The WW exercise can best be accomplished with the DELPHI data set (Hands-on-CERN).

INTRODUCTION:
=============

In the 1/2h introduction to the exercise (which can be done e.g.
by browsing together through the first pages of the exercise package
using a beamer) the scientist should address all important issues, 
but can for sure not cover all eventualities (e.g. muons appearing
in jets, converted photons from pi0 on tau decays, etc.). The students
should encounter problems themselves. Especially the Z0:"tau-tau" class
should not just used as "all what doesnt fit elsewhere", but
the students should be convinced about their choice. 
The properties of the W in the detector should be noted. 
What is the difference to the Z0?

The presence of graduate student tutors for helping the high school
students with their PC exercises is extremely important. For about each
10 students (5 groups) one graduate student as tutor is needed.


CONDUCTING EXERCISES:
=====================

The OPAL and DELPHI WW package contain rouglhy 500 events each. Each Z0 exercise package contains 1000 events. For introduction each pair of students should "reconstruct" 25 to 50 Z0 events covering all possible decay channels if possible. They can log them into the local .xls file, but due to low statistics a reasonable result with low errors cannot be expected. 

Since the WW events are so small, a group of students should not work on less than 50 events, i.e. take 10 groups a 2 students and 50 events each. The students switch to W pair events and measure again 48 - 80 events each group depending on the time needed.

Necessarily, identical data samples are given to different groups, their results should be averaged, before entering the local EXCEL table. 

NB: the EXCEL file does *not* require that the groups work through all of their events. Often some careful groups are much more slower than the others. Then either the faster groups have to get another voluntary exercise or the slower groups just give e.g. the result for 70 scanned events instead of 100.  

STRATEGY:
=========
The given event samples are mixed Z0 and WW decays. Before starting, the student team needs to clearify how many WW decay they really observe. On the basis of these events, the "measurement" can start. Maybe the tutors can suggest an overall measurement technique:
1. see if the event has 4 jets (qqqq)
2. if not, see if it contains 3 jets (maybe Z0->3jets or WW->tau+nu+qqbar)
3. if not, for 2 jets, what is the remaining lepton (electron, mu, tau->pi->electron)


IMPORTANT:
==========
Safe your local .xls data file and transfer the data to the moderator before the video conference via koala!!

GOALS:
======
On average, the students are entitled to measure the branching ratios of the W decaying into leptons(e,mu,tau) and hadrons.

The given LEP WW results are from table G.2 in LEP physics report (PDG 2006).

overall goals of the exercise are:

0) This not school. Common reasoning is needed to identify an event. BUT, there is never a 100% correct answer like in school. Reasoning can produce mistakes which later enter the error calculation. This is physics, this is life!

1) r(evqq) = r(mvqq) = r(tvqq) within the calc. errors
   --> lepton universality

2) calculate the average of all lepton ratios: 
	(sum of all)/ 3

3) determine N(qqqq) / N(lvqq) = N_colour !
   (needs to be corrected for an alpha_s contribution of higher orders, this is already implemented in the .xls sheet)
	=> if somebody asks:
		N(lvlv) / N(lvqq)  = 9 / (4*N_colour)
		(reason: pure combinatorics)	

naive:= LO w/o alpha_s correction

          	LEP/%       	Theory/%   ratio  naive  
r(lvlv/all)   	10.6 +- 0.2   	10.55      0.72   0.75
r(evqq/all)   	14.5 +- 0.2   	14.62      1.00   1.00
r(mvqq/all)   	14.3 +- 0.2   	14.62      1.00   1.00
r(tvqq/all)   	15.2 +- 0.3   	14.62      1.00   1.00
r(qqqq/all)  	45.4 +- 0.4   	45.59      3.12   3.00