  1st Indirect Searches informal meeting, Thursday 15th, May, 1997


Present : M.N. Minard, E. Merle, D. Schlatter, L. Moneta, F. Cerutti,
          V. Buescher, R. Alemany, I. Tomalin, G. Dissertori
_____________________________________________________________________

  Agenda : 


1.- EW measurements at LEP2
    -----------------------  
   o Status report, Things-to-Do list(Marie-Noelle Minard)


2.- A.O.B 
    ----- 
   o Discussion






  
-  Status report, things to do
   -------------------------------

Marie-Noelle first has given an overwiew of the statistical
and systematic errors in the various x-section measurements for
1996, and the statistical errors to be expected for 1997
(Note: She has assumed 10 times more integrated Lumi. compared to
1996, however, with the latest SPS problem this seems to be very very 
optimistic). Here a table of the errors for

 
sqrt(s'/s) > 0.9 :

Channel	1996	1997
qqbar	5% stat 4% syst	2% stat
µ⁺µ^-	1.5% stat 1.1% syst	5% stat
tau⁺tau^-	18% stat 3.1% syst	6% stat
e⁺e^-	3.1% stat 1.5% syst	1% stat
A_FB(µ)	8% stat	2.5% stat

Whereas in 1996 the systematic error was no problem, in the future we should try to reduce it, to reach the level of the statistical one. In the hadronic channel the gamma-gamma background is negligible (for the hard s'-cut). The error on the efficiency is about 1%. A main problem for the s'-cut is the understanding of single and double radiative events. Therefore we have to study ISR and look for a possible s'-cut optimization.

The error from WW and other channels is of the order 2-3 %. Here the main question is what should be counted as signal (ZFITTER), what as backgrounds? Example:

Signal : qqqq via Z-gamma or gamma-gamma qqll via Z-gamma (ll from gamma*) gamma-gamma (qq pass s'-cut)

Backgr : WW, We\nu + all charged current
               eeqq   via    gamma-gamma or Z-gamma (->ee)
                llqq   via    gamma-Z              (ll from Z)
                                   gamma-gamma    (qq fail s'-cut)
               qqqq via     gamma-gamma    (both qq fail s'-cut)
                ZZ

The error from ISR/FSR interference amounts to approx. 1.5 % (studied by switching on and off the contributions from ISR and FSR). The biggest effect is seen at high s', low cos(theta). This has to be studied. Where can we improve?

Clarify impact of gamma-gamma events (tagged
--> gamma-gamma MC HERWIG, PHOT02

Photon radiation modelling to be reviewed, as this affects effeciency and sqrt(s'/s) --> PYTHIA, KORALZ

Strategy with respect to WW,ZZ, 4-fermion
--> EXCALIBUR, KORALW (CC03+all diagrams)

There is room for improvement in the event selection and systematics. Other points to be discussed:

A strategy (a likelihood function) for the limits extraction has to be established.

For the limits derivation one could also use Rb (error 1996: 18%, error 1997: approx 6%) and Rs(from K^* ?) .

The use of angular distribution cos(theta*)(muons) is to be investigated.

Marie-Noelle concludes that we have two goals: In a short term we should extract limits from the measurements at 130 - 172 GeV (and maybe 180 GeV) and submit to the summer conferences. In a longer term the actual analysis could be improved. As systematics are correlated between the experiments, the systematic error should be reduced to the 1% level.



  
-  Discussion
   ----------

Marie-Noelle has been asked to give the fortran code of her standard event selection routines to Guenther who will put them on a newly created Web page, like the ones which are available for the other ALEPH analysis groups.

An abstract should be send to Jerusalem. There they most general contact interaction scenario will be studied (like a recent OPAL paper or the ALEPH LEP1 paper on contact terms, Z.Phys.C59 (1993) 215). In the long term other more specific scenarios like leptoquarks will be looked at. As we expect not so much statistics before Jerusalem, the size of the systematic errors as shown by Marie-Noelle will be fine, and the detailed studies of systematics (ISR, s-' definition, ...) have a longer timescale. What has to be done immediately is described under the following point.

We have to decide about a strategy regarding the problem: What processes are to be considered "Standard Physics" when putting limits on contact terms, i.e., should we put everything (QCD, WW, gamma-gamma, 4-fermion) together or reject WW and/or other processes/backgrounds? The general feeling is that taking all standard processes together should be better, however, it has to be tested. Lorenzo and Fabio are willing to take care of this problem, however, they will be absent for the following two weeks. Dieter, Ian and Guenther should think about it in the meantime.

Ian volunteers to study the problem of the definition of s', to look for an alternative solution.

Guenther volunteers to study the problem of ISR.

How are the limits extracted? Marie-Noelle has already a program. Maybe it should also be put on the Web. Mario Antonelli and Frederic Teubert should be asked if they could take over this task.

Other LEP experiments should be contacted in order to find common definitions of non-standard physics, which in the end would help very much in combining limits.

Volker (and Gerardo Ganis) show interest in the topic, however, in a short term they will not directly contribute. The same holds for Ricard.

- Next meeting ---> In about two weeks. Exact date yet to be decided.

Guenther Dissertori
Thu May 22 19:21:38 MET DST 1997