Thursday 26 November 1998
                                                 Ian Tomalin


                MINUTES OF THE 24TH BEW MEETING
               =================================
                                                   
Agenda
======

E. Simopoulou : Direct search for single leptoquark production.   
M. Swynghedauw: Z' limits.
M.N. Minard   : Removal of acollinearity cut in Bhabha selection. 
M. Maggi      : Limits on gaugino decays to gluino                
                from qq cross section
F. Teubert    : To study or not to study the running of alpha_QED.


E. Simopoulou : Direct search for single leptoquark production    
===============================================================
  Errietta presented the direct search for single leptoquark production
in the 183 GeV data made by the Demokritos group. Their draft ALEPH note
can be found in 

    http://alephwww.cern.ch/LEP2ANALYSIS/BEW/results/Welcome.html

Several suggestions made to improve analysis. (e.g. Place limits on all
types of scalar and vector LQ, find out which cuts are causing loss of
selection efficiency and optimize cuts). Expected to be finished for
Moriond conference.

M. Swynghedauw: Z' limits
==========================
  Marc presented Z' limits based on <= 183 GeV data. A full description
of his analysis can now be found in ALEPH note 98-094.

M.N. Minard : Removal of acollinearity cut in Bhabha selection 
=================================================================
  Old Bhabha selection used tight acollinearity cut. This corresponded
to a very tight cut on sqrt(s'/s), implying large theoretical errors, so
has now been removed. This increases the selection efficiency by about
8%, but also increases radiative background by 30-80%. Comparison of 
old with new results, showed variations at 1 sigma level, which seemed
to have different signs at different energy points. Further checks 
would be made.

M. Maggi : Limits on gaugino decays to gluino from qq cross section
====================================================================
  Marcello used the LEP2 inclusive q-qbar cross section measurements to 
placed limits on the decay chain e+e- -> chi+ chi-, chi -> q qbar gluino.
If the gluino is light, this can dominate over the more conventional decay
mode chi+ -> W+ chi0. The gluino presumably fragments to a glueballino 
particle, but this is not explicitely searched for, plus a gluon jet. 
Hence one gets a multijet event. Providing that the mass difference between 
the chargino and the gluino is not too small, the selection effiency (of the 
standard LEP2 hadronic event selection), is close to 100%. 
  The analysis should be finished in a few months. Not yet included 
is the decay e+e- -> chi0 chi0. It was also suggested that one could 
improve on the standard inclusive hadronic selection by explicitely
rejecting Z return events, or removing the thrust cut. Furthermore, it
was recommended that any publication refer to ALEPH's existing limit on
light gluinos.

F. Teubert : To study or not to study the running of alpha_QED
===============================================================
  At high E_cms, about 50% of cross section is due to s-channel gamma
exchange, so if sigma is q-qbar cross section:

      Delta(sigma)       Delta(alpha_QED)
      ------------ = 2   ----------------
          sigma             alpha_QED

This implies that we can measure alpha_QED(M_Z) with a relative error of 1.6%,
which compares badly with 0.07% deduced from alpha_QED(0) + dispersion
relation. Therefore, not a good measure of alpha_QED. But can we claim
to test running of alpha_QED ? No, since most of running occurs at low
sqrt(s), and therefore experiments like TRISTAN can do much better than us.
Conclusion: forget it.