old definition where events with outgoing q-qbar mass > 0.9*s called signal). Argument for - don't have to worry about FSR photons merging with jet. Argument against - propogator mass is not well defined because of ISR-FSR interference. Long discussion, no decision. 3) Small changes in thrust and visible mass cuts to improve selection.
- Fabiola Gianotti : What to do with leptoquark analysis ------------------------------------------------------ The direct leptoquark search of E.Simopoulou is not competative with results from the TEVATRON or HERA. => Does not justify a paper on its own, but could be published as part of another paper. Feeling was that this should be a paper on Other Searches including for example excited leptons (on which nobody is working at present.)
- Elsa Merle : Bhabhas at LEP2 ----------------------------- Progress report: New cuts have been found which halve the number of radiative background events selected. However, the Ebeam/P distribution for Bhabhas seems badly simulated. Hopefully it will be better with the new GALEPH version. Final report on Bhabha cross-section expected at next week's meeting.
- Ian Tomalin (for Guenther Dissertori) : Status of MC production --------------------------------------------------------------- 183 GeV MC being regenerated with latest GALEPH version to benefit from better calorimeter simulation, TPC coordinate smearing and VDET matter etc. Bhabha production with BHWIDE started last week. By end of ALEPH week should be finished, with 10X luminosity of data. Hadronic events to be produced with KORALZ starting next week and ending on around 7th May (about 15X luminosity of data). No plans for tau or mu or for lower energy MC.
- Corrine Goy : sigma(c-cbar) --------------------------- New analysis to measure sigma(c-cbar) at 183 GeV. Rejection of b events achieved using mixture of lifetime tags. Selection of c events then done using 12 variable N.N. => 50% purity. sigma(c-cbar) = 6.05 +- 0.7 +- 0.3 +- 0.2
- Discussion of publication plans -------------------------------- Wish to produce paper for publication by end of May: Status of analyses - Sigma(q-qbar) (Minard): Will be finished once MC production done (mid-May) Sigma(l+l-) and A_FB(l+l-) (Merle): The dimuon and Bhabha results should be done within a couple of weeks. The ditau by mid-May. ALEPH note will be written. Sigma(b-bbar) (Antonelli): Can be finalized. sigma(c-cbar) (Goy): Can be finalized. A_FB(q-qbar,c-cbar,b-bbar) (Tomalin, Goy, ?): Probably not finished for paper. Maybe -> conference. Results can be used to constrain new physics - Contact Terms (Teubert): OK Leptoquarks (Ganis): OK ? Z' (Swynghedauw): OK, but not during next couple of weeks. R-parity violating squarks : Should be only a couple of days work, but nobody doing it !!! R-parity violating sneutrinos : Nobody !!!
- Mario Antonelli: Single top production --------------------------------------- In S.M. BR(t -> c Z/gamma) < 10**-10. However, experimental limits (from CDF) very weak. CDF limit implies t-cbar production at LEP 183 GeV has cross-section of < 0.1 pb. ALEPH analysis gives 16% selection efficiency. Limit < 0.5 pb. With this year's data, ALEPH should be able to compete with CDF.
- Marc Swynghedauw: EPIDOR ------------------------ The EPIDOR package does electron/muon identification. It was used for dilepton studies etc. at LEP1, and is now updated for LEP2. An ALEPH note will be released very soon. Uses more detailed information of HCAL digital hit pattern than QMUIDO, so e.g. better at identifying low momentum muons, and also achieves higher purities. Works best in low multiplicity events. => Should be useful for gamma-gamma physics too. Only runs on DST/POT.
- Next meeting --->
Thursday 30th April ?
Ian Tomalin
Tue April 21, 17:30:38 MET DST 1998